AMERICAN MUSEUM Novtitates

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024

Number 3322, 44 pp., 5 figures February 27, 2001

Earliest Eutherian Ear Region: A Petrosal Referred to Prokennalestes from the Early Cretaceous of Mongolia

JOHN R. WIBLE,' GUILLERMO W. ROUGIER,”? MICHAEL J. NOVACEK,? AND MALCOLM C. McKENNA?

ABSTRACT

A right petrosal from the ?Aptian or Albian Khoobur locality is referred on the basis of size and morphology to Prokennalestes trofimovi, the earliest eutherian previously known only from dentigerous elements. The petrosal shows a mosaic of primitive and derived features, bearing on the purported therian and eutherian morphotypes. Among the primitive features shared with the Early Cretaceous prototribosphenidan Vincelestes and other more basal taxa that are modified in later eutherians and metatherians are the pattern of basicranial arterial and venous circulation, including a prootic canal and an intrapetrosal inferior petrosal sinus; a vertical paroccipital process; and a fenestra semilunaris, an incomplete wall between the cavum epiptericum and cavum supracochleare. Among the derived features shared with therians is a cochlea coiled through a minimum of 360°, with Prokennalestes extending the range of the oldest occurrence of such a coiled cochlea by at least 10 million years. Shared with Late Cretaceous eutherians is a shallow internal acoustic meatus with a thin prefacial commissure. The petrosal referred to Prokennalestes is intermediate in having a reduced anterior lamina and lateral flange, both of which are well developed in Vincelestes and essentially lacking in later eutherians and metatherians. Features previously held to be part of the therian and eu- therian morphotypes, such as the absence of the anterior lamina and lateral flange, may have been lost independently in metatherians and in post-Prokennalestes eutherians.

‘Research Associate, Division of Vertebrate Zoology, American Museum of Natural History; Associate Curator, Section of Mammals, Carnegie Museum of Natural History, 5800 Baum Boulevard, Pittsburgh PA 15206.

? Research Associate, Division of Paleontology, American Museum of Natural History; Assistant Professor, Depart- ment of Anatomical Sciences and Neurobiology, School of Medicine, University of Louisville, Louisville KY 40292.

3 Curator, Division of Paleontology, American Museum of Natural History.

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INTRODUCTION

Remains of eutherian mammals from the Early Cretaceous are exceedingly rare. The only undoubted eutherians from this interval are Prokennalestes from the ?Aptian or Al- bian of Mongolia (Kielan-Jaworowska and Dashzeveg, 1989; Sigogneau-Russell et al., 1992) and Bobolestes from the late Albian of Uzbekistan (Nessov, 1985; Nessov and Kie- lan-Jaworowska, 1991; Nessov et al., 1994). The former is known from nearly complete lower dentitions and fragmentary uppers (Kielan-Jaworowska and Dashzeveg, 1989; Sigogneau-Russell et al., 1992), whereas the latter is known from a maxillary fragment with two molars (Nessov, 1985). Other pur- ported Early Cretaceous eutherians known from incomplete lower jaws include Ausktri- bosphenos from the Aptian of Australia (Rich et al., 1997, 1998, 1999) and Endo- therium from not later than the Aptian of northeast China (Shikama, 1947; Wang et al., 1995). The eutherian designation of Ausktri- bosphenos has been contested by Kielan-Ja- worowska et al. (1998) and Rougier and No- vacek (1998), and the original specimen of Endotherium has been lost. Other possible Early Cretaceous eutherians include Montan- alestes from the Aptian-Albian of Montana (Cifelli, 1999) and, according to Kielan-Ja- worowska (1992), Tribotherium known from four isolated, incomplete upper molars from the ?Berriasian of Morocco (Sigogneau-Rus- sell). 1991. 1995):

Khoobur (also variously spelled Khobur, Khoboor, and Khovboor), Guchin Us Somon, Mongolia is the ?Aptian or Albian locality yielding Prokennalestes (Kielan-Jaworowska and Dashzeveg, 1989). Discovered by the Joint Soviet-Mongolian Paleontological Ex- peditions (Beliajeva et al., 1974), additional collecting at Khoobur has been conducted by the Geological Institute of the Mongolian Academy of Sciences, and in the summer of 1991, 1997, and 1999, by the Mongolian Academy of Sciences—American Museum of Natural History Expeditions (MABE). In ad- dition to Prokennalestes, a diverse mamma- lian fauna has been reported from fragmen- tary mandibular remains, including multitu- berculates (Trofimov, 1980; Kielan-Jawo- rowska et al., 1987), gobiconodontids

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(Trofimov, 1978), symmetrodonts (Trofimov, 1980, 1997), cladotherians (Dashzeveg, 1979, 1994), and tribosphenidans (Dashze- veg, 1975; Dashzeveg and Kielan-Jaworows- ka, 1984).

In 1995, Wible et al. described a well-pre- served petrosal bone (PSS-MAE 104) from Khoobur yielded through screen washing by the MAE. They concluded (Wible et al., 1995: 10), based on its size and the results of their cladistic analysis, that this specimen belonged “‘to either an as yet unknown tri- conodont or to a primitive holotherian, which in the context of the known Khoobur fauna would be the symmetrodont Gobiodon infin- itus . ... Holotheria includes the common ancestor of Kuehneotherium and _therians plus all its descendants.”’ In a subsequent contribution, Rougier et al. (1996a) reported a second petrosal (PSS-MAE 129), which was identified as the sister group of PSS- MAE 104 in their cladistic analysis. How- ever, the triconodont versus holotherian re- lationships of both specimens were left un- resolved. More recently, another well-pre- served, smaller petrosal has been found in the MAE screen wash collection from Khoobur. A preliminary announcement of this specimen (PSS-MAE 136) was made in Wible et al. (1997). These authors identified this as the petrosal of ?Prokennalestes. A full description of this specimen, the earliest known eutherian ear region, is provided here.

METHODS

For the descriptions of PSS-MAE 136, we employ the anatomical terminology that we have used in reports on basicrania of other Mesozoic mammals (e.g., Wible, 1990; Rougier et al., 1992, 1996a; Wible et al., 1995). In addition to describing the outer sur- faces of the petrosal, we report some of the internal features of the PSS-MAE 136 as re- vealed through radiographic analysis. The specimen was digitally imaged at the Uni- versity of Louisville School of Dentistry us- ing a small dental intraoral charged-coupled device (RadioVisioGraphy Model PCi sen- sor; Trophy Radiography, Vincennes, France) and a dental X-ray generator oper- ating at 70 kVp, 7mA at an exposure time of approximately 0.1 seconds. Multiple projec-

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tions of the specimen were taken and resul- tant images enhanced in Adobe Photoshop. We also reconstruct the major vessels and nerves associated with the petrosal bone. Our research on the anatomy of recent mammals (e.g., Novacek, 1986, 1993; Wible, 1986, 1987, 1990; Rougier et al., 1992; Wible and Hopson, 1995) serves as background for this vascular and nervous reconstruction. Recent- ly, several authors (e.g., Bryant and Russell, 1992; Witmer, 1995) have offered explicit methods for reconstructing soft tissues in fossils and for evaluating levels of confi- dence in those inferences. In formulating hy- potheses about soft-tissue reconstruction here, we accept that PSS-MAE 136 is attrib- utable to Prokennalestes and, following the recent phylogenetic analysis by Rougier et al. (1998), identify Prokennalestes as a basal eu- therian (fig. 5). Consequently, under the ter- minology proposed by Witmer (1995), the extant phylogenetic bracket (minimally, the first two extant outgroups) for Prokennales- tes consists of placentals and marsupials. In- ferences that are based on soft-tissue struc- tures and osteological correlates occurring in both extant outgroups are considered more decisive than those occurring in only one.

INSTITUTIONAL ABBREVIATIONS

AMNH_ Department of Vertebrate Pale- ontology, American Museum of Natural His- tory, New York

MACN Museo Argentino de Ciencias Naturales “‘Bernardino Rivadavia’’, Buenos Aires

MAE Collections of joint Mongolian Academy of Sciences—American Museum of Natural History Paleontological Expeditions

PSS Paleontological and Stratigraphic Section of the Geological Institute, Mongo- lian Academy of Science, Ulaan Baatar

UCMP University of California, Muse- um of Paleontology, Berkeley

DESCRIPTIONS

In Recent mammals, the petrosal houses the organs of hearing and equilibration. For descriptive purposes, the therian petrosal his- torically (e.g., Voit, 1909; Fawcett, 1918) has been divided into two parts: the more anter-

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 3

oventral pars cochlearis, enclosing the co- chlea, and the more posterodorsal pars can- alicularis, enclosing the vestibule and the semicircular canals. We describe the petrosal of Prokennalestes here in three views—ven- tral, dorsal, and lateral—with the orientation based on the presumed position in the skull. However, given that the specimen is isolated and incomplete, its precise orientation in the skull is subject to interpretation. We have provided an estimate in figure 1, but must admit that the angulation to the midline may be considerably different depending on the proportions of the surrounding cranial ele- ments. Following our descriptions, the mor- phology of the osseous inner ear as revealed through radiographic analysis and a recon- struction of the courses of the major vessels and nerves suggested by grooves, canals, and foramina on PSS-MAE 136 are presented.

PSS-MAE 136 is a right petrosal (fig. 1). The pars cochlearis is largely intact, with the only substantive damage being to the antero- medial surface, which has exposed spongy bone within the petrosal. In contrast, the pars canalicularis has suffered considerable dam- age such that roughly the lateral, posterodor- sal half of it is missing. Of the three semi- circular canals, the lateral one is almost wholly enclosed in bone, as is the bulk of the posterior one. However, little remains of the anterior semicircular canal or the bone be- tween it and the other two canals.

VENTRAL VIEW (fig. 1A, D)

In ventral view, the pars cochlearis is rep- resented chiefly by the oval promontorium of the petrosal. The shape and topography of the promontorium closely reflect the en- closed cochlear duct (fig. 2), which does not coil in a single plane but in a ventrally di- rected spiral. Consequently, the ventralmost bulge of the promontorium, which is situated posteromedially, underlies the end of the coil. Extending from the anterior and medial border of the promontorium is a narrow, flat- tened shelf. The full extent of the anterior part of this shelf is uncertain because of dam- age, but the medial part is intact. We identify this shelf as an epitympanic wing following MacPhee (1981), who used that term for out-

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1mm

Fig. 1. Three views of right petrosal referred to Prokennalestes trofimovi, PSS-MAE 136. A, D, Ventral view. B, E, Dorsal view. C, F, Lateral view. Given that the petrosal is both isolated and incomplete, providing the precise orientation for the bone in the complete skull is difficult. One estimate is shown here. Scale = 1 mm.

growths from any basicranial bones contrib- __ nestra vestibuli is recessed slightly from the uting to the tympanic roof. surface of the promontorium; this is most

Two large apertures open into the posterior — pronounced along the lateral border. As a re- aspect of the promontorium. The anterolat- sult of breakage, the posteromedial section of eral and larger of the two is the fenestra ves- _ the rim of the fenestra vestibuli is missing. tibuli or oval window, which in life accom- Despite the damage, the shape and orienta-

modated the footplate of the stapes. The fe- tion of the fenestra can be reasonably recon-

2001 WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES

D greater petrosal nerve

facial foramen ve ramus inferior groove prootic canal

Be ade internal carotid groove ramus inferior foramen

lateral flange epitympanic wing

stapedial artery promontorium foramen

squamosal facet stapedial groove ROSTRAL

epitympanic recess fenestra vestibuli LATERAL

Tse ineudis cochlear canalicularis crista parotica stylomastoid notch paroccipital process stapedius fossa

caudal tympanic process exoccipital facet

Broken : : : ) crista interfenestralis prefacial commissure fenestra semilunaris prootic canal foramen acusticum fem INTERNAL superius prootic sinus groove ACOUSTIC MEATUS ventral ascending foramen acusticum canal inferius a, anterior semicircular canal crus commune anterior ampulla lateral ampulla ROSTRAL inferior petrosal sinus subarcuate fossa MEDIAL

jugular notch lateral semicircular

. { canal cochlear canalicularis

posterior ampulla vestibular aqueduct ; >. posterior semicircular

F exoccipital facet canal subarcuate fossa

posterior semicircular crus commune

canal ha prootic sinus groove

anterior semicircular

canal internal acoustic meatus

ramus superior groove

cavum epiptercium wall

DORSAL ventral ascending canal arteria diploética magna groove

notch for ramus temporalis

CAUDAL

?alisphenoid facet greater petrosal nerve promontorium

squamosal facet fenestra semilunaris

anterior lamina

proctic canal 1 mm

Fig. 1. Continued.

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structed. As a measure of shape, and assum- ing that the outline of the fenestra vestibuli reflects the stapedial footplate morphology, Segall’s (1970) stapedial ratio dength/width of the oval window or footplate) is approxi- mately 1.7, that is, somewhat elliptical. Re- garding orientation, the opening is directed ventrolaterally and slightly posteriorly. The other large aperture is into the posterior as- pect of the promontorium. This is the fenes- tra cochleae or round window, which was closed by the secondary tympanic membrane in life. We identify this aperture as a round window and not a perilymphatic foramen as in monotremes (Kuhn, 1971; Zeller, 1985, 1989, 1991), because PSS-MAE 136 has a separate canal for the perilymphatic duct as in Recent therians (“‘cochlear canaliculus”’ in fig. 1E). The bone flooring this canal in ex- tant therians is derived from the processus recessus of the chondrocranium (De Beer, 1937; Zeller, 1985). In PSS-MAE 136, the processus recessus is the narrow bridge of bone between the medial border of the fe- nestra cochleae and the jugular notch. The fenestra cochleae is directed posteriorly and somewhat ventrally, and is subcircular, slightly wider in the horizontal plane than vertically. Separating the round and oval windows is a narrow, near vertical bar of bone, the crista interfenestralis.

There are two smaller openings into the outer contour of the ventral surface of the pars cochlearis. Anterior to the fenestra ves- tibuli is a posteriorly directed, oval aperture, the facial foramen, which transmitted the main or hyomandibular branch of the facial nerve into the middle-ear space. Running posteriorly from the facial foramen is a very short, shallow facial groove that ends just in front of the oval window. The other opening, barely visible in ventral view, lies at the pos- teromedial corner of the pars cochlearis, just in front of the jugular notch, the petrosal’s contribution to the border of the jugular fo- ramen. As preserved, there are actually two openings in this area: a smaller, circular pos- terior one and a larger, oval anterior one. We believe that the latter is a product of damage and only the smaller, circular foramen was present during life. Visible through both openings is spongy bone laterally and a nar- row canal in the epitympanic wing anteriorly.

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Although the endpoint of this canal cannot be confirmed, we believe that its terminus is in the spongy bone exposed via damage at the anteriormost surface of the pars coch- learis (visible in dorsal and lateral view, fig. 1B, C). As discussed in the vascular recon- struction, and following similar examples in other Mesozoic mammals, we interpret this canal as for the inferior petrosal sinus.

Two moderately developed vascular sulci run nearly the length of the ventral surface of the pars cochlearis. The more medial one starts posteriorly at the medial rim of the fe- nestra vestibuli and extends anteromedially to the end of the epitympanic wing. This is the transpromontorial sulcus for the internal carotid artery (Wible, 1986). The sulcus notches the medial rim of the fenestra ves- tibuli, with the occupant of this notch inter- preted to be the stapedial artery (Wible, 1987; Rougier et al., 1992). The second sul- cus runs along the lateral edge of the pro- montorium, beginning just in front of the fa- cial foramen and extending onto the epitym- panic wing. The major occupant of this groove is interpreted to be the ramus inferior of the stapedial artery (Wible, 1987; Rougier et al., 1992). About halfway along the length of the sulcus for the ramus inferior, a second, much smaller groove flows into it from the endocranial surface. This would have trans- mitted the greater petrosal nerve (palatine ra- mus) of the facial nerve. Continuous poste- riorly with the well-developed sulcus for the ramus inferior and anterior to the facial fo- ramen, there is what might be a very faint sulcus (see Vascular and Nervous Recon- struction).

The pars canalicularis in ventral view is roughly L-shaped, with the short arm poste- rior to the promontorium and the long arm lateral to the posterior half of the promon- torium. The outer contour of both arms of the L is raised as crests and eminences, whereas the inner contour is depressed as troughs and fossae. For descriptive purposes, we treat the short and long arms separately.

The crest on the posterior edge of the short arm, the caudal tympanic process (MacPhee, 1981), forms the back wall of the middle-ear space. It does not extend medially all the way to the jugular notch, but fades out posterior to the middle of the fenestra cochleae. The

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small section immediately behind the fenes- tra cochleae is the only undamaged part of the caudal tympanic process. It is lower than the broken lateralmost part of the caudal tympanic process, but slightly higher than the intervening middle part. The broken base of the caudal tympanic process, which con- tained a pneumatic space, extends laterally from behind the round window and connects with the raised outer edge of the long arm of the L. Between the caudal tympanic process and the fenestra cochleae is a flattened shelf, which forms the posterolateral border of the jugular notch. Lateral to this shelf is a de- pression containing a broad, flat, oval fossa, which is roughly twice the area of the fenes- tra vestibuli. This fossa housed the origin of the stapedius muscle. The bone roofing the stapedius fossa is thinner than that surround- ing it and post mortem damage has opened a hole in the fossa that connects to the en- docranial surface of the petrosal. It is appar- ent from study of the endocranial surface that the rim around the stapedius fossa is formed by the bone containing the lateral semicir- cular canal. Consequently, the caudal tym- panic process, which forms the posterior wall of the stapedius fossa, lies directly ventral to the lateral semicircular canal.

The morphology of the lateral edge of the long arm of the L is more complex than that of the short arm. Posteriorly is the broken base of a broad eminence, which contained a pneumatic space continuous with that in the adjacent caudal tympanic process. This em- inence is equivalent to what is called the lat- eral section of the caudal tympanic process of the petrosal in various placentals (Mac- Phee, 1981) or the paroccipital process in more basal taxa, such as the prototribosphen- idan Vincelestes from the Early Cretaceous of Argentina (Rougier et al., 1992). We em- ploy the latter term here. Although the ven- tral extent of the paroccipital process in PSS- MAE 136 is uncertain, it apparently was well developed and vertical in orientation. Ante- rior to the paroccipital process is a triangular depression, whose apex points posteriorly. The deepest part of this depression is at the apex and in life housed the crus breve of the incus. Anterior to the fossa incudis is the shallower and broader epitympanic recess, which housed the articulation between the

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 7

malleus and incus. Forming the medial wall of the fossa incudis is the crista parotica, continuous with the paroccipital process be- hind. Only the broken base of this crest is preserved, and it also contained a pneumatic space, likely continuous with that in the par- occipital process. In extant mammals, the crista parotica provides the point of attach- ment to the embryonic auditory capsule of Reichert’s cartilage (the second or hyoid arch cartilage), the proximate segment of which often ossifies to form a tympanohyal element in the adult (De Beer, 1937). Although not preserved in PSS-MAE 136, the point on the crista parotica where the tympanohyal (whether cartilaginous or osseous) would have attached is indicated by slight notching in the medial wall of the crest. This stylo- mastoid notch transmitted the facial nerve from the middle-ear space. The tympanohyal would have attached to the crista parotica im- mediately in front of the notch. Forming the lateral wall of the fossa incudis and epitym- panic recess is a thickened, rounded ridge, continuous with the paroccipital process be- hind. Judged by the surface texture, this ridge also likely contained a pneumatic space. Moreover, it is apparent that the bulk of this ridge was covered in life by another bone, presumably the squamosal, which therefore was the major element in the wall lateral to the mallear—incudal articulation. In front of the epitympanic recess, this ridge narrows and continues to the anterior limit of the pars canalicularis. As described below, the narrower anterior section of this ridge floors several prominent vascular foramina. Because of its continuity with the paroccip- ital process and crista parotica, we identify this ridge as the lateral flange of the petrosal, as occurs in more basal taxa (Rougier et al., 1992; Wible and Hopson, 1993).

In the interval between the promontorium on the one side and the paroccipital process, crista parotica, and lateral flange on the other, the pars canalicularis is marked by a broad, smooth-walled, longitudinal trough. This trough ends at a subcircular aperture at the anterior limit of the pars canalicularis. This aperture is directed anterosuperiorly and slightly laterally, and leads into a short, near vertical prootic canal opening on the endo- cranial surface of the petrosal. Lateral to the

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prootic canal, above the lateral flange, is a figure-eight-shaped aperture, which we inter- pret as having accommodated two arteries. The smaller posterior opening of the figure eight was for the stapedial artery; the larger anterior aperture was for the ramus inferior. The figure-eight-shaped aperture, which is directed laterally, leads into a canal that bends superiorly to open on the lateral sur- face of the petrosal. This canal is equivalent in position and general orientation to the ven- tral ascending canal in more basal taxa, such as Vincelestes (Rougier et al., 1992).

DORSAL VIEW (fig. 1B, E)

As in the case of the ventral view, the shape of the pars cochlearis in dorsal view closely reflects that of the enclosed cochlear duct (fig. 2). The most prominent feature is the internal acoustic meatus for the facial and vestibulocochlear nerves, which is an ovoid opening offset laterally from the center of the pars cochlearis. Within the meatus are two unequal-sized, oval apertures separated by a low transverse septum. The smaller, lateral aperture, the foramen acusticum superius, is directed ventrolaterally and ends in two sub- equal-sized, circular structures. The larger anterior one is a canal transmitting the facial nerve. The smaller posterior one is a blind pit with tiny perforations in it. This is the cribriform dorsal vestibular area for the pas- sage of bundles of the vestibular nerve. The larger, medial aperture in the internal acous- tic meatus, the foramen acusticum inferius, is directed ventrally into a pit, whose ante- rior, medial, and posterior walls have three irregular apertures into the inner ear, which we believe are the result of damage. The spicules of bone between these three open- ings have a rough, pitted surface resembling that in the cribriform dorsal vestibular area. The remaining surfaces in the foramen acus- ticum inferius are smooth. We interpret this rough surface as evidence of another cribri- form area, in this case the spiral cribriform tract (tractus spiralis foraminosus), tiny per- forations in a spiral belt that transmit the fas- cicles of the cochlear nerve in other therians (Meng and Fox, 1995a, 1995b).

The surface of the pars cochlearis anterior

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and medial to the internal acoustic meatus is broad and flat. The surface posterior to the meatus slopes posterodorsally into the pars canalicularis (see below). The lateral wall of the meatus is formed by a thin bar of bone, the prefacial commissure. The aspect of the pars cochlearis lateral to the prefacial com- missure slopes steeply ventrally and is more fully visible in lateral view (fig. 1C). This smooth-walled surface formed the postero- medial wall of the cavum epiptericum, the extradural space housing the trigeminal gan- glion and other nervous and vascular struc- tures (Gaupp, 1902, 1905; Kuhn and Zeller, 1987). The only other feature on the endo- cranial surface of the pars cochlearis is just lateral to the jugular notch, where there is a depression with two dorsomedially directed foramina. The larger ventral one transmitted the perilymphatic duct. This foramen is often called the cochlear aqueduct, but following the Nomina Anatomica Veterinaria (1994, Ath ed.) we refer to it as the cochlear cana- liculus. The smaller dorsal foramen likely transmitted a vein accompanying the peri- lymphatic duct.

Less than half of the endocranial surface of the pars canalicularis is preserved; it rises steeply posterodorsally from the pars coch- learis. The area just behind the internal acoustic meatus housed the vestibule of the inner ear, and projecting from that were the three semicircular canals (fig. 2). The most prominent feature on the pars canalicularis is a deep depression in the preserved posterior edge. When closed by the complete pars can- alicularis, this depression would have been the anterior part of a very wide, deep subar- cuate fossa, which housed the paraflocculus of the cerebellum. The loss of the posterior edge of the subarcuate fossa has made visible parts of the bony housing for all three semi- circular canals.

The only complete canal preserved is the lateral (horizontal) semicircular canal, which lies in the floor of the subarcuate fossa. The bone between the lateral semicircular canal and vestibule is very thin and is perforated by a jagged, irregular opening, the artifact within the stapedius fossa described above. A bulge between the lateral terminus of the lateral semicircular canal and the vestibule reflects the underlying lateral ampulla. The

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posterior (inferior) semicircular canal lies in the medial wall of the subarcuate fossa, and that for the anterior (superior) would have formed most of the rim of the now incom- plete opening into the subarcuate fossa. Ven- trally the posterior canal connects with the lateral one just distal to a bulge representing the posterior ampulla. Dorsally the posterior canal is broken open, exposing a groove and two openings. It is in this broken area, in the medial rim of the subarcuate fossa, that the posterior and anterior canals join to form the crus commune. From there the crus com- mune continues forward in the anteromedial rim of the subarcuate fossa to connect with the vestibule. Along the ventral surface of the bone enclosing the crus commune is a small posterodorsally directed foramen, the vestibular aqueduct for passage of the en- dolymphatic duct. The anterior semicircular canal is broken open just distal to the bulge over the anterior ampulla in the anterolateral rim of the subarcuate fossa.

The surface of the pars canalicularis an- teromedial to the subarcuate fossa is smooth. Often in eutherians, this area has a sulcus transmitting the sigmoid sinus to the jugular foramen. Posterior to this smooth-walled sur- face is a roughened, medially facing, cres- centic facet for contact with another bone, presumably the exoccipital. The surface of the pars canalicularis lateral to the vestibule contains a posterodorsally directed, round fo- ramen; this is the endocranial aperture into the prootic canal. Leading into this foramen from above and behind is a broad sulcus for the prootic sinus, which likely continued onto the missing posterior part of the pars canalicularis. Ventrolateral to this sulcus, the endocranial aperture into the ventral ascend- ing canal is visible. As with the tympanic aperture, the endocranial one is somewhat figure-eight shaped, with the posteroventral opening larger than the anterodorsal one. The shape of this aperture is fully visible only in an oblique dorsal view (not shown). Antero- lateral to this aperture is a smooth, crescentic surface exposed on the lateral braincase wall (see below).

LATERAL VIEW (fig. 1C, F)

The pars cochlearis presents two main sur- faces in lateral view. Anteriorly is the broken

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 9

anterior pole with spongy bone exposed by post mortem damage. Posterior to that is the smooth, laterally facing surface that contrib- uted to the posteromedial part of the cavum epiptericum described above. The ventral edge of this smooth surface bears a narrow, ventrolaterally directed ridge, except where is it notched by a narrow groove (see below). The edge of this ridge is flattened, serving as a facet for contact with another bone, pre- sumably the alisphenoid. Just above the pos- terior end of this ridge is a subcircular de- pression, within which is an oval foramen. The foramen opens into a small space within the petrosal that has two other points of egress: the facial foramen in the middle ear and the canal for the facial nerve in the in- ternal acoustic meatus. The space in the pe- trosal is the cavum supracochleare (Voit, 1909), which housed the geniculate ganglion of the facial nerve. We name the opening into the cavum supracochleare visible in lat- eral view the “‘fenestra semilunaris’’, follow- ing Rougier et al. (1992). Running antero- ventrally from the fenestra semilunaris is a narrow groove interpreted to be for the great- er petrosal nerve, a branch of the facial nerve, which notches the ridge bearing the alisphenoid facet and, therefore, would have been closed as a foramen between the petro- sal and alisphenoid in life. The groove con- tinues onto the ventral surface of the pars cochlearis and merges with the groove for the ramus inferior of the stapedial artery.

Two distinct regions of the pars canalicu- laris are visible in lateral view, a ventrolat- eral one (see below), and a more dorsomedial one exposed through post mortem damage. The latter is the deep, medial portion of the subarcuate fossa, rimmed by the semicircular canals. Forming the anteroventral rim of the subarcuate fossa is the crus commune, the conjoined anterior and posterior canals. The remainder of the rim is formed by the pos- terior semicircular canal, including its broken portion at the dorsal apex.

The ventrolateral region of the pars can- alicularis in lateral view shows postmortem damage, and it is uncertain how much of the original bone has been lost through breakage. The ventralmost part of what is preserved has a small, bowed, crescentic surface that we believe was exposed on the sidewall of

10 AMERICAN MUSEUM NOVITATES

the braincase. Consequently, we identify this as an anterior lamina such as occurs in ex- tinct non-therian mammaliaforms (Kermack and Kielan-Jaworowska, 1971; Wible and Hopson, 1993). The ventral edge of the an- terior lamina is the lateral flange (see above under Ventral View, fig. 1A, D). The anterior lamina in Prokennalestes was larger than the preserved crescentic surface, because there is evidence of breakage along most of its an- terior and dorsal border. However, the unbro- ken edges that are preserved, especially pos- terodorsally, reveal that, overall, the anterior lamina in Prokennalestes was smaller than that in extinct non-therian mammaliaforms.

Immediately behind the anterior lamina in PSS-MAE 136 is a deep notch (visible in ventral view lateral to the lateral flange, fig. 1A, D). Most of the bony surface of this notch is roughened for contact with another bone, presumably the squamosal. The only exception is the surface adjacent to the an- terior lamina, which is smooth and may not have been covered.

Running adjacent to the anterior lamina are two major vascular channels. Immediate- ly medial to the anterior lamina is the ventral ascending canal, and anterior to it the sulcus for the prootic sinus. Both of these channels run at about a 45° angle to the horizontal, sloping posterodorsally. The sulcus for the prootic sinus ends ventrally at the endocra- nial aperture of the prootic canal, which is situated between the anterior lamina and fe- nestra semilunaris. Two major vessels were transmitted by the ventral ascending canal in light of grooves emanating from this canal’s figure-eight-shaped endocranial aperture. The larger posterior groove, interpreted to be the posttemporal groove for the arteria di- ploética magna, bends posteromedially, and the smaller anterior one, identified as the dor- sal ascending groove for the continuation of the ramus superior, runs dorsally. A third smaller vessel apparently arose from the ar- teria diploética magna. The posterodorsal border of the anterior lamina (and the figure- eight-shaped aperture into the ventral ascend- ing canal) has a shallow, smooth, concave edge that we interpret as contributing to the ventral border of a foramen on the sidewall of the braincase. Completing the borders of this small foramen was likely the squamosal.

NO. 3322

cochlea vestibule

anterior ampulla

singe lateral ampulla

lateral semicircular canal

cochlear aqueduct

tea anterior semicircular crus commune Nossee . canal

posterior ampulla . oe posterior semicircular

canal Fig. 2. Reconstruction of the osseous laby- rinth of the right petrosal referred to Prokenna- lestes trofimovi, PSS-MAE 136, in dorsal view, based on radiographs. Bone housing lateral semi- circular canal and enclosing subarcuate fossa is missing.

Transmitted via this foramen was a ramus temporalis to the temporal musculature.

OSSEOUS INNER EAR

Figure 2 shows our restoration of the os- seous labyrinth, cavities hollowed out within the petrosal that in life contained perilymph in which the membranous labyrinth was sus- pended. Our restoration is based on radio- graphic analysis of PSS-MAE 136 along with the surface topography as well as the internal morphology exposed through post- mortem damage. The osseous labyrinth con- sists of three parts: the cochlea, which con- tained the cochlear duct; the vestibule, which contained the utricle and saccule; and the semicircular canals, which contained the semicircular ducts.

The most prominent feature of the osseous labyrinth is the cochlea. It is a coiled, hollow tube of uniform diameter that occupies the majority of the available space in the par cochlearis with little room to spare. The con- nection between the cochlea and vestibule is at the posteromedial aspect of the pars coch- learis. Anteromedial to its origin, the cochlea is joined by the short, narrow cochlear can- aliculus, which transmitted the cochlear aq- ueduct of the perilymphatic duct. Beyond the cochlear canaliculus, the cochlea coils in a

2001

clockwise direction, spiraling ventrally and ending anterior to the fenestra vestibuli. In counting the degrees of curvature of the co- chlea, we follow West (1985: 1092), who measured the number of turns in the spiral “starting at the inflection point at the round window, where the cochlear duct leaves the basal hook region to begin its spiral course, continuing up the apex of the cochlea where the cochlear duct terminates.’’ With no in- dication of the round window on the radio- graphic images of PSS-MAE 136, we used the cochlear canaliculus instead, drawing a straight line from the back edge of this canal to the axis of the coil. The apex of the coil intersected the straight line, meaning that the coil was 360°. The basal segments of the os- seous primary and secondary spiral laminae are visible through the oval window. These are delicate laminae that project outward and inward from the inner and outer walls of the cochlea, respectively. The gap between the primary and secondary laminae is filled in life by the basilar membrane, which supports the organ of Corti. In extant mammals, the basilar membrane divides the cochlear duct into two passages, the scala tympani and sca- la vestibuli, which communicate with each other by a small opening, the helicotrema, at the apex of the cochlear duct.

The vestibule communicates with the co- chlea anteriorly and the semicircular canals posteriorly. It consists of an irregular, oval, central space and three distal swellings or ampullae on the semicircular canals at their junction with the vestibule. The anterior and lateral ampullae lie dorsolateral and ventro- lateral to the vestibule, respectively. The pos- terior ampulla is ventromedial, and the crus commune, formed by the union of the non- ampullated ends of the anterior and posterior semicircular canals, is dorsomedial. The po- sition of the ampullae and the semicircular canals was not well resolved in the radio- graphs, but were determinable from the spec- imen, as reported above. The lateral semicir- cular canal lies in a nearly horizontal plane in the floor of the subarcuate fossa. The pos- terior semicircular canal is in a nearly verti- cal plane in the medial wall of the subarcuate fossa; it joins the anterior canal in the crus commune dorsally and the posterior canal ventrally. The anterior ampulla is the only

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES ia |

direct evidence for the anterior semicircular canal, which in life formed the rim for the orifice into the subarcuate fossa. We saw no evidence of the vestibular aqueduct for the endolymphatic duct on the radiographs, but its endocranial aperture is on the crus com- mune in the specimen as expected.

VASCULAR AND NERVOUS RECONSTRUCTION

The foramina, grooves, and canals asso- ciated with the major basicranial vessels and nerves have already been noted in the above descriptions. We provide a comprehensive restoration of the basicranial vessels and nerves here to account for the pathways be- tween the major conduits into and out of the skull (fig. 3). For comparative purposes, Fig- ure 4 shows the major basicranial vessels and nerves in representatives of the three groups of extant mammals—the monotreme Orni- thorhynchus (based on Wible and Hopson, 1995, with amendments from Rougier et al., 1996a), the marsupial Didelphis (based on Wible and Hopson, 1995), and the placental Solenodon (based on McDowell, 1958, with amendments from personal obs.)—plus the prototribosphenidan Vincelestes (based on Rougier et al., 1992; Rougier, 1993). The platypus and Vincelestes are distinguished from therians in having well-developed ves- sels in the posttemporal canal between the petrosal and squamosal and on the basicra- nial surface. In contrast, in therians, the post- temporal vessels are either reduced (e.g., Di- delphis) or lacking (e.g., Solenodon), arma- dillos being a notable exception (Tandler, 1899, 1901; Bugge, 1979). Moreover, in marsupials, the major basicranial arteries are lost and the veins reduced, and in placentals, the major basicranial veins are lost.

VEINS (fig. 3)

PSS-MAE 136 preserves two channels that we interpret to be part of the venous sys- tem: the prootic canal and the intrapetrosal inferior petrosal sinus. The prootic canal transported the prootic sinus from its endo- cranial sulcus to the lateral head vein within the middle-ear space. The lateral head vein ran posteriorly in the trough between the pro- montorium and lateral flange, and turned me- dially to join the inferior petrosal sinus below

12 AMERICAN MUSEUM NOVITATES

NO. 3322

A greater petrosal nerve

ramus inferior

ramus superior

stapedial artery

arteria diploética magna

facial nerve

lateral head vein

ramus superior

arteria diploética magna

ramus temporalis

ramus superior

internal carotid artery

inferior petrosal sinus

internal jugular vein

ramus inferior

greater petrosal nerve

internal carotid artery

stapedial artery

Fig. 3.

Reconstruction of major arteries, veins, and nerves on the right petrosal referred to Proken-

nalestes trofimovi, PSS-MAE 136. A, Ventral view. B, Lateral view.

the jugular foramen to form the internal jug- ular vein. Among extant mammals, a prootic canal occurs only in monotremes and some marsupials (Wible, 1990; Wible and Hopson, 1995). Of these, the prootic canal and asso- ciated sulci in Prokennalestes most closely resembles that in monotremes in size and ori- entation (fig. 4A); the marsupial prootic ca- nal is narrow and laterally directed (fig. 4C).

Our reconstruction of the intrapetrosal in- ferior petrosal sinus in Prokennalestes is more problematic. It is clear that there is a channel of some sort enclosed within the me- dial edge of the pars cochlearis in PSS-MAE 136. Similar channels have been described for the vast majority of extinct non-therian mammaliaforms (fig. 4B; Rougier et al., 1996a; Wible and Rougier, 2000). As noted by Rougier et al. (1996a), comparable struc- tures are not known in extant mammals. There are small venous sinuses within the pe-

trosal in some (e.g., monotremes), but these are highly anastomotic and do not form a sin- gle continuously walled canal. Given that the only substantial venous structure in this vi- cinity in extant mammals is the inferior pe- trosal sinus, Rougier et al. (1996a) suggested that this vessel occupied the intrapetrosal ca- nal in extinct non-therian mammaliaforms. An analogous arrangement is found, for ex- ample, in some living carnivorans in which the inferior petrosal sinus is within a deep sulcus in the medial edge of the petrosal that is closed by the basioccipital and auditory bulla to form a petrobasilar canal (Hunt, 1977; Evans and Christensen, 1979). Prokennalestes likely had a sigmoid sinus, a distributary of the transverse sinus running within the tentorium cerebelli, given that this vein is invariably present in extant mammals. The absence of a sulcus for the sigmoid sinus on the petrosal directed toward the jugular

2001

foramen suggests that the major exit of this vein was via the foramen magnum in Pro- kennalestes, aS in monotremes (fig. 4A) and marsupials (Hochstetter, 1896; Archer, 1976; Wible, 1990).

Veins likely accompanied some of the branches of the basicranial arterial system. Certainly, this was the case for the internal carotid artery and the arteria diploética mag- na, aS veins accompany these vessels in ex- tant monotremes, marsupials, and placentals (Wible, 1984; Wible and Hopson, 1995; per- sonal obs.). It is less certain that veins ac- companied the ramus superior and ramus in- ferior in Prokennalestes. Among extant mammals, such companion veins are known for the platypus (fig. 4A; Wible and Hopson, 1995; Wible and Rougier, 2000), but are thought to be generally lacking in therians preserving the ramus superior and ramus in- ferior. The faint indication of a sulcus ante- rior to the facial foramen in PSS-MAE 136 that merges with the well-developed groove for the ramus inferior might have transmitted a companion vein of the ramus inferior, a post-trigeminal vein, into the lateral head vein. However, to date, a post-trigeminal vein has not been described in extant theri- ans.

ARTERIES (fig. 3)

PSS-MAE 136 has a groove that runs the length of the promontorium, from just medial to the oval window to the anterior pole. Among extant mammals, when such a trans- promontorial groove is present, with few ex- ceptions (Conroy and Wible, 1979) it trans- mits the internal carotid artery with accom- panying vein and sympathetic nerve (fig. 4D; Wible, 1986).

The transpromontorial groove nicks the medial rim of the fenestra vestibuli in PSS- MAE 136, an arrangement which in extant mammals invariably indicates the presence of the stapedial artery, the major extracranial branch of the internal carotid (Wible, 1987). Judging from the size of the notch at the oval window, the stapedial artery was consider- ably smaller than the internal carotid. A small stapedial artery is also suggested by the relatively small caliber of the posterior portion of the figure-eight-shaped foramen

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 13

on the lateral flange. The stapedial artery forms during development in all extant mam- mals, with the exception of the echidna (Hochstetter, 1896), but is lost by the adult stage in all marsupials as well as in various placentals (Wible, 1984, 1987).

The general pattern in those mammals re- taining the stapedial artery in the adult is for that vessel to have two major branches: a ra- mus superior and a ramus inferior (fig. 4A, D; Tandler, 1899, 1901; Bugge, 1974; Wible, 1984, 1987). The ramus superior runs for- ward through the braincase, sends branches to the temporalis musculature and meninges, and enters the orbit as the ramus supraorbi- talis to send branches with the ophthalmic nerve. Dorsal to the ear region, the ramus superior joins a vessel running forward from the occiput in the posttemporal canal be- tween the squamosal and petrosal, the arteria diploética magna. The ramus inferior leaves the middle ear anteriorly and sends branches with the maxillary and mandibular nerves. We believe that Prokennalestes exhibits the essentials of this pattern. The ramus inferior is indicated by the longitudinal groove on the lateral edge of the promontorium of the pe- trosal; this vessel follows a similar pathway in the platypus (Wible and Hopson, 1995) and erinaceomorphs (MacPhee et al., 1988). The ramus superior is indicated by the ven- tral ascending canal, which is remarkably like that transmitting the ramus superior in the platypus (fig. 4A; Wible and Hopson, 1995). Continuous with the endocranial ap- erture of the ventral ascending canal are a well-developed posttemporal groove directed posteromedially for the arteria diploética magna and a dorsal ascending groove direct- ed dorsally for the continuation of the ramus superior. A notch in the anterior lamina rep- resents the petrosal’s contribution to the bor- ders of a foramen for a ramus temporalis, resembling that in the platypus and Vinceles- tes. The figure-eight-shaped tympanic aper- ture of the ventral ascending canal is unusual among mammals, but must have contained components of the stapedial system. We be- lieve that the smaller, posterior opening, which is directed toward the oval window, transmitted the stapedial artery. Because the larger, anterior foramen is directed toward the groove on the promontorium for the ra-

14 AMERICAN MUSEUM NOVITATES

A post-trigeminal vein lamina obturans ramus inferior

ramus superior

glenoid fossa

arter prootic canal ; stapedial arteria dipoética artery magna lateral head

occipital artery é vein

facial nerve paroccipital process

vertebral vein occipital condyle

ramus inferior greater petrosal nerve

lateral trough

internal carotid artery

inferior petrosal sinus

facial nerve lateral head vein

arteria diploética magna

greater petrosal nerve

internal carotid

NO. 3322

C great petrosal nerve

geniculate ganglion

prootic canal rostral tympanic

F process capsuloparietal

emissary vein + Ll inferior petrosal sinus

arteria diploética lateral head vein

magna caudal tympanic process facial nerve mastoid process piriform fenestra at! D ramus inferior ramus superior

artery of

glencid fossa pterygoid canal

sphenoparietal

P : greater petrosal emissary vein

nerve

internal carotid artery

postglenoid foramen

facial nerve

inferior petrosal

ramus posterior sinus

stapedial artery jugular foramen

Fig. 4. Right ear regions in ventral view, with major arteries, veins, and nerves. A, The basicranium of the monotreme Ornithorhynchus anatinus, modified and redrawn from Wible and Hopson (1995: fig. 4B). B, The petrosal of the prototribosphenidan Vincelestes neuquenianus, modified and redrawn from Rougier et al. (1992: fig. 3C). C, The petrosal of the marsupial Didelphis virginiana, modified and redrawn from Wible and Hopson (1995: fig. 5A). D, The basicranium of the placental Solenodon par- adoxus, modified and redrawn from McDowell (1958: fig. 7B). The opossum has an internal carotid artery (not shown), but it does not contact the petrosal, which also occurs in the platypus.

mus inferior, we interpret it as transmitting that vessel. Consequently, the ramus inferior arose from the stapedial artery within the ventral ascending canal.

NERVES (fig. 3)

As in all extant mammals, the internal acoustic meatus in Prokennalestes gave pas- sage to the facial and vestibulocochlear nerves. The latter divided into two bundles that occupied different compartments within the internal acoustic meatus: the larger, me- dial cochlear nerve and the smaller, lateral vestibular nerve. The latter terminated by ramifying into numerous fascicles that en- tered the inner ear through tiny pores in the internal acoustic meatus; the former does not appear to show the same pattern. The facial

nerve entered the facial canal in the lateral part of the internal acoustic meatus. The fa- cial canal ran ventrolaterally into a wider space, the cavum supracochleare, where the geniculate ganglion of the facial nerve was situated. The main stem or hyomandibular branch of the facial nerve left the posterior aspect of the geniculate ganglion and entered the middle-ear space via the facial foramen, anterior to the oval window. The nerve ran posteriorly dorsal to the lateral head vein and left the middle ear via the stylomastoid notch in the crista parotica. This is the pattern ex- hibited by these structures in the platypus and opossum (fig. 4A, C). The greater petro- sal nerve or palatine ramus of the facial nerve left the anterior aspect of the genicu- late ganglion and entered the rear of the ca-

2001

vum epiptericum via the fenestra semilunar- is. It then ran anteroventrally out of the ca- vum in a groove that merged with that for the ramus inferior on the tympanic surface of the petrosal. In the intact skull, this groove would have been closed to form a foramen, a hiatus Fallopu, by the alisphenoid bone. This contrasts with the pattern of the hiatus Fallopii in other mammals, which is entirely within the petrosal, either in the tympanic roof (fig. 1A, B, D) or along the anterior edge (fig. 1C).

DISCUSSION TAXONOMIC ALLOCATION OF PSS-MAE 136

Wible et al. (1997) noted that PSS-MAE 136 exhibits a mosaic of primitive features also found in the prototribosphenidan Vin- celestes from the Hauterivian of Argentina and derived features shared with therians. Among the former is the inferred vascular pattern, whereas among the latter is the coil- ing of the cochlea. These authors also noted that PSS-MAE 136 was intermediate in some features between Vincelestes and _ therians. For example, in contrast to therians, PSS- MAE 136 has an anterior lamina, but one that is reduced compared with the structure occurring in Vincelestes. The anterior lamina in Vincelestes extends forward to the level of the anterior pole of the promontorium, whereas in PSS-MAE 136 it barely extends forward to the level of the back of the pro- montorium. Based on the morphology and size of PSS-MAE 136, Wible et al. (1997) concluded it most likely belonged to Proken- nalestes.

To elaborate on the rationale followed by Wible et al. (1997) regarding size, we offer the following remarks. To associate PSS- MAE 136 with a Khoobur taxon known from dentitions, we used the nearly complete skulls of Vincelestes (Rougier, 1993) and the basal eutherian Asioryctes (Kielan-Jawo- rowska, 1981) to predict the length of the skull associated with an isolated petrosal. Us- ing Vincelestes, which has a relatively longer petrosal than does Asioryctes, we estimate the skull length for PSS-MAE 136 to be 26.5 mm; using Asioryctes, the estimate is 25.3 mm. These estimates fall in the range of the skull length of 24—27 mm predicted for Pro-

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 15

kennalestes trofimovi by Kielan-Jaworwoska and Dashzeveg (1989) from lower jaws and teeth. The smaller species P. minor was es- timated by these authors as having a skull 21 mm in length. The other described Khoobur taxa are either smaller than P. minor (i.e., the tribosphenidan Kielantherium gobiensis, Dashzeveg, 1975; Dashzeveg and Kielan-Ja- worowska, 1984; the cladotherians Arguimus and Arguitherium, Dashzeveg, 1979, 1994; the gobiconodontid Gobiconodon hoburen- sis, Trofimov, 1978; Kielan-Jaworowska and Dashzeveg, 1998; and the multituberculates Arginbaatar dimitrievae, Trofimov, 1980; Kielan-Jaworwoska et al., 1987; and Eoba- taar minor, Kielan-Jaworowska et al., 1987) or considerably larger than P. trofimovi (1.e., the gobiconodontid Gobiconodon borissiaki, Trofimov, 1978; the multituberculate Eoba- taar magnus, Kielan-Jaworowska et al., 1987; and the symmetrodont Gobiodon infin- itus, Trofimov, 1980; see Wible et al., 1995). Consequently, of the currently described Khoobur taxa, PSS-MAE 136 accords best in size with P. trofimovi.

Kielan-Jaworowska and Dashzeveg (1989) contend that the dentition of Prokennalestes is the most primitive known for Eutheria. This view has been supported subsequently by Butler (1990). The morphology of PSS- MAE 136 is congruent with taxonomic as- signment to Prokennalestes, as its petrosal structure is intermediate between Vincelestes on the one hand and Late Cretaceous euthe- rians (e.g., asioryctitheres, zalambdalestids) on the other. Already mentioned is the inter- mediate condition of the anterior lamina of PSS-MAE 136. Another example concerns the lateral flange, which runs the length of the petrosal in Vincelestes and more primi- tive mammals (Rougier et al., 1992, 1996a; Wible and Hopson, 1993, 1995), but is re- stricted to the posterolateral corner of PSS- MAE 136 and is greatly reduced or absent in Late Cretaceous and younger eutherians (Wi- ble, 1990; Rougier et al., 1998). This feature, however, may be at least partially correlated with the presence and development of an an- terior lamina.

An alternative is that PSS-MAE 136 be- longs to an as yet undescribed or unknown Khoobur taxon that occupies a phylogenetic position between Vincelestes and Late Cre-

16 AMERICAN MUSEUM NOVITATES

taceous eutherians. There is, in fact, another form from Khoobur that has been named but not formally described—Prodelttheridium kalandadzei (Trofimov in Reshetov and Tro- fimov, 1984)—and, therefore, is considered a nomen nudum (Kielan-Jaworowska and Dashzeveg, 1989; Kielan-Jaworowska and Nessov, 1990). In light of the name, this form was presumably thought to have affin- ities with the basal metatherian Deltatheri- dium (Gregory and Simpson, 1926; Rougier et al., 1998) and, therefore, to occupy a po- sition between Vincelestes and Late Creta- ceous eutherians. The alternative that PSS- MAE 136 belongs to this or some other un- known form cannot be eliminated, of course, with total certainty. However, because of the reasonable match regarding both size and morphology, we follow Wible et al. (1997) in attributing PSS-MAE 136 to Prokennales- tes trofimovi.

PHYLOGENETIC ANALYSIS OF PROKENNALESTES

The only phylogenetic analysis with a tax- on-character matrix to include Prokennales- tes as a terminal taxon is that by Rougier et al. (1998), a study designed to evaluate the relationships of Late Cretaceous deltathe- roidans. Their matrix included 156 characters (66 dental, 11 mandibular, and 79 cranial) and 48 terminal taxa, 7 of which are eutherians. In addition to Prokennalestes, these included the Late Cretaceous Asian taxa Otlestes, Asioryctidae (Asioryctes + Ukhaatherium), Kennalestes, Zalambdalestes, and Zhelestes- Aspanlestes, and the Early Tertiary leptictids. Prokennalestes was identified as the basal- most eutherian in the most parsimonious trees recovered (fig. 5). In their analysis, Rougier et al. (1998) followed Wible et al. (1997) by including PSS-MAE 136 with Prokennalestes. Altogether, Prokennalestes was scored for 67% of the characters (105 of 156), including 35 basicranial characters. Be- cause of space restrictions, Rougier et al. (1998) were unable to provide detailed de- scriptions and discussions of the characters they considered. We take this opportunity to elaborate on these 35 basicranial characters, providing justification for their coding of Prokennalestes. Based on new interpretations or additional preparation, we also amend the

NO. 3322

scoring of 10 character states; the vast ma- jority of these concern Zalambdalestes, for which there are new specimens (Novacek et al., 1997; Wible et al., 1998) currently being described (Wible et al., in prep.). The char- acter numbers, descriptions, and initial ref- erences below are those used by Rougier et al. (1998), available as supplementary infor- mation at www.nature.com and repeated here in the appendices. The other taxa considered by Rougier et al. (1998) were either inves- tigated by direct observation or taken from the literature, which is listed at the Internet site. For scoring of some characters in the metatherian Andinodelphys, Rougier et al. (1998) acknowledged Dr. Christian de Mui- zon. Among the eutherian taxa, basicrania are not yet described for Otlestes and Zhe- lestes-Aspanlestes.

Since the publication of Rougier et al. (1998), new descriptions of some extinct therians relevant to that analysis have ap- peared. The most pertinent of these for the present discussion is Daulestes nessovi from the Coniacian of Uzbekistan assigned tenta- tively to the Asioryctitheria (McKenna et al., 2000), the clade including Asioryctidae and Kennalestes (Novacek et al., 1997). Daules- tes is represented by a single incomplete skull that includes partial petrosals. Based on the descriptions and illustrations in McKenna et al. (2000), we were able to identify char- acter states in Daulestes for only 11 of the 35 basicranial characters scored for Proken- nalestes in the Rougier et al. (1998) matrix. We include these observations below.

103. Orbitotemporal canal—present (0) or absent (1) (Rougier et al., 1992): In extant mammals, the orbitotemporal canal transmits the ramus superior from its union with the arteria diploética magna forward to the orbit, where it emerges as the ramus supraorbitalis (Rougier et al., 1992; Wible and Hopson, 1995). Prokennaletes preserves the most proximal part of the orbitotemporal canal on its petrosal, the dorsal ascending groove. In light of the size of the dorsal ascending groove (“‘ramus superior groove”’ in fig. 1F), it seems likely that the ramus superior reached the orbit in Prokennalestes. An or- bitotemporal canal is widely present among Mesozoic mammals, with the exception of metatherians (Rougier et al., 1998). Based on

2001

EUTHERIA

THERIA

METATHERIA

ral MARSUPIALIA

Fig. 5.

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 1}

Dryolestoids Peramus

Vincelestes Kielantherium Potamotelses Slaughteria Pappotherium Picopsis Prokennalestes Otlesies Asiorycies Kennalestes Zalambdalestes Leptictids Zhelestids Holociemensia Deltheridium Deitatheroides Sulestes

Gurlin Tsav Skull Dideiphodon Eodeilphis Pariadens Kokopellia Anchistodelphys Pediomys Glasbius Albertatherium Alphadon Turgidodon fugomortiferum Asiatherium iquatadelphis Borhyaenidae Jaskhadelphys Mayulestes Pucadelphys Andinodelphys Marmosa Dideiphis Dasyurids Dromiciops

Strict consensus tree of 144 equally most parsimonious trees, taken from the phylogenetic

analysis by Rougier et al. (1998). Trees were obtained by using PAUP on a database of 156 craniodental characters, representing 365 morphological transformations across 48 taxa (see appendices). Six taxa were subsequently pruned from the study because of incompleteness, all being represented by only one tooth (Aegialodon, Comanchea, Trinititherium, Kermackia, Falepetrus, and Zygiocuspis). Tree length of the individual trees is 570; consistency index = 0.444; and retention index = 0.663.

the incidence of an orbital aperture of the orbitotemporal canal, Rougier et al. (1998) scored this vascular channel present among early eutherians in asioryctids, Kennalestes, and leptictids. This foramen is also present in Zalambdalestes (personal obs.) and likely in Daulestes (’?sinus canal foramen’”’ in Mc- Kenna et al., 2000).

108. Anterior lamina exposure on lIat- eral braincase wall—present (0), rudimen-

tary (1), or absent (2) (Modified from Ker- mack, 1963; Hopson and Rougier, 1993): An extensive anterior lamina contributes to the sidewall of the braincase in Mesozoic mam- maliaforms, with the exception of eutherians and metatherians (Wible and Hopson, 1993; Hopson and Rougier, 1993). Marshall and Muizon (1995) described a reduced anterior lamina for the Paleocene metatherian Puca- delphys, but this element has no exposure on

18 AMERICAN MUSEUM NOVITATES

the sidewall of the braincase. Prokennalestes is unique among therians (including appar- ently Daulestes, McKenna et al., 2000: fig. 8) in that it has an anterior lamina that was exposed on the lateral braincase (fig. 1C, F). Although this element is damaged in Pro- kennalestes, it is apparent that it was reduced compared with the extensive anterior lamina of more basal taxa. In extant monotremes, the anterior lamina forms as an independent intramembranous ossification called the lam- ina obturans (fig. 4A; Kuhn, 1971; Presley, 1981; Zeller, 1989), and this pattern may have been repeated in extinct taxa as well.

109. Cavum epiptericum—/loored by pe- trosal (0), petrosal and alisphenoid (1), pri- marily or exclusively by alisphenoid (2), or primarily open as piriform fenestra (3) (Modified from Wible and Hopson, 1993): The composition of the floor of the cavum epiptericum, including the fossa for the tri- geminal ganglion, varies among Mesozoic mammaliaforms (Wible and Hopson, 1993; Luo, 1994). Among the taxa considered by Rougier et al. (1998), the cavum epiptericum is floored by the petrosal only in Vincelestes. In metatherians, the floor is formed by the alisphenoid, either alone or in concert with the petrosal. In eutherians, there is a well- developed piriform fenestra (fig. 4D) beneath the cavum epiptericum in asioryctitheres and Zalambdalestes, and a floor formed by the alisphenoid and petrosal in leptictids (and presumably Daulestes, McKenna et al., 2000: fig. 8). The posteromedial part of the trigeminal fossa is preserved on the petrosal of Prokennalestes (“‘cavum epiptericum”’ in fig. 1F). The remainder was either in another bone, the alisphenoid, or was open, as a pir- iform fenestra. It appears that bone, presum- ably alisphenoid, contacted the lateral sur- face of the promontorium and floored at a minimum the posterolateral part of the tri- geminal fossa (character state 1).

111. Foramen ovale composition—in pe- trosal (anterior lamina) (0), between petro- sal and alisphenoid (1), or in alisphenoid or between alisphenoid and squamosal (2) (Modified from Gaudin et al., 1996): It has been noted by various authors that the com- position of the foramen ovale, which trans- mits the mandibular division of the trigemi- nal nerve, varies among mammals. The most

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recent detailed accounting of the distribu- tions of morphologies across Mammalia is by Gaudin et al. (1996). Rougier et al. (1998) modified the three states identified by Gaudin et al. (1996) to those above. As presented on the Internet, state 1 of Rougier et al. (1998) inadvertently was “between petrosal and squamosal”’ instead of ‘“‘between petrosal and alisphenoid”’ as above. In the taxa scored by Rougier et al. (1998), the foramen ovale is in the petrosal in Vincelestes; between the petrosal and alisphenoid in most metatheri- ans and Zalambdalestes; and in the alisphe- noid (or between that bone and the squa- mosal) in Dromiciops, asioryctitheres, and leptictids. In addition, the foramen ovale is not in the petrosal in the metatherians Asia- therium and the Gurlin Tsav skull (see Szalay and Trofimov, 1996), but it is unclear wheth- er these forms exhibit state 1 or 2. The char- acter state in Prokennalestes is uncertain.

113. Squama of squamosal—absent (0) or present (1): In his phylogenetic analysis of Mammaliamorpha, Rowe (1988) em- ployed as a character whether the cranial moiety of the squamosal was confined to the zygomatic root or contributed broadly to the cranial wall. Rougier et al. (1998) modified this description to refer to the presence/ab- sence of the squama of the squamosal. In their analysis, the squama is lacking in Vin- celestes and present in therians, including Prokennalestes, asioryctitheres, Zalambda- lestes, and leptictids. A clear facet for the Squama is not preserved in PSS-MAE 136. However, in light of the reduced size of the anterior lamina compared with the condition in non-therian mammaliaforms, the squa- mosal must have contributed broadly to the braincase wall in Prokennalestes. A well-de- veloped squama is tentatively identified for Daulestes by McKenna et al. (2000).

122. Epitympanic wing medial to pro- montorium—absent (0), flat (1), undulated (2), or confluent with bulla (3): An epitym- panic wing extends medially from the pro- montorium in many therians, but is wholly lacking in extinct non-therian mammals (Rougier et al., 1996a, 1998). In Prokenna- lestes (fig. 1A, D), the epitympanic wing is flat, as it is in asioryctitheres, Zalambdales- tes, and leptictids, and certain metatherians as well, including Deltatheridium. The epi-

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tympanic wing presents an undulated mor- phology in the metatherians Didelphodon and Pediomys (following the attribution of isolated petrosals by Wible, 1990), and is confluent with the bulla in Asiatherium and Dromiciops. The flat bony shelf on the an- teromedial aspect of the promontorium in Daulestes identified as a rostral tympanic process by McKenna et al. (2000) is actually a flat epitympanic wing.

123. Tympanic aperture of hiatus Fal- lopii—in roof through petrosal (0), at ante- rior edge of petrosal (1), or absent (2) (Mod- ified from Wible, 1990): In describing iso- lated petrosals of Late Cretaceous metathe- rians, Wible (1990) employed a character concerning the length of the hiatus Fallopii, which transmits the greater petrosal nerve forward from the geniculate ganglion of the facial nerve. Following Rougier et al. (1998), we believe that the position of the distal ap- erture of the hiatus Fallopii more objectively accounts for the differences noted by Wible (1990). In Vincelestes and asioryctids, the distal aperture of the hiatus Fallopii is in the tympanic roof (e.g., fig. 4B, D). In contrast, in leptictids and in the metatherians scored by Rougier et al. (1998) with one exception, the hiatus Fallopii opens distally at the an- terior edge of the petrosal (e.g., fig. 4C). The one exception is Deltatheridium, which has no separate canal for the greater petrosal nerve. The condition in Prokennalestes was scored as in the roof by Rougier et al. (1998), but it differs in one regard from the condition in Vincelestes and asioryctids: the aperture is not wholly in the petrosal, but between the petrosal and another bone, presumably the alisphenoid (fig. 1C, 3A). The hiatus Fallopii in Zalambdalestes is at the anterior edge of the petrosal (personal obs.).

124. Prootic canal—long and vertical (0), short and vertical (1), short and horizontal (2), or absent (3) (Modified from Wible, 1990): Among extant mammals, monotremes have a long, vertical prootic canal that trans- mits the prootic sinus from the cranial cavity to the middle ear where it joins the lateral head vein (fig. 4A), and some marsupials have a short, horizontal prootic canal that en- closes the lateral head vein (fig. 4C; Wible and Hopson, 1995). Prokennalestes is unique among eutherians in having a prootic canal

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 19

(fig. 1), and Rougier et al. (1998) recognized an intermediate condition for it: short and vertical.

125. Position of sulcus for anterior dis- tributary of transverse sinus relative to subarcuate fossa—anterolateral (0) or pos- terolateral (1): In extant mammals, the an- terior distributary of the transverse sinus is the prootic sinus in monotremes, the spheno- parietal emissary vein in marsupials, and the capsuloparietal emissary vein in placentals (Gelderen, 1924; Wible, 1990; Wible and Hopson, 1995). Despite the apparent non-ho- mology of these veins based on embryolog- ical grounds, each occupies a sulcus on the endocranial surface of the petrosal in the vi- cinity of the subarcuate fossa. Rougier et al. (1998) identified this character to account for the difference in the position of this sulcus between metatherians and other mammals, with the sulcus not extending as far forward in the former forms. Although the sulcus is not fully preserved in Prokennalestes, it does extend anterolateral to the subarcuate fossa to the endocranial aperture of the prootic ca- nal (“‘prootic sinus groove”’ in fig. 1E, F) as it does in Kennalestes, Zalambdalestes, and leptictids.

126. Lateral flange—parallels length of promontorium (0), restricted to posterolat- eral corner (1), or greatly reduced or absent (2) (Modified from Rougier et al., 1996a): Running the length of the promontorium in non-therian mammals is an extensive lateral trough, the lateral edge of which is down- turned to form the lateral flange (fig. 4B; Wi1- ble et al., 1995). In therians, the lateral flange is either greatly reduced or absent (Wible et al., 1995). Prokennalestes preserves a well- developed lateral flange, but it is restricted to the posterolateral corner of the petrosal (fig. 1A, D).

127. Stapedial ratio—rounded, less than 1.8 (0) or elliptical, more than 1.8 (1) (Se- gall, 1970): Segall (1970) quantified the shape of the stapedial footplate and oval win- dow as a ratio of length to width for various extant mammals. He discovered that mono- tremes and most marsupials that he consid- ered have a slightly oval footplate, with a stapedial ratio less than 1.8, whereas it tends to be more elliptical in placentals, with a sta- pedial ratio higher than 1.8. Among the the-

20 AMERICAN MUSEUM NOVITATES

rians coded by Rougier et al. (1998), Pro- kennalestes with a value of 1.71 was unique among the eutherians in having a stapedial ratio less than 1.8 and Dromiciops was unique among the metatherians with a sta- pedial ratio greater than 1.8.

128. Complete wall separating cavum supracochleare from cavum epipteri- cum—absent (0) or present (1) (Modified from Wible and Hopson, 1993): In the platy- pus, the geniculate ganglion of the facial nerve lies immediately posterior to the tri- geminal ganglion in the cavum epiptericum (Kuhn and Zeller, 1987; Zeller, 1989). In contrast, in the echidna, the geniculate gan- glion is enclosed in the petrosal bone, cre- ating a cavum supracochleare separate from the cavum epiptericum (Kuhn, 1971; Kuhn and Zeller, 1987). Most extant therians ex- hibit the echidna condition, but the wall be- tween the two cava is incomplete in some marsupials (Wible, 1990). The aperture con- necting the two cava in such instances has been named the fenestra semilunaris by Rougier et al. (1992). Among the taxa con- sidered by Rougier et al. (1998), a fenestra semilunaris (and so an incomplete wall) oc- curs in Vincelestes and Prokennalestes (fig. 1C, F). The fenestra semilunaris in Proken- nalestes is unusual in that it transmits the greater petrosal nerve (fig. 3B). Rougier et al. (1998) scored this character as unknown for Zalambdalestes, but we now know that the wall is complete (personal obs.).

129. Coiling of cochlea—less than 360° (O) or more than 360° (1): Because the co- chlea of Prokennalestes is coiled through just 360°, we propose amending the derived state of this character to “360° or greater’’. As far as we currently know, all therians (including Daulestes, McKenna et al., 2000) have a co- chlea coiled through at least one turn (see below). In contrast, the cochlea of Vinceles- tes is coiled through about three-fourths of one turn (Rougier, 1993).

130. Rostral tympanic process of petro- sal, on posteromedial aspect of promon- torium—absent or low ridge (0) or tall ridge, occasionally contacting ectotympanic (1) (Modified from Wible, 1990): Many the- rians have a low ridge on the promontorium near the fenestra cochleae (fig. 4C; Wible, 1990), which in some forms (e.g., primates,

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MacPhee, 1981; dasyurids, Archer, 1976) ex- pands to a tall ridge making a significant contribution to the tympanic bulla. Wible (1990) employed the term rostral tympanic process of the petrosal for both conditions, following MacPhee (1981). None of the eu- therians considered by Rougier et al. (1998), including Prokennalestes, has a rostral tym- panic process. This is also the condition in Daulestes contra McKenna et al. (2000; see character 122 above). In the Rougier et al. (1998) matrix, the only forms with the de- rived state are the metatherians Didelphodon and Asiatherium.

131. Paroccipital process (sensu Wible and Hopson, 1993) orientation and shape—-vertical (0), slanted, projecting an- teroventrally as flange toward back of pro- montorium (1), or indistinct or absent (2): Non-therian mammaliaforms have a well-de- veloped process on the tympanic surface of the pars canalicularis of the petrosal that serves for muscle attachment (fig. 4A; Rowe, 1988; Wible, 1991; Wible and Hopson, 1993). This process is equivalent to what MacPhee (1981) called the lateral section of the caudal tympanic process of the petrosal in various placentals. In most of the taxa con- sidered by Rougier et al. (1998), the paroc- cipital process is either vertical (e.g., Vince- lestes, Asiatherium) or indistinct to absent (e.g., didelphids, asioryctitheres, leptictids). However, in some metatherians (1.e., Delta- theridium, Mayulestes, and borhyaenids), the paroccipital process is slanted anteriorly to- ward the back of the promontorium. AI- though the paroccipital process is damaged in Prokennalestes (fig. 1A, D), it is apparent that this process was well developed and ver- tical. This condition also was scored for Za- lambdalestes by Rougier et al. (1998), but we now know the bulk of this process is on the squamosal bone (as the posttympanic pro- cess) and that the paroccipital process is in- distinct.

132. Caudal tympanic process of petro- sal development— tall wall behind postpro- montorial recess (O), tall wall decreasing in height markedly medially (1), or notched be- tween stylomastoid notch and jugular fora- men (2) (Modified from Wible, 1990): We use the term caudal tympanic process of the petrosal sensu Wible (1990) for the wall on

2001

the tympanic surface of the pars canalicularis of the petrosal, medial to the paroccipital process and posterior to the postpromontorial recess. In Rougier et al.*s (1998) matrix, the caudal tympanic process is a well-developed wall in Vincelestes and some metatherians, but it decreases in height markedly as it nears the jugular foramen in other metatherians (e.g., Didelphodon, Pediomys; fig. 4C). In contrast, in the eutherians considered by Rougier et al. (1998), including Prokenna- lestes, the caudal tympanic process has a notch in it between the stylomastoid notch laterally and the jugular foramen medially. In reviewing the scoring of Prokennalestes here, we believe that this character should be scored a | or 2. The preserved medial end of the caudal tympanic process in PSS-MAE 136 indicates that a tall wall was not present (character state 0). However, damage to the remainder of this element does not allow us to discriminate between the remaining two states (fig. 1A, D).

133. Crista interfenestralis and caudal tympanic process of petrosal connected by curved ridge—absent (O) or present (1) (Modified from Rougier et al., 1996a): The crista interfenestralis is the strut of bone sep- arating the oval and round windows (Wible et al., 1995). In asioryctitheres and Zalamb- dalestes, the crista interfenestralis is con- nected to the medial part of the notched cau- dal tympanic process of the petrosal by a curved ridge. This condition is lacking in the remaining taxa considered by Rougier et al. (1998), including Prokennalestes (fig. 1A, D).

134. ‘‘Tympanic process”—absent (0) or present (1) (Kielan-Jaworowska, 1981): Kie- lan-Jaworowska (1981) described in the asioryctitheres Asioryctes and Kennalestes a prominent, fingerlike vertical process on the petrosal posterior to the fenestra cochleae, which she dubbed the “‘tympanic process’’. This process also occurs in Zalambdalestes, but is lacking in Prokennalestes (fig. 1A, D) and the remaining taxa considered by Roug- ier et al. (1998).

136. Rear margin of auditory region— marked by a steep wall (0) or extended onto a flat surface (1): In most of the taxa in Rougier et al.’s (1998) matrix, the pars can- alicularis posteriorly ends abruptly at the lev-

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES pa

el of the caudal tympanic process of the pe- trosal, which forms a steep wall for the rear margin of the auditory region. In contrast, in asioryctitheres, Zalambdalestes, and leptic- tids, there is a flat shelf posterior to the cau- dal tympanic process, which extends the rear margin of the tympanic region. Despite the damage to the caudal tympanic process in Prokennalestes, it is apparent that this ele- ment formed a steep wall behind the middle- ear space (fig. 1A, D).

137. Fossa incudis—continuous with (0) or separate from (1) epitympanic recess: An epitympanic recess is restricted to multitu- berculates, Vincelestes, and therians, whereas a fossa incudis is more widely distributed among mammaliaforms (Rougier et al., 1996a). In forms having both an epitympanic recess and fossa incudis, Rougier et al. (1998) noted that these two spaces are either continuous or separated by a distinct ridge. The epitympanic recess and fossa incudis are continuous in most of the taxa considered by Rougier et al. (1998), including Prokenna- lestes (fig. 1A, D), asioryctids, Zalambdales- tes, and leptictids. The derived state is re- stricted to some metatherians, i.e., Didelpho- don, Pediomys, borhyaenids, and dasyurids.

138. Epitympanic recess—with small contribution to posterolateral wall by squa- mosal (0) or with extensive contribution to lateral wall by squamosal (1): In Vincelestes and most metatherians scored by Rougier et al. (1998), the lateral wall of the epitympanic recess is formed largely by the petrosal, and the squamosal is confined posterolaterally. However, in the eutherians Prokennalestes (fig. 1A, D), asioryctids, Zalambdalestes, and leptictids, and in the metatherians Di- delphodon, Andinodelphys, Turgidodon (fol- lowing the attribution of isolated petrosals by Wible, 1990), and the unnamed Gurlin Tsav skull (see Szalay and Trofimov, 1996), the squamosal contributes extensively to the lat- eral wall of the epitympanic recess.

139. Stapedius fossa—twice the size of fe- nestra vestibuli (O) or small and shallow (1): Rougier et al. (1998) observed that the fossa of the stapedius muscle varied significantly in size among the taxa they investigated. In the majority including Prokennalestes (fig. 1A, D), asioryctitheres, and leptictids, the stapedius fossa is roughly twice the size of

22 AMERICAN MUSEUM NOVITATES

the oval window. In contrast, the stapedius fossa is small and shallow in most meta- therians, the exceptions being Deltatheri- dium, Pediomys, Turgidodon, and borhyaen- ids. The derived state was identified as an unambiguous synapomorphy of the grouping of post-borhyaenid South American and Australian metatherians. Rougier et al. (1998) scored Zalambdalestes as unknown, but it exhibits the primitive state (personal obs.).

140. Hypotympanic sinus—absent (0), formed by squamosal, petrosal, and alisphe- noid (1), or formed by alisphenoid and pe- trosal (2) (Modified from Muizon, 1994): A variety of names have been employed in de- scribing the various spaces in the middle ear enclosed by the auditory bulla. Klaauw (1931) used the term hypotympanic sinus to include the parts of the middle ear that do not contain the principal elements, such as the auditory ossicles. Muizon (1994) noted that the alisphenoid hypotympanic sinus, which is situated anterior to the epitympanic recess, in his Borhyaenoidea (borhyaenids plus Mayulestes) is formed by the squamosal and petrosal in addition to the alisphenoid. Rougier et al. (1998) identified three states for the alisphenoid hypotympanic sinus in the taxa they considered. It is absent in Vin- celestes, Deltatheridium, Pucadelphys, An- dinodelphys, asioryctitheres, and Zalambda- lestes; formed by the squamosal, petrosal, and alisphenoid in Didelphodon, Mayulestes, borhyaenids, and leptictids; and formed by the petrosal and alisphenoid in Marmosa, Di- delphis, Dromiciops, and dasyurids. The last state was an unambiguous synapomorphy of Marsupialia. In addition to the above, there are several metatherians that have an ali- sphenoid hypotympanic sinus, but the con- tributing elements are uncertain. Included are Eodelphis, Pediomys, Turgidodon, Asiathe- rium, and the Gurlin Tsav skull (see Szalay and Trofimov, 1996). Finally, the petrosal of Prokennalestes shows no involvement in an alisphenoid hypotympanic sinus (states 1 and 2), and so this taxon was scored 0. The same state occurs in Daulestes (McKenna et al., 2000: fig. 8).

143. Foramina for temporal rami—on petrosal (0), on parietal and/or squama of squamosal (1), or absent (2): The platypus

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has a foramen in the lamina obturans on the lateral braincase wall that transmits an artery off the ramus superior and vein associated with the temporalis muscle (fig. 4A; Rougier et al., 1992; Wible and Hopson, 1995). Vin- celestes has three similarly situated foramina in the anterior lamina that likely had the same function (Rougier et al., 1992). In con- trast, in therians, foramina for the temporal rami are either on the squamosal and/or pa- rietal or are absent (Wible, 1987). The one exception noted by Rougier et al. (1998) is Prokennalestes, which has a temporal fora- men between the petrosal and presumably the squamosal (“‘notch for ramus temporal- is” in fig. 1F). Rougier et al. (1998) scored the absent state for Zalambdalestes, but tem- poral foramina have been subsequently iden- tified in the squamosal (personal obs.).

144. Posttemporal canal—large (0), small (1), or absent (2) (Modified from Wi- ble, 1990): Among extant mammals, the posttemporal canal is found between the pe- trosal and squamosal in monotremes and some therians, where it transmits the arteria diploética magna and accompanying vein (fig. 4A, C; Wible, 1987; Wible and Hopson, 1995). Wible (1990) used the presence or ab- sence of the aperture on the occiput into the posttemporal canal as a character in his anal- ysis of isolated Late Cretaceous metatherian petrosals. Rougier et al. (1998) modified this character to the above, noting the size dis- parity in addition to the presence/absence. In the taxa investigated by Rougier et al. (1998), the posttemporal canal is large only in Vincelestes; small in most metatherians and Zalambdalestes, and absent in borhyaen- ids, dasyurids, Dromiciops, asioryctitheres, and leptictids. Absence of the posttemporal canal was an unambiguous synapomorphy of dasyurids and Dromiciops. Prokennalestes was scored as having a small canal, because the posttemporal groove on the petrosal is small compared with that in Vincelestes (cf. figs. 3B and 4B).

145. Foramen for ramus superior of sta- pedial artery—on petrosal (0), on petrosal— squamosal suture (1), or absent (2) (Modi- fied from Rougier et al., 1992): Various au- thors (e.g., Wible, 1987; Rougier et al., 1992) have noted that the composition of the fora- men by which the ramus superior leaves the

2001

middle ear differs among mammals. Rougier et al. (1998) identified three states: on the petrosal as in Vincelestes (fig. 4B) and Pro- kennalestes (fig. 1D, 3A); on the suture be- tween the petrosal and squamosal as in lep- tictids (e.g., fig. 4D); and absent as in me- tatherians (fig. 4C). Rougier et al.’s (1998) phylogenetic analysis identified absence of the foramen as an unambiguous synapomor- phy of Metatheria. Originally scored as un- known for Zalambdalestes, the foramen for the ramus superior has been found to lie on the petrosal-squamosal suture (personal obs.).

146. Transpromontorial sulcus—present (O) or absent (1) (Wible, 1986): The internal carotid artery in extant mammals follows one of three extracranial courses en route to the cranial cavity, according to Wible (1986): ex- trabullar (medial to the auditory bulla), intra- bullar (through a canal in the bulla), or trans- promontorial (across the tympanic surface of the promontorium). Often the last is marked by a sulcus running forward from the vicinity of the fenestrae cochleae and vestibuli to- ward the anterior pole. Among the taxa con- sidered by Rougier et al. (1998), a transpro- montorial sulcus is present in Vincelestes (fig. 4B), Prokennalestes (fig. 1A, D, 3A), and leptictids; and absent in metatherians (fig. 4C), asioryctitheres, and Zalambdales- tes. McKenna et al. (2000) report a trans- promontorial sulcus in Daulestes.

147. Sulcus for stapedial artery—pre- sent (O) or absent (1) (Wible, 1987): Among adult extant mammals, the stapedial artery is found in the platypus (fig. 4A) and various placentals (fig. 4D; Tandler, 1899, 1901; Bugge, 1974; Wible, 1987). Among the for- mer, the course of the stapedial artery is often marked by a sulcus in the vicinity of the fe- nestra vestibuli. Rougier et al. (1998) report- ed a sulcus for the stapedial artery in Vin- celestes (fig. 4B), Prokennalestes (fig. 1A, D), asioryctitheres, Zalambdalestes, and lep- tictids. A stapedial sulcus also occurs in Daulestes (McKenna et al., 2000). In con- trast, the sulcus was absent in all metatheri- ans preserving the petrosal (fig. 4C) and was identified as an unambiguous synapomorphy of Metatheria.

148. Deep sulcus for internal carotid ar- tery excavated on anterior pole of pro-

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 23

montorium—absent (0) or present (1) (Modified from Muizon et al., 1997): Muizon et al. (1997) figured a well-developed sulcus on the anterior pole of the promontorium in the South American metatherians Pucadel- Phys, Andinodelphys, and Mayulestes. A\- though this sulcus carried the internal carotid artery, it is not equivalent to the transpro- montorial sulcus described above, which runs the length of the cochlear housing (fig. 1A, D). In addition to these three taxa, Roug- ier et al. (1998) scored this sulcus for bor- hyaenids. It is lacking in all other taxa stud- ied, including Prokennalestes (and Daules- tes, McKenna et al., 2000). In Rougier et al.’s (1998) phylogenetic analysis, the presence of this sulcus was an unambiguous synapomor- phy of the clade of South American and Aus- tralian metatherians.

149. Jugular foramen size relative to fe- nestra cochleae—subequal to (O) or larger than (1): Rougier et al. (1996a) employed the size of the jugular foramen relative to the fenestra cochleae (or perilymphatic foramen) in their phylogenetic analysis of Mammali- aformes. Rougier et al. (1998) modified this character to account for the very large jug- ular foramen occurring in some metatherians. The derived state was scored for Mayulestes, Pucadelphys, Andinodelphys, and borhyaen- ids, and was an unambiguous synapomorphy of the clade of South American and Austra- lian metatherians. Given the size of the jug- ular notch on the petrosal of Prokennalestes (fig. 1A, D), the jugular foramen was likely subequal to the round window and was scored accordingly by Rougier et al. (1998). In the remaining eutherians considered by Rougier et al., the primitive state occurs in asioryctitheres and Zalambdalestes and the derived state in leptictids.

150. Jugular foramen—confluent with (O) or separated from (1) opening for inferior petrosal sinus: Extant marsupials have two openings in the jugular fossa: an anterior one transmitting the inferior petrosal sinus to the internal jugular vein and a posterior one for cranial nerves IX, X, and XI (Archer, 1976). Archer (1976) and others (e.g., Muizon et al., 1997) have called the anterior foramen, the internal jugular canal, and the posterior, the posterior lacerate foramen. We believe that the term ‘“‘opening for the inferior petrosal

24 AMERICAN MUSEUM NOVITATES

sinus”’ better reflects the function of the an- terior aperture, as the internal jugular vein forms ventral to the skull base in the dog (Evans and Christensen, 1979), humans (Williams et al., 1989), and presumably mar- supials. Rougier et al. (1998) employed the presence of one versus two foramina in the jugular fossa as a character, with the derived state found only in metatherians and an un- ambiguous synapomorphy of Metatheria. Prokennaletes was scored as unknown, be- cause either state is possible depending on the structure of the exoccipital. Leptictids were scored as confluent, but separate is more appropriate (see Novacek, 1986: figs. 2223, 26).

151. Inferior petrosal sinus—intrape- trosal (0), between petrosal, basisphenoid, and basioccipital (1), or endocranial (2) (Modified from Rougier et al., 1996a): Wible (1983) reported that the inferior petrosal si- nus in extant placentals follows one of three possible courses between the cavernous sinus and internal jugular vein along the suture be- tween the petrosal, basisphenoid, and basi- occipital: intracranial, extracranial, or intra- mural. Rougier et al. (1996a) identified a var- lant on the intramural state that occurs in many Mesozoic mammaliaforms: rather than between the petrosal, basisphenoid, and ba- sioccipital, the sinus is wholly within the pe- trosal. In the taxa considered by Rougier et al. (1998), the inferior petrosal sinus is intra- petrosal in Vincelestes (fig. 4B), Didelpho- don (UCMP 53896), and Prokennalestes (fig. 3A); and between the petrosal, basisphenoid, and basioccipital in other metatherians (fig. 4C) and in asioryctitheres, Zalambdalestes, and leptictids.

152. Ascending canal—intramural (0), intracranial (1), or absent (2) (Rougier et al., 1992): Kielan-Jaworowska et al. (1986) coined the term “‘ascending canal’’ for the intramural canal in multituberculates within the suture between the anterior lamina and the squamosal, dorsal to the middle ear. These authors suggested this canal was more widespread among Mesozoic and extant taxa, and a more detailed evaluation of its homol- ogies was offered by Rougier et al. (1992). Reconstructed as the major occupants were the ramus superior of the stapedial artery and accompanying veins (Wible, 1989; Rougier

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et al., 1992; Wible and Hopson, 1995). In comparisons among cynodonts, Rougier et al. (1992) reported that the ascending canal can be an extracranial sulcus, an intramural canal, an intracranial sulcus, or absent. Among the taxa considered by Rougier et al. (1998), only the last three states are present: intramural in Vincelestes (fig. 4B) and Pro- kennalestes (fig. 1C, EK 3B); intracranial in leptictids; and absent in metatherians (fig. 4C). Absence of the ascending canal was an unambiguous synapomorphy of Metatheria. An ascending canal is present in asiorycti- theres, given the presence of a foramen for the ramus superior in the tympanic roof. However, it is unclear if their ascending ca- nal is intramural or intracranial. Zalambda- lestes was scored as unknown by Rougier et al. (1998). Given that a foramen for the ra- mus superior is now known (personal obs.), then an ascending canal (either intramural or intracranial) must have been present as well.

153. Internal acoustic meatus—deep, with thick prefacial commissure (0) or shal- low, with thin prefacial commissure (1): The presence or absence of an internal acoustic meatus, housing cranial nerves VII and VIII, has been used as a character in phylogenetic analyses of Mammaliamorpha and Mamma- liaformes (e.g., Rowe, 1988; Luo, 1994; Rougier et al., 1996a). A distinct depression for the meatus is present in all the taxa with petrosals considered by Rougier et al. (1998). What varies among them is the relative depth and the thickness of the bone forming the anterolateral wall of the meatus, the prefacial commissure. The meatus is deep with a thick prefacial commissure in Vincelestes and some metatherians (1.e., Deltatheridium, Di- delphodon, Pediomys, and borhyaenids); and shallow with a thin prefacial commissure in the remaining metatherians (1.e., Turgidodon, Pucadelphys, Marmosa, Didelphis, Dromi- ciops, and dasyurids) and in the eutherians Prokennalestes (fig. 1B, E), asioryctitheres, Zalambdalestes, and leptictids. The derived state was identified as an unambiguous syn- apomorphy of Eutheria.

154. Mastoid-squamosal fusion—absent (O) or present (1): Among the taxa studied by Rougier et al. (1998), the mastoid portion of the petrosal exposed on the occiput and the squamosal are fused in the stagodontids

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Didelphodon (UCMP 53896) and Eodelphis (AMNH 14169). Enough of the pars canali- cularis of the petrosal is preserved in Pro- kennalestes to ascertain that the squamosal was not fused to the mastoid.

After making the above changes, the amended Rougier et al. (1998) matrix (ap- pendix 1) was run on PAUP (Swofford, 1993). We followed the parameters enumer- ated in the supplementary information at www.nature.com by Rougier et al. (1998). The number and topology of the 144 most parsimonious trees recovered by Rougier et al. (1998) were not altered in our reanalysis. We made two additional PAUP analyses with the amended matrix. First, we ran the 35 ba- sicranial characters described above with the 21 taxa having scores for these characters. PAUP found 2604 equally most parsimoni- ous trees, the strict consensus of which showed little resolution. Rather than at the base of Eutheria as in the analysis using the complete matrix (fig. 5), Prokennalestes was the outgroup to the remaining therians, be- cause it retains the primitive state for 5 char- acters modified in most other therians. These concern the hiatus Fallopii (#123), the fenes- tra semilunaris (#128), the temporal rami fo- ramina (#143), the inferior petrosal sinus (#151), and the ascending canal (#152). The only resolution within Theria was a unre- solved trichotomy with asioryctids, Kenna- lestes, and Zalambdalestes, supported by de- rived states for the caudal tympanic process (#133) and the “‘tympanic process” (#134). In the second analysis, Daulestes was added to the amended matrix and scored for 11 of the 35 basicranial characters (see above). Af- ter exhauting memory at 10,000 equally most parsimonious trees, the resulting strict con- sensus had no resolution whatsoever. Our fu- ture goal is to include information from the dentition and other cranial regions of Dau- lestes in an expanded phylogenetic analysis to evaluate early eutherian relationships, us- ing the Rougier et al. (1998) matrix as the starting point.

IMPLICATIONS FOR THE EUTHERIAN MORPHOTYPE

Rowe (1988) presented the first compre- hensive phylogenetic analysis of extant

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 25

mammals and near relatives to include a tax- on-character matrix. Extant mammals were represented by three terminal taxa: mono- tremes, marsupials, and placentals. Rowe (1988) did not specify how characters were scored for these diverse groups, but it is like- ly that in the case of polymorphisms a mor- photype was constructed. Since 1988, a num- ber of additional studies addressing the high- er-level relationships of extant mammals have been published (e.g., Rougier et al., 1996a, 1996b; Hu et al., 1997; Ji et al., 1999). These analyses have included more fossil forms and characters than did Rowe (1988). However, the extant forms have con- tinued to be represented by three terminal taxa, with the exception of Rougier et al. (1996a, 1996b) who considered ornithorhyn- chids and tachyglossids separately in order to test monotreme monophyly. Regarding eu- therians, in the studies in which we have par- ticipated (e.g., Wible et al., 1995; Rougier et al., 1996a, 1996b), states have been scored that are either universally present or from a morphotype based on in-group analyses (e.g., Novacek and Wyss, 1986; Novacek et al., 1988; Novacek, 1992; Gaudin et al., 1996). If Prokennalestes falls at the base of Euthe- ria, as proposed by Rougier et al. (1998), then its petrosal will impact the eutherian morphotype employed in higher-level phy- logenetic studies. Below we highlight some of the features of the eutherian morphotype affected by the discovery of the petrosal at- tributed to Prokennalestes.

Anterior Lamina—The presence/absence of the anterior lamina exposed on the side- wall of the braincase has been employed in most phylogenetic analyses of mammalia- morph and mammaliaform relationships (e.g., Rowe, 1988; Wible, 1991; Rougier, 1993; Wible et al., 1995; Rougier et al, 1996a, 1996b). In each instance, the only taxa lacking the anterior lamina are Eutheria and Metatheria, which reflects the state of af- fairs before the discovery of the petrosal of Prokennalestes. However, Prokennalestes has a small exposure of petrosal on the brain- case wall, which we identify as an anterior lamina (fig. 1C, F), as did Rougier et al. (1998). The incidence of this requires that the anterior lamina has either been (1) lost in the common ancestor of eutherians and me-

26 AMERICAN MUSEUM NOVITATES

tatherians, and redeveloped in reduced form in Prokennalestes; or (2) lost independently in metatherians and eutherians other than Prokennalestes.

Lateral Flange—Wible et al. (1995) re- ported that the lateral flange is greatly re- duced or absent in eutherians and metathe- rians, whereas it is a ventrally directed crest extending the length of the promontorium in other mammals, usually in continuity with the crista parotica (see also Rougier et al., 1996a, 1996b). Prokennalestes has the pos- terior part of the lateral flange (fig. 1A, D). As with the anterior lamina, the lateral flange has either been (1) lost at the base of Theria and regained in part in Prokennalestes; or (2) retained in part in Prokennalestes, and lost independently in metatherians and eutherians more derived than Prokennalestes.

Prootic Canal—Features of the prootic ca- nal have been used in most phylogenetic analyses of mammaliamorph and mammali- aform relationships (e.g., Rowe, 1988; Wible and Hopson, 1993; Luo, 1994). When in- cluded, Eutheria has been scored as lacking the prootic canal, with one exception: Rowe (1988) scored the prootic canal present for eutherians, but provided no justification. Wi- ble (1991) questioned this, observing that the prootic sinus and lateral head vein, the pro- otic canal occupants in monotremes and mar- supials (fig. 4A, C), are not known for any extant placentals nor is the canal known for any extinct eutherians (see Wible and Hop- son, 1995). However, Prokennalestes chang- es this conclusion, because its petrosal has a canal that resembles the prootic canal of monotremes and extinct non-therian mam- mals, the only significant difference being its relatively shorter length (fig. 1). Absence of the prootic canal has been considered to be a synapomorphy of Eutheria, but is now a synapomorphy of post-Prokennalestes euthe- rlans.

Posttemporal Canal—Monotremes have a well-developed posttemporal canal that transmits the arteria diploética magna and ac- companying vein (fig. 4A), the artery being a major supplier of the stapedial system (Tandler, 1899, 1901; Wible, 1984, 1987). The corresponding osseous structure is ubiq- uitous among extinct non-therian mammali- aforms, with the arteria diploética magna in-

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terpreted to be larger than the stapedial artery (fig. 4B; Rougier et al., 1992). In contrast, in most metatherians, the posttemporal canal is reduced (fig. 4C; Wible, 1990; Rougier et al., 1998) and, therefore, so is the contribution of the arteria diploética magna to the stape- dial system. The posttemporal canal is lack- ing in most forms considered to be basal eu- therians, such as asioryctitheres (Kielan-Ja- worowska, 1981; Rougier et al., 1998), lep- tictids (Novacek, 1986), and palaeoryctids (Thewissen and Gingerich, 1989). Proken- nalestes (fig. 1K 3) and Zalambdalestes are exceptions (Rougier et al., 1998). Although small compared with that in non-therian mammaliaforms, the posttemporal canal in Prokennalestes is larger than the osseous markings left by the stapedial artery on the petrosal, suggesting that the arteria diploética magna was a more significant contributor to the stapedial system. Consequently, the eu- therian morphotype likely retains a well-de- veloped arteria diploética magna as the major supplier of the stapedial system.

Stapedial Ratio—It is generally (e.g., Se- gall, 1970; Fleischer, 1978) accepted that a round stapedial footplate represents the prim- itive mammalian condition and more ellipti- cal ones are derived. In previous studies (e.g., Segall, 1970, Archibald, 1979; Wible, 1990), eutherians are reported to have a more elliptical footplate than metatherians: with stapedial ratios for the former between 1.8 and 2.9, and for the latter between 1.1 and 1.8 with two exceptions: Dromiciops and Macropus at 2.1 (Segall, 1970). Prokenna- lestes, with a ratio of 1.7, has a rounder foot- plate than Late Cretaceous eutherians, which are in the range of 2.0 and higher (Archibald, 1979; Wible, 1990). Consequently, the basal eutherian morphotype may not be quite so elliptical as previously held.

Fenestra Semilunaris—A fenestra semilu- naris connecting the cavum epiptericum and cavum supracochleare has been reported among extant taxa for caenolestids and some marmosine didelphids (Wible, 1990), and among extinct forms for Vincelestes (Rougier et al., 1992) and PSS-MAE 104 and 129, the isolated petrosals from Khoobur that fell be- tween prototribosphenidans and triconodon- tids in the analyses by Wible et al. (1995) and Rougier et al. (1996a, 1996b). In light of

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this distribution, the fenestra semilunaris in Prokennalestes (fig. 1C, F) either is a re- tained plesiomorphy or is convergent on that in Vincelestes and the non-therian Khoobur petrosals. Supporting the latter is the obser- vation that the fenestra semilunaris in Pro- kennalestes 1s unique in that it transmits the greater petrosal nerve (fig. 3B); the remain- ing forms have a canal in the petrosal leading to the hiatus Fallopii.

Paroccipital Process—Various authors (e.g., Rowe, 1988; Luo, 1994; Rougier et al., 1996a, 1996b) have employed different as- pects of the mammaliamorph paroccipital process of the petrosal in phylogenetic anal- yses, including its orientation and size. In the vast majority of non-therian mammaliaforms, the paroccipital process is a well-developed, vertical muscular process continuous anteri- orly with the crista parotica (fig. 4A; Wible and Hopson, 1993; Rougier et al., 1996a). An exception is the echidna, in which the paroccipital process is essentially lacking (see Wible and Hopson, 1995: fig. 3). Iden- tifying the paroccipital process in therians has been problematic, because by and large these forms do not have a well-developed, vertical muscular process posterior to the crista parotica (Wible, 1990; Rougier et al., 1998). Rather than a paroccipital process, what many therians do have that may serve a comparable function is a more posterolat- erally directed shelf, which is usually termed the mastoid process (Novacek, 1986; Wible, 1990). In contrast to other therians, Proken- nalestes has a well-developed, vertical par- occipital process (fig. 1A, D) resembling that in Vincelestes (not easily visualized in the ventral view in fig. 4B). The parocciptial pro- cess has either been (1) retained in Proken- nalestes and lost independently in other eu- therians and in metatherians; or (2) lost at the base of Theria and redeveloped in Proken- nalestes.

Ascending Canal—In extant placentals, the course of the ramus superior of the sta- pedial artery is largely endocranial (Wible, 1987), whereas in the platypus the proximal part of the artery’s course is intramural and the distal part is extracranial (fig. 4A; Wible and Hopson, 1995). Vincelestes presents an intermediate condition: the distal part of the ramus superior has an endocranial course,

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 27

but the proximal part is enclosed in an ex- tended ascending canal within the petrosal (fig. 4B; Rougier et al., 1992). Extant mar- supials lack the proximal portion of the ra- mus superior (fig. 4C); the distal part is en- docranial (Wible, 1987). In previous phylo- genetic analyses (Wible et al., 1995; Rougier et al., 1996a, 1996b), we have scored the course of the ramus superior as endocranial for Eutheria, because in addition to extant placentals, it is the condition suggested by isolated eutherian petrosals from the Late Cretaceous Bug Creek Anthills of Montana (Wible, 1990). Prokennalestes, however, has an ascending canal within the petrosal (figs. 1, 3), as in Vincelestes and some multituber- culates (Kielan-Jaworowska et al., 1986; Rougier et al., 1992). Unfortunately, the de- tails of the course of the proximal part of the ramus superior, whether intramural or endo- cranial, are not known for many extinct eu- therians, including asioryctitheres and za- lambdalestids. Therefore, it is uncertain whether Prokennalestes has retained the primitive state of an ascending canal or de- veloped one independently.

Foramen for Ramus Temporalis—In the platypus, the foramen for the ramus tempo- ralis is within the lamina obturans on the sidewall of the braincase (see “‘ramus supe- rior” in fig. 4A; Rougier et al., 1992; Wible and Hopson, 1995); extinct non-therians have similar foramina in the anterior lamina (Rougier et al., 1992; Wible et al., 1995). In contrast, in therians the foramina transmit- ting temporal rami are in the squamosal or between the squamosal and parietal (Wible, 1987). As interpreted here, Prokennalestes differs from other therians in that its foramen lies between the petrosal and squamosal (figs. 1C, F; 3B). If this is part of the euthe- rian morphotype, then the position of the fo- ramina for the temporal rami in the squa- mosal and/or parietal has been convergently acquired in metatherians and in other euthe- rians.

Inferior Petrosal Sinus—MacIntyre (1972) found a sulcus along the medial aspect in iso- lated petrosals of North American Late Cre- taceous eutherians and metatherians. He in- terpreted this sulcus for the inferior petrosal sinus and speculated that it was primitive for eutherians and metatherians, and present in

28 AMERICAN MUSEUM NOVITATES

their common ancestor. Although not speci- fied by MacIntyre, the likely course for the inferior petrosal sinus in these forms was largely intramural between the petrosal lat- erally and the basisphenoid and basioccipital medially. Wible (1990) noted that a similar course for the inferior petrosal sinus was pro- posed for the Liassic mammaliaform Mor- ganucodon by Kermack et al. (1981) and suggested that such an inferior petrosal sinus may have predated Theria. More recently, however, Rougier et al. (1996a) have reeval- uated the course of the inferior petrosal sinus in extinct non-therian mammaliaforms. These authors observed that many taxa (e.g., Morganucodon, Kermack et al., 1981: fig. 83B; Vincelestes: MACN-NO5, NO9) have a canal wholly within the petrosal near the ba- sisphenoid-basioccipital contact, with an an- terior opening at the anterior pole and a pos- terior opening at or near the jugular foramen (fig. 4B). Because no other course is indi- cated for the inferior petrosal sinus in these forms (contra Kermack et al., 1981), Rougier et al. (1996a) proposed that vein occupied the intrapetrosal canal. They used this con- dition in a multistate character concerning the course of the inferior petrosal sinus: the other states being endocranial, and intramu- ral between the petrosal, basisphenoid, and basioccipital. Metatherians were scored intra- mural, and eutherians polymorphic: either in- tramural or endocranial. However, Rougier et al. (1998) noted that Prokennalestes and the metatherian Didelphodon have the intrape- trosal canal for the inferior petrosal sinus. This may either have been retained primi- tively or convergently acquired in Eutheria and Metatheria.

Cochlear Coiling—It has long been known (e.g., Pritchard, 1881; Gray, 1908a, 1908b; Fernandez and Schmidt, 1963) that marsu- pials and placentals are distinguished from monotremes in having a fully coiled cochlea. Moreover, the degrees of coiling has been used as a character in Rowe’s (1988) phy- logenetic analysis of Mammaliamorpha and in all subsequent studies of similar taxonom- ic scope (e.g., Rougier, 1993; Hu et al., 1997; Ji et al., 1999). The unique condition in each study ascribed to eutherians and metatherians is having a cochlear duct coiled through a minimum of 360°; Vincelestes at 270° is the

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closest to that (Rougier, 1993). However, what the actual primitive condition is for Theria is uncertain. No living therian has fewer than one and a half turns (Gray, 1908b; Lewis et al., 1985), which is also the con- dition reported for eutherians and metatheri- ans from the Late Cretaceous Bug Creek Anthills (Meng and Fox, 1993, 1995b). Slightly older isolated therian petrosals from the Late Cretaceous Oldman and Milk River Formations, Alberta, have even fewer turns at 1.25 (Meng and Fox, 1993, 1995a); two of the Oldman Formation petrosals are from metatherians and the Milk River specimen is from either an eutherian or a tribosphenidan. The fewest number of turns was reported from an endocast of the eutherian Zalamb- dalestes by Kielan-Jaworowska (1984: 162) as “consisting of only one whorl’’. However, using West’s (1985) method of measuring co- chlear curvature on the illustrations in Kie- lan-Jaworowska (1984: fig. 2C; pl. 31, figs. 1b, c), one and one quarter turns appears to be a better estimate for Zalambdalestes. In a recent description of the skull of the Late Cretaceous? asioryctithere Daulestes from the Coniacian (about 87 million years ago) of Uzbekistan, McKenna et al. (2000) have reported that the cochlea has one full coil, although they (p. 23) admit that the degree of curvature “cannot be determined with pre- cision’’. With a spiral of just 360° (fig. 2), the cochlea of Prokennalestes represents both the oldest example of coiling in the fos- sil record by a minimum of 10 million years and, likely along with Daulestes, the only ev- idence supporting 360° as the primitive eu- therian condition.

Perhaps one of the more surprising fea- tures of the cochlea in Prokennalestes is that it is of uniform diameter to its tip (fig. 2). This has also been reported in a therian from the Late Cretaceous Milk River Formation (Meng and Fox, 1995a) and may be the con- dition in Daulestes (see below; McKenna et al., 2000). In most other mammaliaforms (e.g., Morganucodon, Graybeal et al., 1989; Sinocondon, Luo et al., 1995; Canis fami- liaris, Evans and Christensen, 1979), there is some tapering at the cochlear tip. An excep- tion is monotremes, which have an expansion in the apical part of the cochlear duct, a la- gena as in sauropsids (Pritchard, 1881; Al-

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exander, 1904; Griffiths, 1978). From X-ra- diographs, Fox and Meng (1997) reported for an unidentified multituberculate from the Hell Creek Formation an expansion at the tip of the cochlea, comparable to that of mono- tremes, and suggested that it held a lagena. Interestingly, other multituberculates have a cochlea of uniform diameter (Fox and Meng, 1997) or one that tapers at the tip (Luo and Ketten, 1991). Also from X-radiographs, McKenna et al. (2000) found an unusual co- chlea in Daulestes. Although the cochlear tip was as broad as the base, it was separated by a constriction. McKenna et al. (2000) posed two possible explanations for the constric- tion: it was either an artifact or an isthmus lagena, connecting the cochlear duct with a lagena as in monotremes. We believe the lat- ter explanaton is unlikely, because the “‘la- gena”’ so identified for Daulestes by McKen- na et al. (2000) shows no expansion over the remainder of the cochlea, in contrast to the condition in monotremes. If the constriction seen by McKenna et al. (2000) is an artifact, then the cochlea in Daulestes and Proken- nalestes is very similar. Until more compar- ative data are collected, the meaning of the uniform nature of the cochlea in these Cre- taceous forms is unknown.

Cochlear Nerve—In basal mammali- aforms, such as Morganucodon (Kermack et al., 1981), the cochlear nerve enters the inner ear via a Single aperture in the internal acous- tic meatus. In monotremes, however, the in- ternal acoustic meatus has a cribriform plate through which branches of the cochlear nerve pass (Simpson, 1938; Fox and Meng, 1997). A cribriform plate also occurs in ex- tinct and extant therians investigated to date, but differs from that in monotremes in being displayed in a spiral or radial belt (Meng and Fox, 1995b), possibly as a mechanism so that the nerves destined for the organ of Corti within the coiled cochlea have equivalent lengths and, therefore, equivalent nerve im- pulse conduction times (West, 1985). Meng and Fox (1995b) proposed that a cochlear nerve with such a radial pattern is a therian synapomorphy, although they noted that the condition in Vincelestes is unknown. Our in- terpretations of a cribriform plate in the fo- ramen acusticum inferius in Prokennalestes support the proposal of Meng and Fox.

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 29

Secondary Osseous Spiral Lamina—Fox and Meng (1997) claimed that the develop- ment of primary and secondary osseous spi- ral laminae in the cochlea between which the basilar membrane is stretched is a therian synapomorphy, because these structures are not present in monotremes or extinct non- therian mammaliaforms. Earlier, these au- thors (Meng and Fox, 1995b: 60) stated that ‘“‘the osseous laminae are unknown in the Early Cretaceous non-tribosphenic therian Vinceslestes (Rougier et al., 1992).’’ How- ever, Rougier (1993) reported the presence of the secondary osseous spiral lamina in this taxon, and we have repeated that observation elsewhere (Wible et al., 1995; Rougier et al., 1996a, 1996b). Consequently, contra Fox and Meng (1997), the secondary lamina is not a therian synapomorphy. We are uncer- tain about the level of origin of the primary lamina, because we can neither confirm nor deny the presence of the primary lamina in Vincelestes.

CONCLUSIONS

Prokennalestes has been posited to occupy a phylogenetic position at or near the base of Eutheria (Kielan-Jaworowska and Dashzev- eg, 1989; Sigogneau-Russell et al., 1992). The results of the recent phylogenetic anal- ysis by Rougier et al. (1998) are consistent with this view, identifying Prokennalestes as the basalmost of seven eutherian taxa in a study evaluting the position of deltatheroi- dans. Features uniting Prokennalestes with other eutherians include: tall, trenchant pre- molar in the penultimate premolar position; three molars; the size of the molars not in- creasing posteriorly; the penultimate upper premolar protocone a small lingual bulge; the postprotocrista does extend labially past the base of the metacone; and the internal acous- tic meatus shallow with a thin prefacial com- missure. The other eutherians in the Rougier et al. (1998) analysis (Otlestes, asioryctids, Kennalestes, Zalambdalestes, Zhelestes-As- panlestes, and leptictids) are distinguished from Prokennalestes by a stylar cusp B (styl- ocone) that is vestigial to absent and molar conules that are strong, labially placed, with winglike cristae.

Recent comparative studies (e.g., Nova-

30 AMERICAN MUSEUM NOVITATES

cek, 1986; Wible, 1990; Rougier et al., 1996a) have generated detailed hypotheses about the structure of the ear region in basal eutherians and therians. The petrosal of Pro- kennalestes must be considered in such anal- yses, because of its phylogenetic position at or near the base of Eutheria. Our compari- sons have identified primitive and derived features of the ear region that Prokennalestes shares with various taxa, which impact pre- vious notions of the eutherian and therian morphotypes.

The ear region of Prokennalestes is distin- guished from other therians by the number of primitive features it shares with more bas- al forms and by the intermediate conditions that it has between more basal forms on the one hand and other therians on the other. We believe it likely that Prokennalestes is not unique in this regard among Theria and that ear regions of other forms falling near the base of Eutheria and Metatheria yet to be dis- covered will show similar characteristics. If our prediction is correct, then some of the features of the ear region shared by extant marsupials and placentals long held to be therian synapmorphies will have been con- vergently acquired from basal forms resem- bling Prokennalestes (see below).

Primitive features that Prokennalestes shares with the prototribosphenidan Vince- lestes and more basal forms (PSS-MAE 104 and 129) include (1) an intrapetrosal inferior petrosal sinus (also in Didelphodon); (2) an ascending canal within the petrosal; (3) a well-developed arteria diploética magna within the posttemporal canal (also in most metatherians and Zalambdalestes); (4) a fo- ramen for temporal rami on the petrosal; (5) a vertical paroccipital process; (6) a fenestra semilunaris (also in extant caenolestids and some marmosine didelphids); and (7) the rear margin of the auditory region marked by a steep wall (also in metatherians). All seven of these features are modified in other the- rians, with the exceptions noted above. Re- garding #1-—3, different modifications have occurred in other eutherians on the one hand and in metatherians on the other. Conse- quently, the most parsimonious explanation is that Prokennalestes has retained the prim- itive state, with different modifications oc- curring in the eutherian and metatherian lin-

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eages. Regarding #4—6, the same modifica- tions are found in other eutherians and meta- therians. Consequently, the primitive state in Prokennalestes is either a retention with con- vergent modification in other eutherians and metatherians, or modification at the base of Theria with redevelopment of the primitive state in Prokennalestes. We deem the former the more likely in light of the above. Re- garding #7, only post-Prokennalestes euthe- rians have modified the rear of the auditory region through the addition of a flat surface; Prokennalestes and metatherians have re- tained the primitive condition with a steep wall here. Another feature that Prokennales- tes shares with Vincelestes is the absence of a groove for the sigmoid sinus extending to the jugular foramen. This also occurs in metatherians, asioryctitheres, and zalambda- lestids, but has been modified in most extant placentals.

For four characters, Prokennalestes exhib- its an intermediate condition between that in Vincelestes on the one hand and in other therians on the other. (1) The anterior lamina has an expansive exposure on the sidewall of the braincase in Vincelestes and more basal forms, a small exposure in Prokennalestes, and is wholly lacking in other eutherians and metatherians. (2) The lateral flange extends forward from the crista parotica the length of the promontorium in Vincelestes and more basal taxa, a short distance in Prokennales- tes, and is essentially lacking in other theri- ans. (3) The cochlea is coiled through 270° in Vincelestes, 360° in Prokennalestes (and also likely Daulestes), and 450° to 540° in Late Cretaceous eutherians and metatherians. (4) The prootic canal is vertical and long in Vincelestes and more basal taxa, vertical and short in Prokennalestes, horizontal and short in basal metatherians, and wholly lacking in other eutherians. Again, in light of the above, we deem it likely that the intermediate con- dition exhibited by Prokennalestes is primi- tive for Eutheria and perhaps even Metathe- ria. Consequently, metatherians and eutheri- ans other than Prokennalestes have conver- gently lost the anterior lamina and lateral flange, and coiled the cochlea beyond 360°. The eutherian and metatherian lineages have modified the prootic canal differently.

As follows from the above discussion, the

2001

petrosal of Prokennalestes is very general- ized, with most features representing either a primitive therian condition or an intermediate condition not found in the crown-group Pla- centalia. The only petrosal synapomorphy linking Prokennalestes and Placentalia in the stem group Eutheria is the presence of a shal- low internal acoustic meatus with a thin pre- facial commissure. The eutherian affinities of Prokennalestes are, therefore, only weakly supported by the basicranium (a single syn- apomorphy), but more strongly by the den- tition (eight synapomorphies in Rougier et al., 1998). Although partitioning the morpho- logical data sets is probably not defensible from a philosophical point of view, the fossil record itself partitions the evidence available, with ensuing phylogenies generally empha- sizing either dental or basicranial characters. In Rougier et al. (1998) and in the revised analysis presented here, the position of Pro- kennalestes responds to the signal provided by the dentition. An analysis with the basi- cranial characters from the Rougier et al. (1998) matrix identified the ‘‘intermediate”’ morphology of Prokennalestes as the therian plesiomorphic condition and placed Proken- nalestes as the sister taxon to Theria. There- fore, the petrosal attributed to Prokennalestes seems to contradict the position suggested by the dentition. This may be because: (1) the attribution of the petrosal in question is er- roneous and, therefore, we are merging two phylogenetic signals into one; (2) the denti- tion is precocial with regard to the basicra- nium and the basal eutherians early on ac- cumulated dental synapomorphies; or (3) the supposed discrepancies between the basi- cranium and the dentition are a sampling ar- tifact. The relatively more complete dental record of extinct mammals, in particular on the nodes relevant to the position of Proken- nalestes, may distort the diagnostic utility of the characters. The petrosals of the immedi- ate outgroups of Theria are not known, the prototribosphenidan Vincelestes being the first outgroup reference. The morphology of the petrosal of forms such as Pappotherium and Potamotelses are crucial for attaining a more complete picture of the basicranial evo- lution of early therians and close relatives. Until a more even sampling of dental and basicranial characters is obtained for basal

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 31

therians, the few petrosal characters known well enough to be diagnostic at some level are likely to be like ornaments that can be alternatively hung from one branch or anoth- er on a tree constructed mostly on dental characters. Based on the arguments presented here, we believe that proposition #1 is un- likely, but we cannot be certain. With the ev- idence at hand, we are unable to evaluate the impact of #2 and #3, and either a decoupling of the dental and basicranial features, a sam- pling problem, or both are possible.

ACKNOWLEDGMENTS

For access to specimens and information, we thank: Christian de Muizon, Muséum na- tional d’ Histoire naturelle, Paris; William A. Clemens, Museum of Paleontology, Univer- sity of California, Berkeley; and José Bona- parte, Museo Argentino de Ciencias Natura- les “‘Bernardino Rivadavia’’, Buenos Aires. For comments on an earlier version of this manuscript we thank Inés Horovitz, Zofia Kielan-Jaworowska, and an anonymous re- viewer. The illustrations of Prokennalestes were skillfully done, as ever, by Claire Van- derslice. We thank Ed Heck of AMNH for formatting them for publication and William Scarfe, Department of Diagnosis/General Dentistry, University of Louisville School of Dentistry for the X-rays. This research was supported by NSF Grants BSR 91-—19212, DEB-930070, DEB-940799, DEB-9527811, DEB-9625431, DEB-9996051, and DEB- 9996172, and a Ralph E. Powe Junior Fac- ulty enhancement award from Oak Ridge As- sociated Universities to GWR.

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tremes, early therians, and marsupials: 45-62. New York: Springer. Homologies of the prootic canal in mammals and non-mammalian cyno- donts. J. Vertebr. Paleontol. 15: 331-— 356;

Wible, J. R., and G. W. Rougier

2000. Cranial anatomy of Kryptobaatar dash- zevegi (Mammalia, Multituberculata) and its bearing on the evolution of mammalian characters. Bull. Am. Mus. Nat. Hist. 247: 124 pp.

Wible, J. R., M. J. Novacek, and G. W. Rougier

1998. New data on skull structure in the Mon- golian Late Cretaceous eutherian mam- mal Zalambdalestes. J. Vertebr. Paleon- tol. 18(suppl. 3): 86A.

Wible, J. R., G. W. Rougier, M. J., Novacek, M. C., McKenna, and D. Dashzeveg

1995. A mammalian petrosal from the Early Cretaceous of Mongolia: implications for the evolution of the ear region and mammaliamorph _ interrelationships. Am. Mus. Novitates 3149: 19 pp.

Wible, J. R., G. W. Rougier, M. C., McKenna, and M. J. Novacek

1997. Earliest eutherian ear region: a petrosal of ?Prokennalestes from the Early Cre- taceous of Khoobur, Mongolia. J. Ver- tebr. Paleontol. 17(suppl. 3): 84A.

Williams, P. L., R. Warwick, M. Dyson, and L. H. Bannister (eds.)

1989. Gray’s anatomy, 37th ed. Edinburgh:

Churchill Livingstone. Witmer, L. M.

1995. The extant phylogenetic bracket and the importance of reconstructing soft tissues in fossils. Jn J. Thomason (ed.), Functional morphology in vertebrate paleontology: 19-33. New York: Cam- bridge Univ. Press.

Zeller, U.

1985.

1995.

Die Ontogenese und Morphologie der Fenestra rotunda und des Aquaeductus cochleae von TJupaia und anderen Saugern. Gegenbaurs Morphol. Jahrb. 131: 179-204.

Die Entwicklung und Morphologie des Schiadels von Ornithorhynchus anatin- us (Mammalia: Prototheria: Mono- tremata). Abh. Senckenb. Natforsch. Ges. 545: 1-188.

1991. Foramen perilymphaticum und Recessus scalae tympani von Ornithorhynchus an- atinus (Monotremata) und anderen Saug- ern. Verh. Anat. Ges. 84: 441-443.

1989.

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES

Dryolestids 0020000110 0010000000

2222222727072

Peramus 0012000??1 0?0???000?

2222722727702?

Vincelestes 3002102100 12?1??01000 0000000000 Aegialodon

Kielantherium 1??1000???

Potamotelses 22??100????

Slaughteria 2?00?701????

Picopsis aber tirarara:, 2929999990900

Holoclemensia

222201222? 2222722727707?

2000??0??0 0000000000

222222277072?

00010?0012 2021001??? 2222222227?

0000000022 0001100000 0000000000

222272272727?72?

2221000???

2222222727072?

2222001122 2222222227?

222?112222 ODD Dil O so PDD

2222013220 222222222?

2?2?2?110121 22222222??

2222110000 2222222227?

APPENDIX 1

2000??00?? 001010010?

222222277079?

0000000100

2222722727 7?70?

0001002100 0000000101 0?00000000

2222722277070?

0000210101

2222722277072?

0111210100 222222222?

0100200200

2222722277972?

0011121101 2222222227?

0001220101

20 Dad DDE DO Pi?

202??00???0 0?000?0???

2222722

00??00???0

2222722727797?

0000010001 0000000000 000000

21222C2?22? 2222722777970?

0000021001

2222722727772?

000?130001

FD, DED Dior D2?

20222C??2? 222222277970?

20222C??2? 2222722777070?

20222C2?22? 2227227277072?

01000100A1

LD DEON DO DOr DD.

0000130001

22222277070?

0000130001

DED DE D0! Osi Dad?

0000000000

2227227277072?

0?01011000

2227227277070?

1101000000 0000000000

222??20?A010 222222222?

Data matrix for selected non-tribosphenic mammals and therians taken from Rougier et al. (1998) and available at www.nature.com. The character list follows in Appendix 2. Changes to the original matrix are indicated here with an underline. Codes for polymorphic taxa are: A = 0&1; B = 1&2; C = 2&3; D = 0&2. Real polymorphism and polymorphism due to uncertainty or missing data are not distinguished in the matrix.

00?0000102

2222722777070?

0100000100

2222222727070?

0100000001 0000000000

01?0000?00

2222722727772?

0100000000

DED sR PE OO DT OFO

0101000701

222222277?72?

01?2000001

2222227277072?

01?1000?01

2227227277070?

01?1000?02

2222227272772?

11?1001?12 22222222??

11?1000?01 2222222227?

38

Deltatheridium

2001021111 1112111001 2122120110

Deltatheroides

2001021?7?1 12?12???1??1

2222227727070?

Sulestes 222202222? 9222999990990

AMERICAN MUSEUM NOVITATES

2000110112 1011111111 1000?00?00

27000110112 22?2?1111?1? MOOP2.9.9'2:29

2222110710 222222222?

Gurlin Tsav Skull

20:.0U O21 Wor adler cate data arias 12222227770 Pariadens

PP 2VVQ2 2:21?

Kokopellia 2001011??? O?0???1?7?71

2227227272702?

2000111012

0?00?0?1?B

2222012210 2222222227?

aol Ue a era

2222722277072?

222201322? 222222222?

2222104??? 2222222227?

Anchistodelphys

2221011???

2222111010

Tugomortiferum

222211222? Iqualadelphis 2071011???

Didelphodon 2U2V O02 Weed O20 227LLE111 2212120111 Eodelphis 22 VOQTNL 21 0?0??711111 2922999090900

Pediomys

2001011??? Om OFee eeValy lal 2212120110

2222001??2 222222222?

2222112020 2222222227?

2?00011201D 1011111??? 0100?01111

2000112012 1011111??? 222??00???B

2000113000 2?A11111??? 0100??100B

APPENDIX 1 Continued.

0000221100 0100100001 2221211001

000??21100

0001221100

2227227277079?

0200222201 1100100?01

0011222101 222222222?

2000271101 2222222227?

2100222111 2222222227?

OOAA222201 2222222227?

0000222101

2222722727797?

0001221111 222222222?

0?212110?1

0200222201

2222227277972?

??B12110?1

0110120001 00000?10?? 1200??

01?0?20001

0110120001

222792277700?

0110230101 0011001???

0111130101 22222222??

0011230111 2222222227?

0011231101 2222222227?

0011231101 2222222227?

0011231101 222222222?

0011231101 2222222227?

0201??

0110231101

22212?

0011241101 27011171??? 1200??

1111000010

11?1000010

2227227277970?

2201100112 222222222?

0111100212 22222222??

0??1100112 22127??2?22?

0101010000 2??1?1A0121

0101010000

2222722277072?

01?1010000

2227227277070?

B1111B0121

B202110010

2227227277070?

1102110001 2222222227?

1212100102 2222222227?

1202101002

2222722727072?

1212101700 2222222227?

2202111010 2211100101

1202111010 ??111001B1

2202111002

NO. 3322

2001

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES

Albertatherium

rane Ol arcs 222222222?

Alphadon 2001011??1

0101111101 DPD oO POP?

Turgidodon ZOU LLL? 22 2212120110 Glasbius 2001101???

Asiatherium 2021011??1 0?0???1001 13????0??1 Mayulestes 2001011001 0113720710 Borhyaenids 2001021111 0?0??711001 0113120110 Pucadelphys 2001011001 0?0???71101 0112120110

Andinodelphys

2001011001 Ove Opes? 2 L.Ove 0112720710 Jaskhadelphys 222201222? 2992799990900

2001011001 OnOde eA 1: 1112120111 Didelphis

2001011001 0101111111 1112120111 Dasyurids 200101111 010111111 111212011 Dromiciops 202101101 2 OTL P3113 227221

Pr FP

OrRF

2222111010 22222222??

27000111010 1011111??? Redo Re eho ate e)

2000111010

2227222727072

0?00?0010B

2000111010 22221112??? 02 ete 2dD “ealtee dE 1000111001 ODD EL? 2-2 010000???B

2000111011

1000000011

BO00111010 1011111111

10001001011 02B21111

2000111001 1011111111 0000000010

2000111000 201?7?1?111 0A00?001?0

2222111010 DED ED DADs DDD Dit

2000112002 at Ee Wea Poles Ex a ea 2000100012

2000112002 1111110111 2000100012

0000112012 a Bp [pa a al Ea a 2000101012

0000112012 1011111101 20000000?2

APPENDIX 1 Continued.

00A1222111 2222222227?

??B121100?

OOA2222B01 2222222227?

0002221101 POLL 22012

0001222201 1101100001 oye aa BC Lp

0000222201 1101100111

021121221 010011000 122221111

Fr rR

021122221 010111??? 1???21111

FP RF rR

0211222111 Dede DDD" D 2D 19.9

0211122211 0111111001 0021211001

0211112211 0111111011 0021211001

0210222211 A101111011 0222211001

0100222201 0111111001 0222211000

0011231101 2222222227?

0011231101 2222222227?

121???

0011241111 2222222227?

0011230111 0?11001??? Pee 2 0110231001 0001101001 122001

0110221001 0000011011 120010

1010241101 00011010?1 1210?1

1010231101 OOTLETO? 241 1 2220??

T1053 1-031 DDD pt toe Dr DDD

1010031101 0011101111 121011

1010031101 0011101111 121010

1010030101 0011110011 121011

0010041101 0011100111 12101?

2001000112

2222722777072?

A 222A OW AZ 22222222??

0011100111 00?00?0211

0111100111 10100?0221

0011100112 00100?0211

0011700112 00110??2B1

22272277070?

0011010112 0011010221

0011010112 0011010221

1111000112 0011010221

1001000112 0010010221

1222111712 22222222??

1202111000

2222722277070?

2202111012 22222222??

2202101002 B1111A?1??

2211101001 11111011?1

1211111000 1111100121

2212101012 1111101111

2211111012 11111011B1

2222722277070?

2202011002 1111101111

2202011002 1111101111

2202111102 LET Oa? a:

1202011102 2111101121

39

40

AMERICAN MUSEUM NOVITATES

Prokennalestes

0012000??? 0?0???0000 2101010010 Otlestes

0012000??? 0?0???7?000

222222272702

Asioryctids

1012001010 0?0???0001 0103721110 Kennalestes 1012001710 000??00001 01?3021110

Zalambdalestes

1012001120 0?0???0001 0113021110 Zhelestids

0012000??? 0?0???000?

PULP AP PAP PAP PAP Ps

Leptictids

1012001121 0000001001 0113021110

27102111000 2021001??? B200?00100

22???114000 22721000??? Di De eds ote baee 208 0212114001 0021111101 2211010100

0212114001 2021111701 2211017700

0212114022 1101111101 2211010100

2102114020 20227?2?12?0? Dyoptety Bee Pelee 0212114022 1121110101 2200010101

APPENDIX 1 Continued.

1001120100

2??A100000?

0100220100

222722777079?

0100220100 0110101010 0112710000

1100220100 011010?010 0112710000

0100211101 0100100170 2011110000

1100211101

0100211101 0111110110 0012100011

0011130001

0010??

001123?7?01 2222222227?

0011230101 1100101001 1A1001

0011230111 0100101001 1A100?

1001230101 0100001711 1B1001

0011231111

0011240111 1100101011 Tae Oats

00?0100111

2207 222712

2???1001111 2222222227?

OOOA000211 1100??0231

0100000211 110???0231

1101002212 1100071231

0??1?701212 2222222227?

0101012212 1100111221

0111001002

1111001002 2222222227?

11110010A2 2011010112

1111001002 2211070112

2101001012 1011A201B2

1171001012 22222222??

1111001012 2011120111

NO. 3322

2001

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 41

APPENDIX 2

Character list for the data matrix in appendix 1 taken from Rougier et al. (1998) and available at www.nature.com. Changes to characters #58, 111, and 129 from the original list are indicated here in italics. Multistate characters are unordered un- less otherwise noted.

DENTITION—GENERAL

1. Number of premolars—five (0), four (1), three (2), or less than three (3). Ordered.

2. Premolar cusp form—sharp, uninflated (0) or inflated, with apical wear strongly developed (1).

3. Tall, trenchant premolar—in last premolar position (0), in penultimate premolar position (1), or absent (2) (Upper dentition considered when possible).

4. Number of molars—more than four (0), four (1), or three (2). Ordered.

5. Molar cusp form—sharp, gracile (O) or in- flated, robust (1).

6. Size of molars increasing posteriorly—ab- sent (0), moderate posterior increase (1), or marked posterior increase (2) (all molars consid- ered in lower jaw, and all but the last considered in upper jaw).

7. Number of postcanine tooth families—eight or more (0), seven (1), or less than seven (2). Ordered.

DENTITION—UPPER

8. Number of upper incisors—five (0) or less than five (1).

9. First upper incisor—enlarged, anteriorly pro- jecting, separated from I2 by small diastema (0), subequal or smaller than remaining incisors, with- out diastema (1), or lost (2).

10. Number of roots on upper canine—two (0) or one (1).

11. First upper premolar—erect, without diaste- ma (OQ), erect, with a short diastema (1), or pro- cumbent, separated by diastema (2).

12. Penultimate upper premolar protocone—ab- sent (0), small lingual bulge (1), or with an en- larged basin (2). Ordered.

13. Number of roots on penultimate upper pre- molar—two (O) or three (1).

14. Last upper premolar—simple (0), complex, with small protocone (1), or molariform (2). Or- dered.

15. Upper molar shape—as long as wide, or longer (O) or wider than long (1).

16. Upper molar outline in occlusal view—does (O) or does not (1) approach isosceles triangle.

17. Stylar shelf—uniform in width, 50% or more of total transverse width (0), uniform in width, but less than 50% of total transverse width

(1), slightly reduced labial to paracone (2), strong- ly reduced labial to paracone (3), or strongly re- duced or absent (4) (penultimate molar considered when present).

18. Metastylar area on penultimate upper mo- lar—tlarge (0) or reduced (1).

19. Deep ectoflexus—present only on penulti- mate molar (0), on penultimate and preceding mo- lar (1), or strongly reduced or absent (2).

20. Stylar cusp A—distinct, but smaller than B (QO), subequal to larger than B (1), or very small to indistinct (2) (penultimate molar considered when available).

21. Preparastyle—absent (0) or present (1).

22. Stylar cusp B size relative to paracone— smaller but distinct (O), vestigial to absent (1), or subequal (2).

23. Stylar cusp C—absent (0) or present (1).

24. Stylar cusp D—absent (O), smaller or sub- equal to B (1), or larger than B (2).

25. Stylar cusp E—directly lingual to D or D position (O), distal to D (1), or small to indistinct (2).

26. Preparacingulum—absent (0), interrupted between stylar margin and paraconule (1), or con- tinuous (2) (penultimate molar considered when available).

27. Metacone size relative to paracone—notice- ably smaller (0), slightly smaller (1), or subequal to larger (2).

28. Metacone position relative to paracone— labial (0), approximately at same level (1), or lin- gual (2).

29. Metacone and paracone shape—conical (0) or subtriangular, with labial face flat (1).

30. Metacone and paracone bases—adjoined (O) or separated (1).

31. Centrocrista—straight (0) or V-shaped (1).

32. Salient postmetacrista—weakly developed (O) or strongly developed, with paraconid en- larged and metaconid reduced on lower molars (1).

33. Preprotocrista—does not (0) or does (1) ex- tend labially past base of paracone (double rank prevallum/postvallid shearing).

34. Postprotocrista—does not (0) or does (1) extend labially past base of metacone (double rank prevallum/postvallid shearing).

35. Conules—absent (0), small, without cristae (1), or strong, labially placed, with wing-like cris- tae (2). Ordered.

36. Protocone on upper molars—lacking (0), small, without trigon basin (1), small, with dis- tinct trigon basin (2), somewhat expanded antero- posteriorly (3), or with posterior portion expanded (4). Ordered.

42 AMERICAN MUSEUM NOVITATES

37. Procumbent protocone—absent (O) or pre- sent (1).

38. Protocone height—low (QO) or tall, ap- proaching para- and/or metacone height (1).

39. Protocingula—absent (0) or pre- and/or postcingulum present (1).

40. Lingual root position—supporting paracone (O) or supporting trigon (1).

41. Last upper molar width relative to penulti- mate upper molar—subequal (QO) or smaller (1).

DENTITION—LOWER

42. Number of lower incisors—four (0) or less than four (1).

43. Staggered lower incisor—absent (0) or pre- sent (1).

44. Roots on lower canine—biradiculated (0) or uniradiculated (1).

45. First lower premolar—oriented in line with jaw axis (O) or oblique (1).

46. Second lower premolar—smaller than third premolar (O) or larger (1).

47. Last lower premolar—simple (0), complex, with a partial trigonid and/or talonid (1), or mo- lariform (2). Ordered.

48. Trigonid configuration—open, with para- conid anteromedial (0), more acute, with paraco- nid more posteriorly placed (1), or anteroposteri- orly compressed (2).

49. Lower molar talonid—small heel (0) or multicuspidated basin (1).

50. Talonid width relative to trigonid—very narrow, subequal to base of metaconid, developed lingually (O), narrower (1), or subequal to wider (2). Ordered.

51. Lower molar cristid obliqua—incomplete, with distal metacristid present (O), complete, at- taching below notch in metacristid (1), or com- plete, labially placed, at base of protoconid (2). Ordered.

52. Hypoconulid—absent (0), in posteromedial position (1), or lingually placed and “‘twinned”’ with entoconid (2). Ordered.

53. Hypoconulid of last molar—short and erect (O) or tall and sharply recurved (1).

54. Entoconid—absent (0), smaller than (1), or subequal to larger than (2) hypoconid and/or hy- poconulid.

55. Labial postcingulid—absent (O) or present Gia

56. Paraconid and metaconid—metaconid at extreme lingual margin (O) or aligned (1).

57. Metacristid orientation to lower jaw axis— oblique (O) or transverse (1).

58. First lower molar paraconid, low and con- fluent with precingulid—absent (0) or present (1).

NO. 3322

59. Protoconid height—tallest cusp on trigonid (O) or subequal to para- and/or metaconid (1).

60. Paraconid height relative to metaconid— taller (O), subequal (1), or shorter (2) (molars oth- er than the first considered when available).

61. Last lower molar size relative to penulti- mate lower molar—subequal (QO) or smaller or lost (1).

62. Rotation of last lower molar during erup- tion—absent (0) or present (1).

63. Space between last lower molar and coro- noid process—present (QO) or absent (1).

TOOTH REPLACEMENT

64. Deciduous incisors—present (0) or absent (1).

65. Deciduous canine—present (0) or absent (1).

66. Replacement of dP1l/dpl and dP2/dp2— present (O) or absent (1).

LOWER JAW

67. Masseteric fossa—trestricted dorsally by crest reaching condyle (QO) or extended ventrally to lower margin of dentary (1).

68. Posterior shelf of masseteric fossa—absent (O) or present (1).

69. Convex ventral margin behind tooth row continuous to condyle—absent (0) or present (1).

70. Labial mandibular foramen—present (0) or absent (1).

71. Condyle shape—ovoid (OQ) or cylindrical (1).

72. Condyle position relative to tooth row— above (0) or very high (1).

73. Lower jaw angle—posteriorly directed (0), medially inflected (1), or posteroventrally directed (2).

74. Mandibular foramen—below (0) or poste- rior to (1) anterior edge of coronoid process.

75. ““Meckelian’’? groove—present (0) or absent (1).

76. “‘Coronoid’’ facet—present (0) or absent (1).

77. Two large mental foramen, one under sec- ond and third premolar and the other under first and second molar—absent (0) or present (1).

SKULL

78. Septomaxilla—present (O) or absent (1).

79. Premaxilla, palatal process—does not (0) or does reach nearly to (1) canine alveolus.

80. Premaxilla, facial process—does not (0) or does (1) reach the nasal.

81. Lateral margin of paracanine fossa— formed by maxilla (0) or maxilla and premaxilla

(1).

2001

82. Exit(s) of infraorbital canal—multiple (0) or single (1).

83. Flaring of cheeks behind infraorbital fora- men, as seen in ventral view—present (0) or ab- sent (1).

84. Naso-frontal suture with medial process of frontals wedged between nasals—present (0) or absent (1).

85. Nasal foramina—present (0) or absent (1).

86. Frontal-maxillary contact—absent (0) or present (1).

87. Lacrimal tubercle—present (0) or absent (1).

88. Lacrimal foramen exposed on face—pre- sent (OQ) or absent (1).

89. Lacrimal foramen number—double (0) or single (1).

90. Preorbital length relative to postorbital length—two-thirds or more (0) or less than two- thirds (1).

91. Maxillary-jugal contact bifurcated—absent (O) or present (1).

92. Zygomatic arch—stout (0) or delicate (1).

93. Palatal vacuities—absent (O) or present (1).

94. Palatal expansion behind last molar—ab- sent (QO) or present (1).

95. Postpalatine torus—absent (0) or present (1).

96. Palate and basicranium at same level, con- nected by broad choanal ridges—absent (0) or present (1).

97. Minor palatine (postpalatine) foramen— small (0) or large, with thin, posterior bony bridge (1).

98. Palatine reaches infraorbital canal—present (O) or absent (1).

99. Pterygoids contact on midline—present (0) or absent (1).

100. Pterygopalatine crests—present (0) or ab- sent (1).

101. Ectopterygoid process of alisphenoid—ab- sent (Q) or present (1).

102. Optic foramen—absent (0) or present (1).

103. Orbitotemporal canal—present (0) or ab- sent (1).

104. Transverse canal—absent (O) or present (1).

105. Carotid foramen—within basisphenoid (0) or between basisphenoid and petrosal (1).

106. Dorsum sellae—tall (O) or low (1).

107. Alisphenoid canal—absent (0) or present (1).

108. Anterior lamina exposure on lateral brain- case wall—present (0), rudimentary (1), or absent (2).

109. Cavum epiptericum—floored by petrosal (O), petrosal and alisphenoid (1), primarily or ex-

WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 43

clusively by alisphenoid (2), or primarily open as piriform fenestra (3).

110. Exit for maxillary nerve relative to ali- sphenoid—behind (0) or within or in front (1).

111. Foramen ovale composition—in petrosal (anterior lamina) (0), between petrosal and ali- sphenoid (1), in alisphenoid or between alisphe- noid and squamosal (2).

112. Foramen ovale—on lateral wall of brain- case (O) or on ventral surface of skull (1).

113. Squama of squaamosal—absent (0) or pre- sent (1).

114. Position of jaw articulation relative to fe- nestra vestibuli—at same level (QO) or in front (1).

115. Glenoid fossa shape—concave, open an- teriorly (O) or troughlike (1).

116. Glenoid process of jugal—present, with articular facet (0), present, without facet (1), or absent (2). Ordered.

117. Glenoid process of alisphenoid—absent (O) or present (1).

118. Postglenoid process—absent (0) or present (1).

119. Postglenoid-suprameatal vascular sys- tem—absent (QO), present, below squamosal crest (1), or present, above squamosal crest (2).

120. Postglenoid foramen—absent (0), present, behind postglenoid process (1), or present, medial to postglenoid process (2).

121. Alisphenoid tympanic process—absent (0) or present (1).

122. Epitympanic wing medial to promonto- rium—absent (0), flat (1), undulated (2), or con- fluent with bulla (3).

123. Tympanic aperture of hiatus Fallopii—in roof through petrosal (0), at anterior edge of pe- trosal (1), or absent (2).

124. Prootic canal—long and vertical (0), short and vertical (1), short and horizontal (2), or absent (3).

125. Position of sulcus for anterior distributary of transverse sinus relative to subarcuate fossa— anterolateral (O) or posterolateral (1).

126. Lateral flange—parallels length of pro- montorium (Q), restricted to posterolateral corner (1), or greatly reduced or absent (2).

127. Stapedial ratio—rounded, less than 1.8 (O) or elliptical, more than 1.8 (1).

128. Complete wall separating cavum supra- cochleare from cavum epiptericum—absent (0) or present (1).

129. Coiling of cochlea—less than 360° (O) or 360° or greater (1).

130. Rostral tympanic process of petrosal, on posteromedial aspect of promontorium—absent or low ridge (0), tall ridge, occasionally contacting ectotympanic (1).

131. Paroccipital process (sensu Wible and

44 AMERICAN MUSEUM NOVITATES

Hopson, 1993) orientation and shape—vertical (O), slanted, projecting anteroventrally as flange toward back of promontorium (1), or indistinct to absent (2).

132. Caudal tympanic process of petrosal de- velopment—tall wall behind postpromontorial re- cess (OQ), tall wall decreasing in height markedly medially (1), or notched between stylomastoid notch and jugular foramen (2).

133. Crista interfenestralis and caudal tympanic process of the petrosal connected by curved ridge—absent (O) or present (1).

134. “‘Tympanic process’’—absent (0) or pre- sent (1).

135. Tall paracondylar (“‘paroccipital’’) process of exoccipital (sensu Evans and Christensen, 1979)—absent (0) or present (1).

136. Rear margin of auditory region—marked by a steep wall (0) or extended onto a flat surface (1).

137. Fossa incudis—continuous with (0) or separated from (1) epitympanic recess.

138. Epitympanic recess—with small contri- bution to posterolateral wall by squamosal (O) or with extensive contribution to lateral wall by squamosal (1).

139. Stapedius fossa—twice the size of fenestra vestibuli (0) or small and shallow (1).

140. Hypotympanic sinus—absent (0), formed by squamosal, petrosal, and alisphenoid (1), or formed by alisphenoid and petrosal (2).

141. Medial process of squamosal in tympanic cavity—absent (0) or present (1).

142. Ectotympanic—ringlike (O), fusiform (1), or expanded (2).

NO. 3322

143. Foramina for temporal rami—on petrosal (O), on parietal and/or squama of squamosal (1), or absent (2).

144. Posttemporal canal—large (OQ), small (1), or absent (2).

145. Foramen for ramus superior of stapedial artery—on petrosal (0), on petrosal-squamosal su- ture (1), or absent (2).

146. Transpromontorial sulcus—present (O) or absent (1).

147. Sulcus for stapedial artery—present (O) or absent (1).

148. Deep groove for internal carotid artery ex- cavated on anterior pole of promontorium—ab- sent (O) or present (1).

149. Jugular foramen size relative to fenestra cochleae—subequal (0) or larger (1).

150. Jugular foramen—confluent with (0) or separated from (1) opening for inferior petrosal sinus.

151. Inferior petrosal sinus—intrapetrosal (0), between petrosal, basisphenoid, and basioccipital (1), or endocranial (2).

152. Ascending canal—intramural (0), intracra- nial (1), or absent (2).

153. Internal acoustic meatus—deep, with thick prefacial commissure (0) or shallow, with thin prefacial commissure (1).

154. Mastoid-squamosal fusion—absent (O) or present (1).

155. Interparietal—absent (O) or present (1).

156. Dorsal margin of foramen magnum— formed by exoccipitals (O) or by exoccipitals and supraoccipital (1).

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