Earliest eutherian ear region : a petrosal referred to Prokennalestes from the early Cretaceous of Mongolia

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.

2 AMERICAN MUSEUM NOVITATES

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

NO. 3322

(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-

2001

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-

4 AMERICAN MUSEUM NOVITATES NO. 3322

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.

6 AMERICAN MUSEUM NOVITATES

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.

NO. 3322

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

2001

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

8 AMERICAN MUSEUM NOVITATES

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

NO. 3322

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

2001

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

NO. 3322

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-

2001

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,

NO. 3322

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

NO. 3322

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

NO. 3322

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

2001

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-

NO. 3322

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

2001

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

NO. 3322

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-

2001

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-

NO. 3322

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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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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Novitates and Bulletin published during the last five years are available at World Wide Web site

http://nimidi.amnh.org. Or address mail orders to: American Museum of Natural History Library,
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