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Number 2972, 35 pp., 7 figs., 3 tables
10024 June 26, 1990
The Auditory Region of Reithroparamys delicatissimus (Mammalia, Rodentia) and its
Systematic Implications
JIN MENG'
ABSTRACT
The significance and phylogenetic position of the rodent subfamily Reithroparamyinae is con- troversial. This taxon has been included variously in Ischyromyidae (Paramyidae, infraorder Protro- gomorpha) or in the infraorder Franimorpha. Its placement in the Franimorpha was based on an interpretation of the mandible as incipiently hys- tricognathous; reithroparamyines were therefore regarded by some workers as at least part of the ancestral stock of the Hystricognathi. Others con- sidered the Reithroparamyinae to be the most primitive ischyromyids, giving rise to later North American forms. Reithroparamys delicatissimus discussed herein presents several derived features of the auditory region. These include: (1) inflated
bullae with internal septa, (2) apparent loss of the promontory artery, (3) the internal carotid artery crossing over the fenestra rotunda, (4) the presence of a meato-cochlear bridge, (5) a somewhat swol- len promontorium, (6) bony tube for the stapedial artery and facial nerve partially formed, (7) fe- nestra ovalis large and tilted, and (8) epitympanic recess dorsally expanded. Analysis of these de- rived characters allows the ancestor-descendant relationship between reithroparamyines and hys- tricognathous rodents to be rejected. For the same reason, Reithroparamyines do not represent the most primitive ischyromyids; instead, a close re- lationship of reithroparamyines with sciurids, aplodontids, and glirids is proposed.
INTRODUCTION
The genus Reithroparamys was first pro- posed in 1920 by W. D. Matthew. Although Matthew designated AMNH 12561 the ge- notype of Reithroparamys, the type was in
fact Paramys delicatissimus (Leidy 1871) (Wood, 1962; Korth, 1984). Wood (1962) based a new subfamily, the Reithroparamyi- nae, on the genus and placed it in the family
' Graduate Student, Department of Vertebrate Paleontology, American Museum of Natural History; Department
of Geology, Columbia University, New York, N.Y. 10027.
Copyright © American Museum of Natural History 1990
ISSN 0003-0082 / Price $3.75
Paramyidae, superfamily Ischyromyoidea. Later, Black (1971) grouped the Reithropara- myinae, Ischyromyinae, Paramyinae and Prosciurinae in the family Ischyromyidae AI- ston (1876). Wood (1975) proposed a new infraorder, the Franimorpha, under the suborder Hystricognathi, and placed the Rei- throparamyinae within it. Subsequent au- thors have either retained the Reithropara- myinae as a subfamily of the Paramyidae or Ischyromyidae (e.g., Korth, 1984) under the suborder Protrogomorpha or Scituromorpha, or placed it as a subfamily or family in the infraorder Franimorpha under Hystricogna- thi (e.g., Chaline and Mein, 1979; Patterson and Wood, 1982).
It has long been argued that the North American protrogomorphs are the most primitive rodents. Their relationships with later groups remain problematic (e.g., Har- tenberger, 1980: fig. 2; Wood, 1985: fig. 1), because few if any derive characters are known for these rodents. Among the North Ameri- can protrogomorphous rodents, reithropara- myines play a crucial role in the reconstruc- tion of rodent phylogeny. They are central to the debate on fundamental problems in the intraordinal relationships among rodents, such as the definition, composition, and or- igin of the Hystricognathi and the early di- vergence of rodents in North America and Asia. The controversial systematic position of the reithroparamyines has been more or less a direct result of divergent opinions about their mandible structure.
A major problem in the origin of hystri- cognaths is finding a temporal and geographic link between the Early Oligocene hystrico- gnaths of Africa and South America, and the earliest Hystricidae from southern Asian Middle Miocene (Flynn et al., 1986). Wood (1974) presumed that Eocene reithropara- myines present in North America, and pre- sumably present in Asia, provide the linkage, while Lavocat (1980) endorsed direct trans- oceanic dispersal of hystricognaths from Af- rica to South America. Recently, Korth (1984) proposed a hypothesis that reithropara- myines are the most primitive protrogomor- phous stock in North America, and that they give rise to all other ischyromyids and later groups. Auditory features of Reithroparamys
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delicatissimus described in this report present evidence that sheds new light on these prob- lems.
Reithroparamys delicatissimus (AMNH 12561), the best and most completely pre- served specimen of the subfamily, has been studied by many authors (most extensively by Wood, 1962). No agreement has been reached about the nature of the angular pro- cess of the mandible, and therefore its sys- tematic position remains uncertain. Further preparation of this specimen, especially its ear region, has revealed morphological attri- butes previously unknown in Reithropara- mys and known in few other paramyids.
ACKNOWLEDGMENTS
I am grateful to Dr. M. C. McKenna (American Museum of Natural History) for suggesting that I study this subject; he en- couraged me throughout this study and in- cisively criticized an earlier version of this paper. I am also grateful to Dr. M. J. Novacek (AMNH) for patiently introducing mam- malian auditory structures to me and for per- mitting me to study the specimens under his care. For further critical and detailed reading I thank Drs. P. Luckett (University Puerto Rico), L. Flynn (Peabody Museum), A. Wyss (AMNH) and M. Norell (AMNH); this paper is greatly improved and more readable with their valuable comments. I profited enor- mously from discussion with Dr. J. H. Wah- lert (AMNH) on rodent anatomy and system- atics. I have also benefited greatly from discussions with J. R. Wible (University Chi- cago) on the internal carotid complex in var- ious groups of mammals. Thanks are given to Dr. C.-k. Li (Institute of Vertebrate Pa- leontology and Paleoanthropology, Beijing) for permitting me access to specimens under his care. J. Shumsky (AMNH) is gratefully acknowledged for ably preparing the speci- men described here. I wish to express my deep appreciation to many people at the De- partment of Vertebrate Paleontology, AMNBH, for their help in various ways. Fi- nally, I thank my wife, Yu Liu, for her assis- tance in preparation of manuscripts. This work was supported by a faculty fellowship from Columbia University and by the Frick Laboratory Endowment Fund, AMNH.
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11cm
Fig. 1.
MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 3
‘S Ge ES ek Pa ET a ah ity 4, he ~— Sgt NG
X
JF
Ventral view of the basicranial region of Reithroparamys delicatissimus (AMNH 12561). B,
bulla; BO, basioccipital; BS, basisphenoid; EAM, external auditory meatus; EPER, ectotympanic part of the epitympanic recess; FM, foramen magnum; FR, fenestra rotunda; FSM, fossa for stapedius muscle; HF, hypoglossal foramen; I, incus; JF, jugular foramen; MP, mastoid process; OC, occipital condyle; PPER, petrosal part of the epitympanic recess; SMF, stylomastoid foramen.
DESCRIPTION OF THE EAR REGION OF REITHROPARAMYS DELICATISSIMUS
Reithroparamys delicatissimus (AMNH, 12561) is represented by an almost complete skull, articulated mandibles, and some post- cranial skeleton (see Wood, 1962). The bullae on both sides of the skull are preserved al- though both are broken at their posterior end (fig. 1). The bullae are inflated and slightly flat lateroventrally, completely covering the tympanic cavity. The ectotympanic does not expand laterally to form an external auditory canal. The suture between the bulla and the surrounding cranial elements is distinct, in- dicating that the bulla is formed entirely by the ectotympanic. The bulla is tightly joined but not yet fused to the petrosal. The bullar wall is thin but gradually thickens toward the external auditory meatus. Internally a verti- cal bony septum is present at the anterome- dial corner.
On the lateral side of the bulla, the contact between the ectotympanic and the squamosal is limited to the posterodorsal margin of the bulla. The squamosal is completely excluded
from the tympanic cavity by the ectotym- panic. A marked imprint on the ventral mar- gin of the squamosal indicates the overlap- ping of the dorsally expanded ectotympanic above the external auditory meatus (fig. 2). The anterolateral part of the bulla is sepa- rated from the squamosal by a narrow band of the petrosal (fig. 2). The medial side of the bulla is lacking foramina, fissures, or grooves that might be interpreted as an entrance or pathway for a medially placed internal ca- rotid artery or even venous drainage. At the posteromedial end of the bulla, immediately ventral to the jugular foramen, is a foramen which was identified as the stapedial foramen by Wahlert (1974) because of its posterior position, but is herein referred to as the in- ternal carotid foramen (see discussion be- low). The jugular and carotid foramina are separated by a narrowly exposed ridge of the petrosal. The carotid foramen is emarginated ventrally by the ectotympanic and dorsally by the petrosal. Breakage at the posterior end of the bulla has permitted removal of most of the matrix filling the tympanic cavity. The internal carotid artery enters the tym- panic cavity through the carotid foramen, and
SAT PPER
TTF
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MCB FR HE
Fig. 2. Lateral (slightly posteroventral) view of the basicranial region, emphasizing the auditory region, of R. delicatissimus. B, bulla; BO, basioccipital; PPER, petrosal part of the epitympanic recess; FO, fenestra ovalis; FR, fenestra rotunda; FSM, fossa for the stapedius muscle; HF, hypoglossal foramen; IB, imprint left by dorsal extension of the bulla; JF, jugular foramen; M, mastoid; MCB, meato-cochlear bridge (only base preserved); MP, mastoid process; OC, occipital condyle; P, petrosal; PF, postglenoid foramen; PM, promontorium; SAG, groove left by the stapedial artery; SAT, tube for stapedial artery; SMF, stylomastoid foramen; SQ, squamosal; TTF, fossa for tensor tympani muscle.
runs in an open groove until it crosses the fenestra ovalis. The large size of the carotid foramen and associated groove suggest that the internal carotid and stapedial artery also may have been fairly large. The promonto- rium is somewhat inflated. There is no groove or any other trace on the surface of the pro- montorium to indicate the existence ofa pro- montory artery. The fenestra rotundum is small and nearly circular. It is located at the posterior end of the promontorium and faces posteroventrally. The internal carotid artery runs laterally along the posterior end of the promontorium, shielding most ofthe fenestra rotundum. After passing the fenestra rotun- dum, the internal carotid artery continues as
the stapedial artery, while the promontory artery 1s assumed to have been lost (see char- acter analysis section for more details). One of the most distinctive features of the tym- panic cavity 1s the pathway of the stapedial artery. Lateral to the fenestra rotundum, a marked trough for the stapedial artery leads to the fenestra ovalis. In mammals where it is present, the stapedial artery normally trav- els along the ventral rim of the fenestra ro- tundum and approaches the fenestra ovalis from its ventral side. In this case, the long axis of the fenestra ovalis is usually oriented horizontally. The stapedial artery in R. de- licatissimus, however, approaches the fenes- tra ovalis from its posterior rim. Therefore,
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the fenestra ovalis is somewhat tilted so that the stapedial artery can easily run through the stapes so oriented. The fenestra ovalis is relatively large and its rim forms a slightly raised lip.
Another distinctive feature of the tympan- ic cavity is a prominent osseous process de- veloped on the promontorium at the ventral rim of the fenestra ovalis. This process oc- cupies the normal pathway of the stapedial artery. In the left tympanic cavity, this pro- cess is broken at its base, but on the right side it is a distinctly cone-shaped structure pro- jecting posterolaterally. Due to breakage, it does not touch any other structure but in life the distal end of this process probably con- tacted the ectotympanic at the posterior end of the external auditory meatus. This struc- ture is here referred to as the meato-cochlear bridge. The stapedial artery runs posterodor- sal to this bridge. In ventral view, the fenestra ovalis is concealed by the osseous process. A small process posterior to the fenestra ovalis, but dorsal to the groove for the stapedial ar- tery, is also developed. The stapedial artery is thus confined by the meato-cochlear bridge ventrally and this process dorsally before it passes over the fenestra ovalis.
After crossing the fenestra ovalis, the sta- pedial artery enters a bony canal. This canal is formed by the petrosal and is excavated ventrally in the tympanic roof. Its ventral side is cracked, indicating that either the ca-
‘nal was not completely formed in life or was broken in preservation. The canal extends anterolaterally, and the stapedial artery ex- ited the tympanic cavity at its anterolateral corner. There is no indication of the bifur- cation of the superior and inferior ramus of the stapedial artery within the tympanic cav- ity. Posterior to the fenestra ovalis, the lateral wall of this canal continues posteriorly to the stylomastoid foramen; the medial wall of the canal is incomplete so that it opens medially into the fossa assumed for the stapedius mus- cle. Thus, the facial nerve is exposed medially into the tympanic cavity after crossing the stapedial artery dorsally. In the right tym- panic cavity, the bony tube for stapedial ar- tery and facial nerve is not preserved; the tympanic aperture of the facial nerve is vis- ible at the anterior end of the fenestra ovalis; and a groove originating from it runs poste-
MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 5
riorly to the stylomastoid foramen. At the exit of the facial nerve on the right, the ec- totympanic curves inward and covers the ventromedial side of the facial nerve. Because most of the posterior region of the bulla is broken, it is difficult to estimate the extent to which the facial nerve was covered by the ectotympanic. The posterior portion of the path for the facial nerve may have consisted of a bony tube formed by the petrosal later- ally and the ectotympanic medially.
Posterior to the fenestra ovalis, a deep fos- sa, posteriorly bounded by a tubercle of the petrosal, is the fossa assumed for the stape- dius muscle. In life this fossa would have been covered ventrally by the bulla, and the stapedius muscle would have been complete- ly enclosed in the tympanic cavity. Posterior to the stapedius fossa, the mastoid process of the petrosal is small and not horizontally or ventrally expanded.
The epitympanic recess in R. delicatissi- mus, if examined through the external au- ditory meatus, lies between the tegmen tym- pani of the petrosal and the ectotympanic. The petrosal portion of the epitympanic re- cess is located on the lateral surface of the ventrally projecting tegmen tympani of the petrosal and is well separated from the fe- nestra ovalis. Immediately lateral to the pe- trosal portion of the epitympanic recess is the expanded ectotympanic portion of the epi- tympanic recess, which forms a distinct el- liptical fossa in the ectotympanic. This fossa is situated dorsal to the external auditory meatus but is not distinctly separated from - it.
Lateral to the stapedial canal and antero- dorsal to the epitympanic recess, there is a deep depression roofed by the petrosal and medially bounded by the stapedial artery tube. This depression probably implies a dorsal ex- pansion of the epitympanic recess. Because of this depression, the bony tube for the sta- pedial artery appears even more prominent. A tiny bone, possibly the displaced incus, lies within it. A broad depression assumed for the insertion of the tensor tympani muscle lies at the anterolateral side of the promon- torium and medial to the stapedial tube. An- terior to this depression is a breakage which cannot be ascertained to be the piriform fe- nestra.
CHARACTER ANALYSIS
In this section, attention is given to the determination of character polarity. Focus centers on the derived auditory characters found in early rodents in reconstructing their phylogeny. Whether a character is primitive or derived depends on the systematic level being considered. Because R. delicatissimus has been regarded either as a member of the most primitive ischyromyids (Korth, 1984), or in the ancestral stock of the Hystricognathi (Wood, 1985), its derived auditory features are of special significance at the intraordinal level of rodents. In the following discussion, the Reithroparamys auditory region is com- pared with those of Paramys, Sciuravus, Co- comys, and the Theridomorpha. The ear re- gions of these groups are well known and generally accepted as representing the prim- itive morphotypes in rodents (Lavocat and Parent, 1985; Li et al., 1989). Comparison is also made between Reithroparamys and mu- roids, especially when characters are not available in early fossil forms due to frag- mentary materials. This is because muroids are thought to have the most primitive au- ditory region among living rodents (Lavocat and Parent, 1985). The primitiveness of the auditory characters in the groups mentioned above is obtained through outgroup com- parison (for instance, Cocomys, Lietal., 1989). The character polarity of Reithroparamys is basically determined by comparison with these rodent groups. However, when the po- larity cannot be ascertained on comparison with these groups, or when the character states are unclear in these groups, other mamma- lian groups, especially those that may be closely related to rodents such as lagomorphs and macroscelidids (Novacek, 1985; Nova- cek and Wyss, 1986; Novacek et al. 1988) or primitive eutherians such as Leptictis, are employed to determine polarity.
ECTOTYMPANIC BULLA INFLATED WITH INTERNAL SEPTUM. Novacek (1977, 1980) pointed out that the primitive condition of the auditory bulla in eutherians was probably one similar to that of monotremes; there the bony bulla is not present and the ectotym- panic rests at a low angle to the horizontal plane of the skull. An auditory bulla formed completely by the ectotympanic is present in
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Rodentia and Lagomorpha. It is widely agreed that an ectotympanic bulla represents a de- rived condition in eutherians. In rodents a small ectotympanic bulla loosely attached to the petrosal, such as occurs in Sciuravus and Cocomys, is taken as the primitive condition.
In early rodents, an osseous bulla is not commonly found associated with the skull. An osseous bulla has not yet been found in Eocene ischyromyids except Reithroparamys (Korth, 1984). Sciuravus may have a poorly developed bulla, preserved in one specimen (USNM 22477) and illustrated in a recon- struction by Dawson (1961) and Wahlert (1974). The bulla of Sciuravus must have been loosely attached to the petrosal because no markings on the petrosal are detected. The bulla was not inflated and was completely confined to the ventral side of the skull. It even failed to cover the middle lacerate fo- ramen. Cocomys, from the early Eocene of South China, displays a similar bullar con- dition (Li et al., 1989). A bulla co-ossified with the skull, like that of the R. delicatissi- mus, has been regarded as a derived condi- tion in rodents. In R. delicatissimus, how- ever, the bulla is not only co-ossified with the skull but also inflated to a considerable de- gree. The squamosal of R. delicatissimus is blocked, in ventral view, by the inflated bulla (fig. 1); the bulla was also tightly attached, though not completely fused, to the petrosal.
‘Additionally, a vertical septum is present in
the bulla of R. delicatissimus; this is definitely a derived condition. This condition is very much like that of sciurids, where a few septa are present in the bulla and one of them is always located at its anteromedial corner within the bulla.
A bony bulla is assumed to have been pres- ent in at least some paramyines (Wood, 1962). If such a bulla was present, it was probably similar to that of Sciuravus and Cocomys, and more primitive than that of R. delicatis- simus. This may be inferred on two grounds. When viewing the lateral side of the R. de- licatissimus skull, one can see a distinct im- print on the ventral margin of the squamosal (fig. 2), that accommodates the dorsal expan- sion of the ectotympanic. Such an imprint was not observed on any available skulls of Paramys. This implies that the lateral wall of the bulla in Paramys, if indeed the bulla
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was present, must have been entirely con- fined to the ventral side the skull.
In addition, Paramys (AMNH 12508, for example) has a distinct tympanohyal, or a mastoid tubercle (which may be represented in part by an outgrowth of the mastoid and may fuse with the tympanohyal) (Novacek, 1986). The tympanohyal is the most cranial element of the hyoid bar, which often fuses to the petrosal in late developmental stages (van der Klaauw, 1931; De Beer, 1937; McDowell, 1958; MacPhee, 1981). Similar to the condition in Lepftictis, there is a round- ed fossa on the ventral surface of the tym- panohyal in Paramys. A prominent tympa- nohyal is generally associated with a poorly developed bulla, as in marsupials, Leptictis, creodonts, and primitive carnivorans. In ro- dents where the bulla is considerably devel- oped, the tympanohyal, or even the mastoid process where the tympanohyal is attached, fuses with the bullar wall so that a distinct tympanohyal is not usually present. If Para- mys had a bulla, it was most likely very small and loosely attached to the petrosal. Lavocat and Parent (1985) claimed that the most primitive rodent auditory region is that of the Theridomorpha. Theridomorphs have a somewhat inflated bulla with the external au- ditory meatus slightly projected laterally (Lavocat, 1967, pl. 2). This bulla is clearly more derived than that of Sciuravus and Co- comys, and possibly that of paramyines.
Loss OF THE PROMONTORY ARTERY (PA). The classic consideration of the primitive morphotype of the internal carotid artery (ICA) in eutherians is that the ICA system has two main trunks: the medial ICA courses medially to the tympanic cavity while the lateral ICA runs along the ventral surface of the promontorium and later gives off the sta- pedial artery (Matthew, 1909; Van Valen, 1965, 1966; McKenna, 1966; Szalay, 1975; Archibald, 1977; Parent, 1980, 1983). An in- ternal carotid artery and stapedial artery can be traced back in various other tetrapods (Goodrich, 1930; Romer 1956; Romer et al., 1977). An alternative hypothesis about the ICA system has been proposed recently by several workers (Presley, 1979; Cartmill and MacPhee, 1980; MacPhee, 1981; Wible, 1983, 1986, 1987). Based on the observation of the auditory ontogeny and comparative
MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 7
anatomy of living mammals, it is argued that there is only a single ICA trunk in all fossil and living mammals, and that the single ICA simply takes different positions in the tym- panic region in different groups of mammals (fig. 3). The artery may be either in a medial or lateral position; there is no example in extant mammals showing the simultaneous presence of the ICA in both positions. This interpretion is followed in this paper, even though the more traditional position was tak- en by Lavocat and Parent (1985).
The terminology of the internal carotid ar- tery system needs to be clarified because dif- ferent terms have been used in the literature. Besides the usage in the two-trunk model of Matthew and others (see above), the internal carotid artery is regarded as the portion that terminates at the point where it bifurcates into stapedial and promontory arteries (No- vacek, 1986). This portion is taken as equiv- alent to the lateral internal carotid artery by MacPhee (1981). In the usage of others (e.g., Bugge, 1985; Wible, 1986), the internal ca- rotid artery includes the portion before the stapedial-promontory bifurcation and the promontory artery. In this paper (fig. 3), in- ternal carotid artery system (ICAS) is used to indicate the proximal internal carotid artery, after it stems off the external carotid artery, and all its terminal branches including the stapedial artery. Proximal internal carotid ar- tery (PICA) is used to indicate the portion before the stapedial-promontory bifurcation, while distal internal carotid artery (DICA) is used for the portion beyond the bifurcation point, no matter which position it is in. When DICA is in the promontory position, it is referred to as promontory artery (PA); when it is in the medial position, it is referred to as medial distal internal carotid artery (MDI- CA).
It is widely agreed that the reduction of the ICA system is a general evolutionary tenden- cy in eutherians, apparently occurring inde- pendently, in various lineages. Similarly, it is a common evolutionary pattern in euthe- rian mammals that the stapedial artery was reduced and then lost while the MDICA or PICA may still exist. The final stage of this hypothesized transformation series is the complete loss of the ICA system, and its func- tional replacement by the external carotid
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EC PA P sQ PP SA EOFN EAM BP FO FR ER PICA FN FSM A
MDICA
SAT
FNC
FOT
MCB
Fig. 3. Block diagrams showing character states in the ventral view of the tympanic cavity. A, Primitive eutherian condition; B, muroid condition; C, reithroparamyine condition. See text for details. Abbreviations: BP, branching point of SA and DICA; PA, promontory artery; EAM, external auditory meatus; EC, ectotympanic; EOFN, external opening of the facial nerve; ER, epitympanic recess; FN, facial nerve; FNC, facial nerve canal; FO, fenestra ovalis; FOT, fenestra ovalis tilted; FR, fenestra rotunda; FSM, fossa for stapedius muscle; MCB, meato-cochlear bridge; MDICA, medial distal internal carotid artery; P, petrosal; PICA, proximal internal carotid artery; PP, promontorium proper; SA, stapedial artery; SAT, tube for stapedial artery; SQ, squamosal.
system (e.g., Carnivora, Artiodactyla, and some Primates). Persistence of a functional stapedial artery while the promontory artery is completely absent is a rare occurrence in eutherians, recorded in some microchirop- terans (Buchanan and Arata, 1969) and some rodents. A similar situation may (Szalay, 1975; MacPhee, 1981) or may not occur (Conroy and Wible, 1978) in lemurs.
In living rodents, muroids are the group that presents the most primitive ICA pattern, i.e., a MDICA and stapedial artery are both present (Bugge, 1974b, 1985) although the bifurcation of the DICA and stapedial artery is medial to the bulla (Goodrich, 1930; Wi-
ble, 1987), a rare condition in eutherians (fig. 3B). Among early rodents, sciuravids surely had a promontory and stapedial artery (Wah- lert, 1974; personal obs. on AMNH 11614). Theridomorpha and some paramyines may have both arteries (Lavocat and Parent, 1985), but the promontory artery has not been clear- ly observed in Paramys. No matter which position the DICA is in, the condition in which both the distal internal carotid artery and the stapedial artery are present is prim- itive relative to the condition in which either is lost.
It may be asked from which condition, sci- uravid-like or muroid-like, the R. delicatis-
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simus condition was derived? If derived from the muroid condition, then the DICA would have been lost as a MDICA and the stapedial artery within the tympanic cavity would be homogeneous. If derived from the sciuravid condition, the vessel within the tympanic cavity is not completely the stapedial artery because its proximal end is the terminal end ofthe proximal internal carotid artery. It may further be asked which position of the inter- nal carotid artery, PA or MDICA, is primi- tive for rodents? In answering this question, the single ICA hypothesis encounters some difficulty. Recently, Wible (1986) has at- tempted to clarify this problem. Several lines of evidence suggest that a promontory artery (i.e., an artery in the transpromontoral po- sition) is more primitive than a medially placed distal internal carotid artery for pla- centals. Other workers argued a contrary view (e.g., Presley, 1979; Novacek, 1985, 1986). Wible’s hypothesis is adopted here. The promontory artery is probably present in ear- ly rodents such as Sciuravus, Cocomys (Li et al., 1989), and possibly Paramys. The prom- ontory artery is also present in macroscel- ids and possibly in early lagomorphs. Based on the present evidence, it seems likely that the promontory artery is primitive in rodents and the reithroparamyine condition is de- rived from an ICA system which had the dis- tal internal carotid artery in the promontory position. Therefore, the proximal portion of the vessel in the tympanic cavity is part of the proximal internal carotid artery (fig. 3). In other words, the vessel in the tympanic cavity is not completely homologous to a sta- pedial artery and the foramen through which the artery enters the tympanic cavity is more precisely called the carotid foramen instead of the stapedial foramen.
Interestingly, it has been observed with in sciuroids (Tandler, 1899; Bugge, 1971b, 1974b; Wible, 1984) and gliroids (Bugge, 1971b, 1974b) the internal carotid artery does enter the tympanic cavity, as evidenced by its accompanying internal carotid nerve (Wi- ble, 1984). In front of the cochlear fossula, the artery and nerve diverge: the artery (now the proximal stapedial artery) runs laterally toward the fenestra ovalis, and the nerve runs forward. Wible (1984) pointed out that Guth- rie (1963) mistakenly identified the sciuroid
MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 9
and gliroid internal carotid as the stapedial artery and the posterior carotid foramen as a stapedial foramen.
An extratympanic bifurcation of the DICA and stapedial artery has been poorly dem- onstrated (Ting and Li, 1984; Li and Ting, 1985) in the eurymylid Rhombomylus. The authors (Ting and Li, 1984: 98) stated: “‘the stapedial artery, after stemming from the ex- ternal carotid artery, enters the tympanic cavity through the stapedial foramen at the posteromedial corner of the bulla.” and “‘The ICA, after stemming from the external ca- rotid artery, enters the cranial cavity through a fissure-like foramen at the anteromedial corner of the bulla (originally in Chinese).” The systematic implications of this feature are profound. In placental mammals, an ex- tratympanic bifurcation of the DICA and sta- pedial artery is found only in some muroids (Wible, 1987) and probably in Heteromyidae (Bugge, 1974b, 1985; Lavocat and Parent, 1985). This means that these rodents may have evolved from an ancestor that had an ICA pattern similar to that of Rhombomylus. If this interpretation is further confirmed, it may serve as additional evidence for eury- mylid-rodent relationship. Nevertheless, the condition as described in Rhombomylus by Ting and Li (1984, 1985) is questioned be- cause the “internal carotid canal’’ seems too anteriorly located and too small to be a ca- rotid foramen.
In the skull of Paramys, there is a very large canal between the petrosal and the basioc- cipital, that was interpreted by Lavocat and Parent (1985) as a possible course for the medial internal carotid artery. Moreover, none of the specimens of Paramys in the col- lection of the American Museum of Natural History displays a groove on the promon- torium obviously branching from the groove for the stapedial artery to indicate the pres- ence of the promontory artery. A medial ca- nal is not present in R. delicatissimus. An internal carotid artery located between the petrosal and the basioccipital has never been recorded (Wible, 1983, 1984, personal com- mun.). The medial canal in Paramys likely housed the inferior petrosal sinus. It is pos- sible that the promontory artery in Paramys was reduced or lost.
THE INTERNAL CAROTID ARTERY SHIELD-
10 AMERICAN MUSEUM NOVITATES
ING THE FENESTRA ROTUNDA. An internal ca- rotid artery with the stapedial artery has been reported in many groups of living and fossil mammals (Wible, 1987). Its shielding the fe- nestra rotunda is uncommon. The common and primitive condition is that the proximal internal carotid artery enters the tympanic cavity through the carotid foramen and then bifurcates into the promontory and stapedial arteries on the ventral surface of the prom- ontorium. The promontory artery travels anteriorly while the stapedial artery runs lat- erally along the ventral rim of the fenestra rotundum toward the fenestra ovalis (fig. 3A). This pattern has been reported to be present in a few fossil mammals (MacIntyre, 1972; Szalay, 1975; Cifelli, 1982; Coombs and Coombs, 1982; Novacek, 1986), and in some early rodents (Wahlert, 1974; Parent, 1980; Bugge, 1974a; Lavocat and Parent, 1985; Li et al., 1989). Lavocat and Parent (1985) re- ported the stapedial artery of aplodontids as being lost, but in their character distribution chart (ibid., ch. 13, fig. 4), they coded the fenestra rotunda crossed by the stapedial ar- tery. Nevertheless, the stapedial artery is present in some primitive aplodontids, such as prosciurines (Wahlert, 1974; Luckett and Hartenberger, 1985). In Allomys nitens (AMNH 6997) evidence of this vessel also exists: a bony tube partially crosses the fe- nestra rotunda, similar to the condition in sciurids. The proximal end of the stapedial artery is probably the distal end of the prox- imal internal carotid artery as postulated in this paper.
It has been reported that the lateral internal carotid artery (=PICA in this paper) shields the ventral part of the fenestra rotundum in a few placental groups. This condition is re- garded as primitive in placentals (Szalay, 1972, 1975; Archibald, 1977). Novacek (1980, 1986), however, provided an alter- native explanation for the shielding in certain groups. He argued that in many groups this shielding may result from specialized modi- fications including the marked enlargement of the stapedial artery or its enclosure in bony tubes or both. In R. delicatissimus, the shield- ing clearly results from the development of the meato-cochlear bridge (see below), and likely represents a derived condition. If the condition of the internal carotid artery in R.
NO. 2972
delicatissimus is derived from a sciuravid pattern, the shielding is at least partly formed by the proximal internal carotid artery (fig. 3C).
THE OSSEOUS MEATO-COCHLEAR BRIDGE. This structure is very narrowly distributed in rodents; it is reported only in Sciuridae and Aplodontidae (Lavocat and Parent, 1985). No such condition has been observed in other mammalian groups. A similar structure may be formed by the fusion of the tympanohyal with the mastoid tubercle in some mammals. In the adult crania of some mammals, the proximal end of the tympanohyal fuses to the mastoid of the petrosal and its distal end may extend to the promontorium, ventrally arch- ing over the exit groove of the facial nerve. In some cases, this process may touch or fuse to the promontorium, to form a bridge which looks superficially like the meato-cochlear bridge. These two conditions are clearly not homologous. The tympanohyal fuses with the mastoid and touches the promontorium at the ventral side of the fenestra rotundum as in the insectivore Apternodus, or at the pos- teroventral side of the fenestra ovalis as in some creodonts and primitive carnivorans. In all these cases, this osseous connection does not block the stapedial artery at the ventral side of the fenestra ovalis. The stapedial ar- tery runs anteroventrally to the connection. If the meato-cochlear bridge is present, how- ever, it joins the ectotympanic at the external auditory meatus and the stapedial artery runs posterodorsal to it (figs. 2, 3). Cifelli (1982) has proposed, alternatively, that in primitive carnivorans, the tympanohyal fuses to a ven- tromedial elongation of the squamosal. I have reexamined several specimens of primitive carnivorans and creodonts and found that specimens with clear sutures show a mastoid- tympanohyal connection instead of a squa- mosal-tympanohyal connection.
BONY TUBE FOR FACIAL NERVE AND STA- PEDIAL ARTERY. A bony tube for the stapedial and promontory artery has been reported in various groups of placentals and has been regarded as a derived condition (Szalay, 1975; Archibald, 1979; Novacek, 1980). The prim- itive condition is a sulcus or groove on the promontorium, i.e., the stapedial artery is ex- posed ventrally into the tympanic cavity.
As described by Lavocat and Parent (1985),
1990
the stapedial artery, after passing the fenestra ovalis, enters the facial canal and exits the tympanic cavity by several possible routes. In fact, the stapedial artery does not enter the facial canal but just crosses it. In rodents where the stapedial tube is present, the facial nerve and the stapedial artery are enclosed in two separate tubes (fig. 3). The stapedial artery crosses the facial nerve ventrally in all eu- therians, whether or not it is enclosed in a bony tube. The facial nerve runs posteriorly and usually parallel to the lateral side of the promontorium while the stapedial artery runs anterolaterally away from it. Their intersec- tion occurs at a very short distance inside the tympanic cavity. If the bony tubes for the stapedial artery and facial nerve are present, they may be confluent with each other at the crossing point, but this does not mean the stapedial artery enters the facial canal.
After it exits the petrosal and enters the middle ear cavity posteriorly, the facial nerve is covered by a thin bony lamina for a short distance. The tensor tympani muscle usually inserts on the ventral surface of this bony lamina. Primitively, the course of the facial nerve in the tympanic cavity is open. In a more derived condition, however, the facial nerve is completely enclosed in a bony tube. In rodents most of this tube is formed by the petrosal, and only its posteromedial portion is formed by the ectotympanic. When the ec- totympanic is well developed, it sinks dor- sally into the tympanic cavity, and finally covers the stapedius muscle fossa and the ventromedial side of the facial nerve. When the ectotympanic fuses to the petrosal, the facial nerve is thus completely enclosed in a bony tube.
Similarly, the primitive condition of the stapedial artery in placentals is that the sta- pedial artery is conveyed in a sulcus and is thus exposed ventrally in the tympanic cav- ity. After it crosses the fenestra ovalis and then the facial nerve, the stapedial artery bi- furcates into the ramus superior and ramus inferior within the tympanic cavity. In a few groups (rodents, lagomorphs, bats, and ele- phant shrews) the ramus superior/inferior bi- furcation is within the cranial cavity, which is accepted as derived condition for Eutheria (Wible, 1986).
In primitive rodents, such as Paramys, Sci-
MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 11
uravus, Cocomys, and Theridomorpha, the facial nerve, after coursing through the in- ternal auditory meatus, appears in the tym-
panic cavity at an opening anterior or lateral
to the fenestra ovalis. After passing the sta- pedial artery, the lateral side of the facial nerve is bounded by a bony wall formed by the petrosal until it exits the tympanic cavity through the stylomastoid foramen. Com- pared to the condition in R. delicatissimus, this wall in those forms is very short. In ad- dition, the wall in the primitive forms is much lower so that the fenestra ovalis and the epi- tympanic recess is not significantly separated by it. In all these forms, then, the stapedial artery is completely exposed through the tympanic cavity. After passing by the fenestra ovalis, the proximal stapedial artery imme- diately pierces the petrosal and exits the tym- panic cavity.
R. delicatissimus displays a more derived condition. Initially, the stapedial artery is ex- posed in the tympanic cavity as it courses over the fenestra rotundum, but after passing through the stapes it immediately runs into a bony tube. As in sciurids, the bony tube for the stapedial artery projects ventrally to a remarkable degree from the ventral roof of the tegmen tympani. The facial nerve enters the tympanic cavity after it passes the fenes- tra ovalis, but only its medial side is exposed. Before it exits the tympanic cavity, the facial nerve may well be enclosed medially by the ectotympanic. The lateral wall for the facial nerve projects so much that it separates the fenestra ovalis from the epitympanic recess as in sciurids.
ORIENTATION OF THE FENESTRA OVALIS. The orientation of the fenestra ovalis in ro- dents is generally classified as either vertical or horizontal (Parent, 1980, 1983; Lavocat and Parent, 1985). A vertical fenestra ovalis may refer to its vertical orientation relative to the frontal (horizontal) plane of the skull, while a horizontal fenestra ovalis is parallel to the frontal plane. The vertical fenestra Ovalis is regarded as the primitive condition (ibid.). The orientation of the fenestra ovalis can also be described by the orientation of its long axis. The fenestra ovalis in mammals is more or less oval. A small and somewhat rounded fenestra ovalis occurs in some groups as a primitive condition (Archibald, 1979;
12 AMERICAN MUSEUM NOVITATES
Prothero, 1983). Orientation of the long axis of the fenestra ovalis parallel to both frontal and sagittal planes is likely to be primitive. This condition is present in primitive euthe- rians and most living mammals, whether or not the fenestra ovalis is vertical or horizon- tal. Paramys, Sciuravus, and Cocomys have this condition. In R. delicatissimus, however, the fenestra ovalis is somewhat tilted to the frontal plane possibly because of the presence of the meato-cochlear bridge and the course of the stapedial artery approaching from its posterior side (figs. 2, 3A). In addition, the fenestra ovalis is larger than that in para- myines and sciuravids. A large fenestra ovalis is also believed to be a derived condition (Lavocat and Parent, 1985).
EXPANDED EPITYMPANIC REcEsS. Unlike many mammals where the epitympanic re- cess is formed medially by the petrosal and laterally by the squamosal, the epitympanic recess in rodents is laterally bounded by the ectotympanic, which completely excludes the squamosal from the tympanic region (figs. 2, 3). Among living rodents, muroids retain a primitive ear region that compares closely with that of the Theridomorpha (Lavocat and Parent, 1985). The epitympanic recess in mu- roids is located primarily in the tegmen tym- pani of the petrosal and the ectotympanic contributes only a narrow process to the lat- eral wall of the recess. The recess is not ex- panded into the ectotympanic nor is it sep- arated from the external auditory meatus. In other living rodents the ectotympanic, which makes up the lateral part of the epitympanic recess, is greatly excavated dorsally to form a deep fossa or notch. A bony lamina is also developed from the ectotympanic, separating the epitympanic recess from the external au- ditory meatus. In R. delicatissimus there is a very marked fossa in the ectotympanic (figs. 1, 3A). This fossa may function as the lateral part of the epitympanic recess. Primitively, however, it is confluent with the external au- ditory meatus. Anterodorsal to the epitym- panic recess and lateral to the stapedial artery canal, a deep depression is formed in the pe- trosal. This depression provides the space for a further expansion of the epitympanic re- cess. In other primitive rodents such as Para- mys and Sciuravus, a complete bulla is not known and the condition of the epitympanic
NO. 2972
recess is difficult to ascertain. As discussed in the section on the bulla above, the ecto- tympanic bulla in these forms was possibly only poorly developed and not yet dorsally expanded, so that a deep epitympanic recess probably did not exist. In Cocomys, the epi- tympanic recess in the petrosal is distinct and there is no expanded fossa in the ectotym- panic (Lietal., 1989). In other living rodents, the epitympanic recess in the tegmen tym- pani becomes less important while its ecto- tympanic part dominates. In addition, with the projecting facial canal, the epitympanic recess in the tegmen tympani is well sepa- rated from the fenestra ovalis.
INFLATION OF THE PROMONTORIUM. In primitive mammals, such as Leptictis (No- vacek, 1986), ?Protungulatum (MacIntyre, 1972), and marsupials (Clemens, 1966), the promontorium proper is generally low or flat. In most mammals, the elongated cochlea is coiled into a spiral; presumably this reflects a need for retaining an elongated labyrinth within the confined space of the ear region. Although monotremes have less than one cochlear turn (Griffiths, 1978), most mam- mals have at least one or two (MaclIntyre, 1972). The shape of the promontorium prob- ably reflects to some extent the number of cochlear coils and the orientation of the co- chlea. However, there appears to be no direct relationship between the height of the cochlea and the number of half-turns in some mam- mals (Pye, 1979), nor such a relationship in rodents, although in general, the greater the cochlear height, the greater the number of half-turns in selected groups of rodents (Pye, 1977).
The cochlea coils around a central axis and this axis may point ventrally or anteriorly, indicating a rotation of the cochlear orien- tation. Generally, when the axis points an- teriorly, the promontorium appears more in- flated, while the promontorium looks low and flat when the axis points lateroventrally. The cochlear capsules undergo certain rotation and shift during ontogeny. Different groups of mammals present various patterns of rota- tion and shift (Zeller, 1987). The detailed knowledge of the cochlear rotation and shift during rodent ontogeny is unfortunately not yet available. Whether the cochlea coils tight- ly or loosely may also affect the shape of the
1990
promontorium, such as in some rodents where the last part of the cochlea remains uncoiled (Lavocat and Parent, 1985). In fossil rodents, Paramys, Sciuravus, and Cocomys have alow and flat promontorium, while R. delicatissi- mus has a more inflated one. Nevertheless, we do not know whether or not the latter has more cochlear turns or has a more anteriorly pointed cochlear axis than the former, unless broken cochlear specimens are available.
THE STAPEDIUS MUSCLE. Lavocat and Par- ent (1985) believed that Paramyinae are somewhat derived in that the stapedius mus- cle lies completely within the bulla, and the ear region is located anteriorly in the skull. They considered Sciuravus and Theridomor- pha to be more primitive because their ear region is located at the very posterior part of the skull and the stapedius muscle extends outside the tympanic cavity. The ear region is located less posteriorly in Paramys than in other rodents because its mastoid process is horizontally and posteriorly elongated. There is a distinct trough on the ventral side of the mastoid process. This condition is very sim- ilar to that of Adelomys (Lavocat, 1967: fig. 2; pl. 2), although in Paramys this trough is probably longer. A corresponding structure, for the insertion of the posterior belly of the digastric muscle, is present in Leptictis and such a trough was regarded as a primitive condition in various groups (Novacek, 1986). Early erinaceomorphs, such as Pholidocercus hassiacus and Diacodon alticuspis, have a similar trough on the ventral side of the mas- toid process (MacPhee et al., 1988). This trough can also be found in early carnivorans and creodonts. In living rodents, the mastoid process is horizontally shortened or in some cases may be vertically protruded. When the mastoid process is reduced, the auditory re- gion appears to be more posteriorly located. It is an open question whether the condition in Paramys is primitive or derived.
The stapedius muscle in Paramys likely ex- tended outside the tympanic cavity. This is because the fossa for the stapedius muscle in Paramys is less restricted posteriorly than in R. delicatissimus, and the bulla is too small to completely cover the tympanic cavity. R. delicatissimus has a reduced mastoid process and the stapedius muscle is assumed to be within the tympanic cavity because the fossa
MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 13
is deep and its posterior side is sharply bounded by a tubercle formed by the petro- sal. In addition, the bulla is expanded pos- teriorly, covering the tympanic cavity and the stapedius muscle fossa.
OTHER CHARACTERS. There are some other characters that are basically primitive in Reithroparamys and will not be discussed in detail. Such discussions may be found in Par- ent (1980, 1983) and Lavocat and Parent (1985). These characters include small and circular fenestra rotundum, presence of the stapedial artery, the external auditory meatus not separated from the epitympanic recess, and tensor tympani muscle not covered.
METHODS AND RESULTS
Table 1 lists the auditory features consid- ered in this analysis. Except for those per- taining to reithroparamyines, most features in other selected rodent groups have been taken from Parent (1980, 1983), Lavocat and Parent (1985), Wahlert (1974), Bugge (1985), George (1985), and Luckett (1985). The dis- tribution of these characters in selected groups of rodents is listed in table 2. This data set
' was analyzed using both PAUP (3.0) provid-
ed by D. Swofford to M. Novacek and HEN- NIG 86 (version 1.5) by J. Farris. The McClade (2.87c) Test was used for data input and character tracing. Data were run on the heuristic and branch-and-bound searches in the PAUP and on ie calculations in HENNIG 86. The PAUP program is convenient in many aspects (see also Novacek, 1989), although its branch-and-bound search is slow in han- dling this particular data set. Calculation un- der the command ie in HENNIG 86 gener- ates trees by implicit enumeration algorithms and the results are certain to be of minimal length (Farris, 1988; see Platnick, 1989, for more details).
Originally, most of the multistate charac- ters have been coded as open, diagonally lined or solid squares for primitive, intermediately derived and derived conditions, respectively (Lavocat and Parent, 1985). The coding is replaced in this paper by 0, 1, and 2 for the purpose of calculation. Apparently, when an intermediately derived condition is specified, transformation of character states is as- sumed, i.e., from primitive to intermediately
14
AMERICAN MUSEUM NOVITATES
TABLE 1 Selected Auditory Characters and Character Polarity as Discussed in Text
. Bullae are small (0), moderately inflated (1), or in-
flated (2).
. Bulla-petrosal contact is very loose (0), tight (1), or
fused (2).
. Promontorium proper is low (0), slightly expanded
(1), or swollen (2).
. Epitympanic recess is a small and shallow fossa (0),
or expanded dorsally into the ectotympanic (1).
. Epitympanic recess overlies the roof of external au-
ditory meatus (0), or is separated from the external auditory meatus (1).
. Coils of cochlea are uniform (0), or its last part un-
coiled (1).
. Cochlea is in a normal shape (0), slightly bent (1),
or strongly bent (2).
. Internal septa of the bullae are absent (0), weakly
developed (1), or well developed (2).
. Distal internal carotid artery is present (0), or absent
(1).
. Stapedial artery is present (0), reduced (1), or absent
(2).
. Fenestra ovalis is vertical (0), inclined (1), or hori- zontal (or tilted) (2).
. Fenestra ovalis is small (0), moderately large (1), or large (2).
. Fenestra rotunda is small (0), moderately large (1), or large (2).
. Fenestra rotunda is regular (rounded) (0), or twisted (1).
. Proximal internal carotid artery runs ventral to the fenestra rotunda (0), partially shielding (1), or greatly shielding the fenestra rotunda (2).
. Stapedius muscle is large (0), reduced (1), or absent (2).
. Stapedius muscle is exposed in the tympanic cavity (0), partially covered (1), or in a closed fossa (2).
. Posterior part of the stapedius muscle is outside the bulla (0), or completely inside the bulla.
. Tensor tympani muscle is uncovered (0), partially covered (1), or in a closed fossa (2).
. Facial nerve is exposed in the tympanic cavity (0), partially enclosed by bony element (1), or complete- ly enclosed in a bony tube (2).
. Hypotympanic recess is absent (0), present but small (1), or large (2).
. Petrosal orientation is horizontal (0), tilted (1), or more vertical (2).
. Meato-cochlear bridge is absent (0), or present (1).
. Maleus and incus are separated (0), tightly jointed (1), or fused (2).
. Stapedial tube is absent (0), partially developed (1), or complete (2).
NO. 2972
derived and then to derived state (or 0-1-2), although an intermediately derived state is more or less arbitrarily determined, such as the fenestra rotunda being small (0), mod- erately large (1), or large (2). Calculations are done based on two character-type sets: all characters unordered (nonadditive) and all characters ordered (additive). As an ordered character type, the character states are or- dered as 0-1-2. When a character changes in a cladogram from state 0 to 2, or vice versa, it will be counted as two steps. In other words, the character must proceed progressively through state 1. However, there is no re- quirement that state 0 be the ancestral state (Swofford, 1989). As an unordered character type, any state is capable of transforming di- rectly to any other state, and character state 1 in a multistate character is no longer treated as intermediately derived but simply a de- rived state. When a character transforms from state 0 to 2, or vice versa, it will be counted as only one step.
Figure 4 is the strict consensus (nelson) tree derived from six equally most parsimonious trees that result from a heuristic search of PAUP and ie algorithms of HENNIG 86, based on unordered (nonadditive) characters. Branch-and-bound search of PAUP is too slow to be completed in this particular data set. All the six trees have 77 steps of tree length and 0.56 overall consistency index. The consensus tree has 77 character changes, equal to the tree length.
Figure 5 is the most parsimonious tree from branch-and-bound search of PAUP and ie algorithms of HENNIG 86, based on ordered (additive) characters. It has 93 steps and 0.46 overall consistency index, but 81 character changes. Character changes are not equal to tree length because when a character changes from state 0 to 2, or vice versa, it is counted as two steps.
Figure 6 is the strict consensus of four equally most parsimonious trees from branch- and-bound search, based on irreversible characters of PAUP. The irreversible char- acters are equivalent to ordered characters with the additional constraint of irreversi- bility being imposed, i.e., transformations from a more derived state to a less derived state are prohibited. In this analysis, the “‘ir-
1990 MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 15
n 3 BloocococoNONANDCOCOCOeH 3 > alorerooconoocoCOnNANS s fae) = al}oooooooconmnn CooooCoo- ~~ % AQ 2 Nicoccocooo-~coccoNnNnoo += = ue, 5 nl/ocoooonmoCONSCOAOeR g Slocooeooonnnnnoonanaae Z On is SBIODOOCH ANN DOR FRA NNO ‘E a Plon DOO OH HHH ORR AHS Dr =a) “locooooocONcDOoOACOCCOCO 2.8 che SPlooeoocoonoNnNnonOoNnean SES C1900 6-06 Nanooooon eoUs eaTU + Be aollg|tlocoocooo one Toe q J <3 ll € oO 2a g[|=|ocooooo- SA oA CS ota Zee \lé 2 A, OIN|[oceo oO cC OH ONAN NA Ae BAe W > I oe “lon ooooCoOHn HANAN ORR AAN Pe OD .n nea wor Oo as § & SlcoceccocoNnonHONNNANS aEBaz taect y ae ES) AlODDCCCDCOCOH HH DOOR Bee ee PEE r~mloooonoononnnoonnaHn evs —™o ° m™LOCODSOADANNANNANAN HANS sav qsap oS9 hs ee) wmwlooddod © ew wt wt wt mt wt we et et = OO Sm 3 ga Ti oocOoOO CO ws) wt mm mm mt met et ee Mo) g° mlocooocoORHNNNANANNANANA] os As NlLODCDOOR NN AN eR ee NANA ll Q a=l1O0ONOnaRNNANNANANAAANAN be) a o _ ES on 3 oOo wo om os = 2 rs br cm) a SUaé oS Ee 3 a3 s s » 5 Y so) 5s 3S FREES 7) a 8 6 eso 5 oy ee é SECCEEL ELIT EPEEE F ° wae Of8 i] gvg Ss £ BEES ESESESR RE SSSE _— —_ — & BS eS SR ORS AS AVE SRS I OZGEASNTOSCKOKAORSEe Il 2. CmQOAwWHUDHnAMHNAZOLO
16 AMERICAN MUSEUM NOVITATES NO. 2972 wu C4 Ai z Lu = uw a: y ¢ = r a < + <q o. ue re) ao 69S - © 5 w & w aS < w E > £u & & € «< @g gs Ww Oo a = e544 @¢2 o 2 2 z~Z2 g < <@eqe24¢co6o2282 928 2 8 &€ 2 B f£zeSogt zt 28 FS Ee =z 8 9 So a@ 5a go 2 eke ER ee SS ” cc ” < © < Sg a. © O = © b= a. = NY 2-2 5-0 1-1 | 10-1) [17-1 11-0 7-1 | 411-2 7.1 13-0 15-1 |18-0 13-0 14-1 | 12-2 14-1 20-1 25-1 19-1 16-2 17-2 25-1 14.1 |21-2 19-1 23-0 ies 16-2 rane 16-0 13-2 3-2,7-2 , 2461 6-1,12-2 21-1 ® 1-2 2-1,9-1,11-2 22-2, 2-1 8-1 2-1 15-2,19-0,23-1 14-1,19-1 9-] 71 3) 20-0 6) 24-2 22- 3-1, 8-2 oe 1 10-0,11-0,25-2 73 5) 16-1 12-1,13-1 2 1-2,2-2,3-2,4-1 5-1,10-2,19-2 Il Bhi 19-1 11-1,18-2
Q)
Fig. 4. Cladogram depicting the strict (nelson) consensus tree for selected rodent groups, derived from six equally most parsimonious trees resulting from heuristic search of PAUP and implicit enu- meration algorithms of HENNIG 86. All characters unordered. Tree length = 77; overall consistency index = 0.56; character changes = 77. Character changes result from accelerated transformation (ACCT- RAN) optimization of PAUP. Number preceding dash = character; number after dash = character state. * only recent members of the family; **, Erethizontidae not included. See text for more details.
rev.up” command is used to specify that states higher in the symbols order are derived rel- ative to states lower in the symbols order, such as state 2 being derived relative to state 1. The four trees have 102 steps and 0.42 overall consistency index. The consensus tree has 83 character changes.
The character changes in figures 4, 5, and 6 are also results of the accelerated transfor- mation (ACCTRAN) optimization of PAUP.
If a derived state is not consistently present in all members of a given group, ACCTRAN prefers reversals over parallelisms whereas DELTRAN (delayed transformation) can be thought of as preferring parallelisms over re- versals. Both optimizations produce clado- grams with the same topography and equal tree length, but with different explanations of character changes. For example, as a result of ACCTRAN, the derived condition (2) of
1990 MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 17 r > WwW = uw = a % r Ww — * a, < <t < 3; =< © < < 6 w § ; eb ee 28 cE Oo 2 ¢g6<a«25u4 & es 09 @ &# § § Y@ =z <a@285998 842 8 2 5 §S zg 2k ec = O oc ao = re) s ea Ww z= Y > F- < a a ee ee on £- = N = 8 eae © ee & 6 -E 6 Se OF Fg 2-2 10-1 | 17- 11-1 | 45-1 | 18-0 ms 1-2 25-4 | 19-1 141 | 12-2 @ 21-2 es 11-0 | 43-4 14-1 23 16-1 | 24-4 16-2 5&0 [19-1 oo 419) 13-0 11-0 init me MLAS) 11-1 ee 16-2 ae 25-1 3-2 12-0 21-1 21-1 17-2 cau Ore 6-1 Q 16-0,19-0,23-1 as 7-2 3-1,7-0,8-2 7-2 22-2 10-0,25-2 vee 8-1,9-1,13-1 14-0, 16-1 2-2,19-2 20-2,22-0 1-2,3-2,4-1,5-1 10-2,19-1,12-1 2-1,7-1 14-1,18-1 (3) 22-1 19-1
Fig. 5. Cladogram depicting the most parsimonious result from branch-and-bound searches of PAUP and implicit enumeration algorithms of HENNIG 86, based on ordered characters. Overall consistency index = 0.46; tree length = 93; character changes = 81. Character changes are not equal to the tree length because when a character changes from state 0 to 2, or vice versa, it is counted as two steps. Character changes are results of ACCTRAN optimization of PAUP.
character 10 occurs at node 4 in figure 4, reverses to 10-0 at node 7, and then appears in Aplodontidae as 10-1. Asa result of DEL- TRAN, however, character state 10-2 will occur at node 6 and in Castoridae, and state 10-1 in Aplodontidae. These character states can be interpreted as acquired independently
in these taxa and groups. Nonetheless, char- acter 10 displays three steps of changes in both ACCTRAN and DELTRAN optimi- zations. Because the results (the cladograms) are the same, only those of ACCTRAN op- timization are illustrated.
Table 3 lists the character consistency in-
18 AMERICAN MUSEUM NOVITATES NO. 2972 < Z w = wi Tow Ww = o a % w oO. <x q < onl a < < wu f£ 4 ra) cw er Ff} = a ud < oOo § = <q <« 5 s; 2 ff w wu << Pt i22uy58 e963 ¢ & gz fem a) = Q — qa?aqge<s¢éE = a = 2 S ao zr & € 8 G@etegsgsg¢geoges&§&ss8 @ 5 Pb st Go 2k Ee 2G 5 & SZ 7 ra crc on» <€ © £ OF 0 F OO € a2 S&S N 2-2 7-4 11-2 11-1 | 10-1 14-1 12-2 15-1 147-1179 [60 Jig. [19-1 ay 2-2 25-1 178.0 | 179 | 74 | 49, [21-0 20-1 11-0 2-1 ie oe 14-1,16-2)19-1 | 94, | 120 16-1 7-1 18-1 11-0 23-4 21-2 aE 17-2 8-1,9-1 wo 15-0 13-2,14-0 (3) 16-2 3-2,6-1 16-1,21-1 ie 742,122 Gy — 242 20-0 (10) 2-2,19-2 22-2 8-2,9-1,11-2 13-1,15-2,25-2 vai e 6) 19-1 ) 2-1,3-1,4-1,5-1 11-1,12-1,18-1,20-2 19-1
1-1
Fig. 6. Cladogram depicting the strict consensus tree derived from four equally most parsimonious trees. The four trees are obtained through branch-and-bound search based on irreversible characters of PAUP. Tree length = 102; overall consistency index = 0.42; character changes = 83.
dices for all characters in figures 4, 5, and 6. Some of these characters have very low con- sistency indices (CI), especially characters 7, 11, 14, 16, and 19. The CI of these characters are consistently lower than 0.5 in all the three algorithms. Figure 7 provides the consensus results of the most parsimonious trees re- sulting from the branch-and-bound searches of PAUP on unordered, ordered, and irre- versible characters, respectively, with the above five characters excluded from the al- gorithms. Figure 7A is the consensus of 81 equally most parsimonious trees of unor-
dered characters. The branch-and-bound search of PAUP is possible in this case, al- though it took 20 hours to complete the cal- culation. The 81 trees have 51 steps tree length and 0.68 overall CI. Figure 7B is the strict consensus result from eight equally most par- simonious trees of ordered characters, each of them having 61 steps and 0.56 overall CI. Figure 7C is the strict consensus of four equally most parsimonious trees of irrevers- ible characters. The four trees have the same 67 steps tree length and 5.07 overall CI.
As one can see, Figures 4—7 present similar
1990
results. A few groups are stable in all these cladograms. These are Anomaluridae-Pedet- idae, Ctenocactylidae-Thryonomyidae-Ca- viomorpha, and the group consisting of Sciuridae, Aplodontidae, Gliridae, Reithro- paramyinae, and Heteromyidae. The occur- rences of Anomaluridae-Pedetidae and Cas- toridae vary considerably in these cladograms, indicating uncertain systematic positions of these taxa (see discussion).
The auditory features do not contribute anything to the monophyly of Rodentia, i.e., no derived character appears at node 1 in figures 4, 5, and 6. Auditory features are only used to reconstruct the relationships within the group. Rodent monophyly is, however, recognized elsewhere by some other charac- ters. In this analysis, Rodentia (ingroup) is assumed to be monophyletic and a hypo- thetical ancestor (outgroup) is employed for the purpose of analysis. This outgroup is not indicated in the cladograms but is implied by the rooted trees.
- DISCUSSION
A cladistic analysis of rodent phylogeny based on the auditory features is attempted in this paper, although such an analysis is generally regarded as difficult, as cautioned by Wilson (1986), because parallelism is thought to be an important factor in rodent evolution. Character distribution in rodents is so inconsistent that it is almost impossible to obtain a single shortest cladogram on any data set by a manual procedure. This is prob- ably why some workers (e.g., Parent, 1980; Lavocat and Parent, 1985) have provided a table of character distributions but failed to present a cladogram out of their distribution data.
I agree with Luckett and Hartenberger (1985) that those phylogenetic hypotheses corroborated by data from several different (and preferably unrelated) organ systems are more likely to reflect the true phylogeny ofa group, than are those hypotheses corrobo- rated by single character complexes. As has already been mentioned, previously pro- posed phylogenetic relationships of reithro- paramyines to other groups of rodents have been based primarily on dental and a few cranial features. Data from the auditory re-
MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS
19
TABLE 3 Consistency Indices for Characters in figure 4 (CI- A), figure 5 (CI-B), figure 6 (CI-C) (Asterisk indicates character that has CI consis- tently lower than 0.5 in all three algorithms)
Character CI-A CI-B CI-C 1 0.667 0.667 1.000 2 0.400 0.500 0.400 3 0.667 0.500 0.667 4 1.000 1.000 1.000 5 0.500 0.500 0.500 6 0.500 0.333 0.333 7* 0.400 0.286 0.400 8 1.000 1.000 1.000 9 0.500 0.500 0.333
10 0.667 0.400 1.000 11* 0.333 0.286 0.296 12 0.667 0.500 0.500 13 0.500 0.500 0.400 14* 0.250 0.250 0.250 15 1.000 0.667 0.667 16* 0.400 0.286 0.400 17 1.000 0.667 1.000 18 0.500 0.333 0.333 19* 0.333 0.333 0.333 20 0.667 0.667 0.667 21 0.667 0.500 0.500 22 1.000 0.667 1.000 23 0.500 0.500 0.500 24 1.000 0.667 1.000 25 0.667 0.550 0.667
gion in major groups of rodents can be used to test previous hypotheses. Although most of these auditory features are known by some authors, a more explicit resolution of rela- tionships based on these auditory features is provided in this paper. This data set in turn produces hypothetical relationships, which are open to further testing. The following dis- cussion will focus on the phylogenetic posi- tion of reithroparamyines and related groups.
REITHROPARAMYINAE CONTROVERSY
The controversy about the relationship of reithroparamyines to other rodent groups be- gan when they were implicated in the origin of the South American Caviomorpha. The Caviomorpha are first known from the Oli- gocene (Deseadan) of South America, and were already fully hystricognathous and hys-
20 AMERICAN MUSEUM NOVITATES NO. 2972 < wi
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Fig. 7. Consensus results after characters 7, 11, 14, 16, and 19 are excluded (see table 3). All trees are obtained through branch-and-bound searches of PAUP. A, Strict consensus of 81 equally most parsimonious trees of unordered characters; tree length = 51; CI = 0.67. B, Strict consensus of 8 equally most parsimonious trees of ordered characters; tree length = 61; CI = 0.56. C, Strict consensus of 4 equally most parsimonious trees of irreversible characters; tree length = 67; CI = 5.07. A, B, and C are similar in configuration to figs. 4, 5, and 6, respectively, but with a higher overall consistency index.
tricomorphous (Wood, 1985). Two widely differing hypotheses concerning the origin of Caviomorpha are held by Wood and Lavo- cat. Views of these authors have been strong- ly and consistently expressed over the last quarter century. According to Lavocat (1973; 1974a, 1974b; 1976; 1980), caviomorphs originated from the African Phiomorpha, a hystricomorphous and hystricognathous group. Phiomorpha, in the usage of Lavocat, includes Old World hystricognaths but is re- stricted to the Thryonomyoidea by others (Patterson and Wood, 1982; Wood 1985). Lavocat’s hypothesis is based largely on the
morphological similarities between Cavio- morpha and Phiomorpha, particularly of the middle ear region (Lavocat, 1973, 1976; Par- ent, 1980, 1983; Lavocat and Parent, 1985). According to Wood (1974, 1975, 1977, 1980, 1981, 1983, 1984, 1985; Patterson and Wood, 1982), the South American cavio- morphs are descended from the reithropara- myines or franimorphs, a North or Middle American group with incipient to full hys- tricognathy.
Beyond simply representing the ancestral stock of the Caviomorpha, Reithroparamyi- nae, included in Franimorpha, have also been
1990
postulated as progenitors for all Hystrico- gnathi. As early as 1975, Wood suggested that all the special similarities shared by the living hystricognaths that were absent in the Frani- morpha must have evolved independently, by parallelism (except the incipient to full hystricognathy). He also believed that from Late Paleocene or early Eocene common ancestors similar to Franimys, the frani- morphs evolved independently in both North America and Asia, and that the nearest ap- proach to Eocene hystricognaths were the Reithroparamyinae. The New and Old World hystricognaths were regarded as indepen- dently derived from New and Old World sub- hystricognathous rodents, presumably all members of the Reithroparamyinae, that had reached North America and Asia by the Early Eocene. Furthermore, Patterson and Wood (1982: 453) stated: ““The Order Rodentia was certainly of northern origin. Members of it first appear in the latest Paleocene of western North America. These forms, which had al- ready acquired all the basic ordinal charac- ters, are referable to two very closely related families, the scilurognathous Paramyidae and the incipiently hystricognathous Reithro- paramyidae, the latter, in our opinion, the ancestral stock of all later members of the Hystricognathi.”
Hystricognathi monophyly has been sup- ported by evidence including blood vascular, reproductive, chromosomal, and skeletal fea- tures (George, 1985), fetal membranes and placenta (Luckett, 1980, 1985), internal ca- rotid pattern (Bugge, 1985), and features at the molecular level (Sarich, 1985; Shoshani etal., 1985; Beintema, 1985; De Jong, 1985). As to the content of the suborder Hystrico- gnathi, however, there are two different view- points. As defined by Wood (1975, 1985), Hystricognathi includes the Hystricidae, the Thryonomyoidea, the Bathyergoidea, the Caviomorpha, and the Eocene-Oligocene Franimorpha. Recently, Wood (1985) argued that the Hystricognathi are a natural group, but only if one includes the basic hystricogna- thous stock, the Franimorpha, in which many of the features secondarily associated with hystricognathy had not yet developed. In contrast to this, the inclusion of Frani- morpha has been rejected by others (Korth, 1984; Luckett and Hartenberger, 1985).
Clearly, the grouping of franimorphs and
MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 21
the transition of hystricognaths from frani- morphs relies heavily on the dubious feature of incipient histricognathy which is believed to be present in reithroparamyines by Wood. That the condition seen in reithroparamyines is indeed incipient histricognathy has been questioned by others (e.g., Dawson, 1977) who regarded it instead as sciurognathy. Korth (1984) has even argued that none of the species included in the Franimorpha appears to have attained histricognathy as defined by Tull- berg (1899), that the “incipient’’ histricogna- thous condition appears quite common among early rodents, and that it may in fact represent the primitive condition for rodents. Apparently, if this feature is disregarded, there remains little else to suggest that reithropara- myines are particularly closely related to oth- er groups of rodents.
REITHROPARAMYIDAE AND HYSTRICOGNATHI
As introduced above, two views are held about the inclusion of reithroparamyines in the Hystricognathi, based on differing inter- pretations of mandible structure. Two ques- tions addressing this problem may be posed: (1) are reithroparamyines the ancestral stock, or at least within the ancestral stock, of the histricognaths? (2) are franimorphs a mono- phyletic group? The answers in both cases appear to be no.
While the mandible of Reithroparamyinae can be viewed as either “incipiently” histri- cognathous or sciurognathous, derived fea- tures of the ear region of Reithroparamys clearly favor a closer relationship of reithro- paramyines to sciurids, aplodontids, and glirids, shown in figures 4—7. In other words, reithroparamyines are more closely related to sciurognathous rodents than to the Hystri- cognathi. In figures 4-7, reithroparamyines are far distantly separated from histricogna- thous groups. There is no shared derived character in the auditory region demonstrat- ing any close relationship between reithro- paramyines and histricognaths. Based on the features of the auditory region, reithropara- myines as the ancestral stock of histricogna- thous rodents is rejected.
This raises the question of “‘Franimorpha”’ monophyly, particularly considering the fact that some of the members included in the
22 AMERICAN MUSEUM NOVITATES
*Franimorpha”’ do develop typical hystri- comorphy and histricognathy, but others have features typical of sciuromorphous and sci- urognathous rodents. Franimorpha was pro- posed by Wood (1975) as an infraorder that first included Reithroparamyinae, Prolapsus, Protoptychus, and Guanajuatomys. After- ward, Wood (1980, 1985; Patterson and Wood, 1982) added Cylindrodontidae to this infraorder and the Reithroparamyinae was elevated to the family Reithroparamyidae. Among the “‘Franimorpha,”’ the late Eocene Protoptychus (Scott, 1895; Wilson, 1937) is recognized as fully histricomorphous (Wah- lert, 1973), although differing views have been published concerning its mandibular struc- ture (Wood, 1975; Dawson, 1977; Korth, 1984). This may indicate that Reithropara- mys and Protoptychus actually represent two different evolutionary lineages, i.e., Protopty- chus evolved to a hystricomorph while the protrogomorphous Reithroparamys to a sci- uromorph because the latter bears some de- rived characters only found in sciuromorphs. Wood (1977, 1981) described the skull of Prolapsus, another member of “Franimor- pha,” as hystricomorphous. But Korth (1984) believed that the skull of Prolapsus was sim- ilar to that of the Bridgerian Sciuravus and that both are clearly protrogomorphous. Un- certainty also exists about the structure of the mandibles of Prolapsus and Mysops, although Flynn et al. (1986) thought that histricognathy occurs in the latter Eocene Prolapsus. Unfortunately, basicranial mor- phology in these forms is poorly known. The ear region in cylindrodontids seems quite different from that of Reithroparamys. As described by Wahlert (1974), there is no stapedial artery in cylindrodontids but the internal carotid artery is probably present. The posterior opening of the carotid artery is very small and is separate from the jugular foramen. In reithroparamyines, as in sci- urids, the carotid foramen occurs roughly in the same depression with the jugular fora- men. The carotid canal, at least in Ardynomys and Cylindrodon, enters and runs anteriorly through the periotic. The circulation system indicates that Reithroparamys and cylindro- dontids are widely divergent: the stapedial artery is retained but the DICA is lost in the former while the reverse is true of the latter. All these features suggest that Franimorpha
NO. 2972
is not a monophyletic group, and there is not any shared and derived feature to support such a grouping. Korth (1984) has actually returned the Reithroparamyinae back to the Ischyromyidae and relegated other members of Franimorpha to various rodent groups.
Recently, a new genus, Marfilomys, from Central Mexico has been described by Fer- rusquia-Villafranca (1989). According to the author, this new genus shows greatest resem- blance to 7 groups among 27 rodent families compared: reithroparamyine ischyromyids, cylindrodontids, protoptychids, octodontids, cocomyids, yuomyids, and chapattimyine ctenodactylids. The author concluded that “the resemblances were interpreted as phy- logenetically related in the case of the reithro- paramyines, cylindrodontids, protoptychids, and the early octodontids, because these four taxa are both hystricognathous and histri- comorphous (at least incipiently) and the first three, which are united as the Franimorpha, have an Eocene record and inhabit North and Middle America” (ibid., p. 114). Therefore, the discovery of Marfilomys in Middle Amer- ica lends strong support to Wood’s hypoth- esis that the franimorphs were essentially a Middle American group, and that from this group stemmed the ancestral caviomorphs. Apparently, the conclusion reached by the author is based on nothing but the same as- sumption provided by Wood that the Frani- morpha are of at least incipient histricog- nathy and of Middle or North America distribution. Additionally, some of the char- acters believed to be shared by Marfilomys and franimorphs by the author (Ferrusquia- Villafranca, 1989: 112) may not be unique to these groups, but have a wide distribution in rodents (for instances: upper cheek teeth 1-0- 2-3; deciduous premolars normally replaced; premaxillae forming a substantial part of the rostral dorsum, and meeting the frontals; and the relatively posteriorly set incisive foram- ina transected by the premaxillo-maxillary suture or being limited posteriorly by this suture).
REITHROPARAMYINAE AND OTHER ISCHYROMYIDS
Differentiating taxonomic groups is much easier than specifying their similarities, es- pecially their synapomorphies. Recognizing
1990
the Franimorpha as a nonmonophyletic group leaves open the question of reithroparamyine relationships to other groups. Although the family Reithroparamyidae has been included in the Franimorpha, many authors usually regard it as a subfamily within the Paramyi- dae (Wood, 1962) or Ischyromyidae (Black, 1971; Korth, 1984). In these cases, the as- sociations have been based for the most part on primitive characters. The diagnosis of the Ischyromyidae proposed by Black (1971) and adopted by Korth (1984), illustrates this point: ““Cheek teeth basically low-crowned and tri- tubercular with hypocone, when present, sec- ondary in importance to protocone; lophate condition rare, found only in a few advanced forms; talonid basins generally large and un- divided; infraorbital foramen generally small, rounded, not compressed; zygoma heavy; masseter arises from ventral surface of zygo- ma; skull quite narrow in postorbital region; nasals usually long; temporal fossa large; braincase small, not inflated; bulla co-ossified only in a few species; tibia and fibula separate; humerus with entepicondylar foramen.” Recently, it has become widely accepted that Heomys from the Middle or Late Paleo- cene of China is the mammal closest to the ancestry of the Rodentia and in particular to the early Eocene ctenodactyloids (Li, 1977; Li and Ting, 1985; Li et al. 1987; Li et al. 1989; Korth, 1984; Flynn et al. 1986). Korth postulated that, if the ctenodactyloids rep- resent the primitive condition, then reithro- paramyines are the most primitive ischyro- myids because they maintain a hypocone on P4, a distinct hypoconulid on lower molars, and relatively larger hypocones on the upper molars than in paramyines. The simpler mo- lar pattern in paramyines would be consid- ered more derived. Korth (1984) has also considered four features of skull and man- dible to be primitive for rodents: (1) nasal bones extended posteriorly to a level even with the posterior margin of the premaxil- laries; (2) double mental foramen on the mandible; (3) posterior margin of the anterior root of the zygoma even with the posterior margin of P4; and (4) auditory bulla not os- sified to the skull. Korth believed that the nasals extending farther posteriorly than the premaxillaries in the Paramyinace is a derived condition, because in several eurymylids (un- known in Heomys) the premaxillaries extend
MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 23
farther posteriorly than the nasals. A similar situation, i.e., the premaxillaries extending to or beyond the posterior edge of the nasals, is present in reithroparamyines, pseudopara- myines, sciuravids, and early ctenodacty- loids. Therefore, Korth concluded that all paramyines could be derived from a primi- tive reithroparamyine stock by lengthening of the nasal bones and simplification of the occlusal pattern of the cheek teeth. In con- trast, the ear region of Reithroparamys deli- catissimus tells a very different story—that reithroparamyines are probably a more de- rived group than paramyines, as shown in figures 4-7.
It is to be noted that some members in- cluded in the genus Acritoparamys Korth, 1984, such as A. atavus, A. atwateri, and A.
francesi, had been placed in Paramys by oth-
ers (Jepsen, 1937; Wood, 1959, 1962; McKenna, 1961; Rose, 1981). Acritoparamys is included in Reithroparamyinae by Korth (1984), and therefore, reithroparamyine ro- dents become the earliest known in North America. As pointed out by Korth (ibid.), the most primitive paramyine is Paramys taurus, which possesses some of the primitive rei- throparamyine characters such as a hypocon- ulid on the lower molars and hypocone on P4. This makes the separation of the two groups difficult.
Double mental foramina on the mandible are present in the Heomys and nearly all early ischyromyids and sciuravids and so are likely primitive. R. delicatissimus has only one mental foramen.
According to Korth (1984), the posterior margin of the anterior root of the zygoma in primitive eutherians is level with the poste- rior molars, and may even be farther poste- rior than M3. In Heomys it is level with the posterior margin of M1. In early ctenodac- tyloids and most primitive paramyines, it is level with the posterior margin of P4, while in reithroparamyines it progressively moves forward until it is in line with the anterior margin of P4 as in Microparamys and Apa- tosciuravus. Reithroparamyines therefore have a more derived zygomatic condition. If paramyines were derived from reithropara- myines, one would have to assume a reversal in this character.
Among Eocene ischyromyids, a bulla co- ossified with the skull is only found in reithro-
24 AMERICAN MUSEUM NOVITATES
paramyines. Korth suggested that this might bar the Reithroparamyinae from ancestry of paramyines, but that the earliest reithropara- myines may not have possessed this char- acter. At least, it may not have developed in the Reithroparamyinae until the Paramyinae had already split off. This statement is a spec- ulation for which there is no evidence, al- though the systematic position of Reithro- paramys and Acritoparamys may be separate issues.
Among the four features of the skull and mandible listed by Korth, only the first seems to be primitive in reithroparamyines. How- ever, whether the nasal bones extending pos- teriorly to a level with the posterior margin of the premaxillaries is primitive or derived remains an open question. First, “eurymy- loids” may have premaxillaries extending to or more posterior than the posterior margin of the nasals, but this does not necessarily imply a primitive condition for rodents be- cause some eurymyloids (Rhombomylus, for instance) are already too specialized (Li and Ting, 1984, 1985; Li et al., 1989) to present the primitive morphotype for rodents. Sec- ondly, in Cocomys lingchaensis, the nasal bones extend more posteriorly than the pos- terior margin of the premaxillaries (Li et al., 1989). Thirdly, and most important, the con- ditions of a small premaxillary and the pos- terodorsal process of premaxillary not ex- tending to the frontals are widely distributed in mammals and are generally regarded as primitive (Novacek, 1985, 1986; Li and Ting, 1985). In most mammalian groups, the pre- maxillary is a small element much less ex- tended posteriorly than the nasal. It is more acceptable that a more posteriorly extended premaxillary in some rodents represents a de- rived condition.
Simplification of the occlusal pattern of the cheek teeth does not seem well defended. P3 in Heomys and Cocomys is relatively larger, with two cusps and two roots (at least in Heo- mys). The evolutionary tendency is more likely the simplification of this tooth, because in all ischyromyids the P3 is single-cusped and single-rooted. Comparatively, however, Reithroparamys has a more reduced P3 than does Paramys. As has been pointed out by various workers (Li, 1977; Li et al., 1989; Dawson et al., 1984), the P4 in both Heomys and Cocomys is nonmolariform, and the non-
NO. 2972
molarized premolar, essential in pointing to the relationship between Cocomys and Heo- mys, 1S another important plesiomorphous feature. Such a P4 has only a single buccal cusp (paracone), and lacks a metacone and hypocone. Flynn (personal commun.) prefers to call the P4 of Heomys “submolariform.” P4 is submolariform in Paramyinae and mo- lariform in Reithroparamyinae (Korth, 1984). If Korth is correct, i.e., paramyines are de- rived from reithroparamyines, the transfor- mation of P4 must be from non- or submo- lariform to molariform and then back to submolariform.
Several other features may also be more derived in reithroparamyines than in para- myines. In Reithroparamys, the masseteric fossa of the mandible is bounded by much heavier ridges and terminates more anterior- ly. The postglenoid foramen in Reithropara- mys is more reduced than in Paramys. In Cocomys this foramen is also very large (Li et al., 1989). The upper and lower incisors in Reithroparamys are more laterally com- pressed, and the anterior surface of the upper incisors is flat. The snout is shorter and more tapered than that of Paramys.
These features, plus the derived auditory region represented by R. delicatissimus, sug- gest that the reithroparamyines are more de- rived than the paramyines. The earliest is- chyromyoids, such as A. atavus (or P. atavus), may truly represent the ancestral stock of the ischyromyoids, but there is little evidence to suggest their placement among reithropara- myines and therefore to suggest that reithro- paramyines are ancestral to paramyines. In- stead, a possible relationship that requires mention is that reithroparamyines may be derived from a morphotype similar to Para- mys. Besides the characters discussed above, the possible absence of the promontory artery in Paramys may suggest such a possibility.
REITHROPARAMYINAE, SCIURIDAE, APLODONTIDAE, AND GLIRIDAE
As shown in the cladograms (figs. 4 and 5), one interesting grouping consists of Reith- roparamyinae, Gliridae, Sciuridae, and Aplodontidae. A unique character at this node is character 15, i.e., the proximal internal ca- rotid artery shields the fenestra rotunda. [Wahlert (personal commun.) considers that
1990
this feature derives independently in glirids because it does not occur in all of them.] Characters supporting this grouping but also occurring in other groups are 2-1, 11-2, and 23-1.
Although aplodontids, sciurids, geo- myoids, and castoroids were previously in- cluded in Sciuromorpha by Simpson (1945), not all these groups appear to be closely re- lated (Wood, 1937, 1955, 1959; Stehlin and Schaub, 1951; Schaub, 1953; Lavocat 1956). An exception seems to be the Sciuridae and Aplodontidae, where a close sister-group re- lationship has been proposed by many re- searchers (Vianey-Liaud 1985; Wahlert, 1985b; Lavocat and Parent, 1985), based on the dental and cranial evidence of early fos- sils. This sister-group relationship is also sup- ported by molecular evidence (Sarich, 1985), although both forms share the most primitive fetal membrane complex in rodents (Fisher and Mossman, 1969; Luckett, 1971, 1985; Luckett and Mossman, 1981). Moreover, Lavocat and Parent (1985) pointed out that these two groups share a few derived char- acters in the auditory region, including (1) cochlea bent; (2) meato-cochlear bridge; (3) fenestra rotunda twisted; (4) absence of the internal carotid artery, although some of these characters are also shared with other groups. These authors also believed that the auditory region of living Aplodontidae is more ad- vanced than that of sciurids in two regards: loss of the stapedial artery, and loss of the stapedial bony tube crossing the fenestra ro- tunda (Lavocat and Parent, 1985). However, it has been noted by Luckett and Hartenber- ger (1985) that the stapedial artery is present in primitive prosciurine aplodontids as de- scribed by Wahlert (1974). As mentioned above, in a specimen of Allomys nitens (AMNH 6997), the ear region displays a very similar condition to that in Palaeosciurus and other sciurids. In this specimen, the stapedial is apparently present and enclosed in a bony tube along its entire course through the tym- panic cavity. The pathway of this artery is similar to that of sciurids, i.e., the stapedial artery (probably the portion of the proximal internal carotid artery) partially crosses over the fenestra rotunda in a bony tube. This in- dicates that the other aplodontids may have their stapedial artery secondarily reduced from a condition present in Allomys. It is
MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 25
likely that sciurids and aplodontids form a sister-group based on the known evidence.
Sciurids appeared in Europe immediately after the “Grande Coupure’’ (Lopez and Thaler, 1974; Vianey-Liaud, 1979, 1985; Hartenberger, 1983), and cannot have orig- inated in Europe. Wilson (1949) suggested that the Aplodontidae, Sciuridae, and Cas- toroidea were derived directly from Eocene paramyines. Lavocat and Parent (1985) held that the Sciuroidea appeared simultaneously in the Oligocene of North America, Europe, and Asia, suggesting a previous Asiatic his- tory, but information showing their connec- tion to earlier forms is lacking. In general, sciurids and aplodontids have been regarded as derived from the Eocene protrogomor- phous ischyromyids or sciuravids of North America (Korth, 1984; Vianey-Liaud, 1985), but no specific lineage has been proposed to support this.
Emry and Thorington (1982) described a specimen (USNM 243981), which they thought to be closely related to, if not con- specific with, Protosciurus jeffersoni. They re- garded this specimen from the Oligocene White River Formation of Wyoming as not only the oldest fossil squirrel, but as almost certainly a tree squirrel. Although this spec- imen is basically protrogomorphous, the au- ditory region contains some derived features of modern sciurids: periotic and tympanic bulla fused into a single unit, bulla enlarged, transbullar septae present, and stapedial ar- tery enclosed in a bony conduit through the middle ear. Vianey-Liaud (1985) pointed out these authors’ failure to recognize the Euro- pean sciurid Palaeosciurus goti (Vianey- Liaud, 1974a, 1974b, 1975), from Mas de Got, Quercy. This species is also known in the Early Oligocene locality of Aubrelong 1 (Quercy). Summarizing some of the dental, cranial, and postcranial features which define the family Sciuridae, Vianey-Liaud acknowl- edged that the earliest Oligocene European ‘‘squirrel’’ was clearly a sciuromorphous sci- urid and rejected Protosciurus jeffersoni as a sciurid.
Wood (1980b) held that the transition from protrogomorphy to sciuromorphy should mark the boundary between the Paramyidae and the Sciuridae; and this assumption was followed by Vianey-Liaud (1985). However, sciuromorphy as presently conceived is not
26 AMERICAN MUSEUM NOVITATES
unique to Sciuridae, being independently at- tained in geomyoids and castorids (Emry and Thorington, 1982), although geomyoids may differ in some details, such as position of the infraorbital foramen. The specializations of the auditory region of sciurids could have taken place before the appearance of sciuro- morphy, and may represent a good diagnosis for the group. Lavocat and Parent (1985), based on characters of the ear region, pro- posed that the Sciuroidea (family Sciuridae) are monophyletic and recognizable by two rare auditory features that are always asso- ciated: (1) the stapedial artery (PICA used herein) crossing the middle part of the fe- nestra rotunda within a bony canal; and (2) a strong osseous bridge connecting the prom- ontorium to the auditory meatus (osseous meato-cochlear bridge) and hiding the ossi- cles in ventral view. These two character were also regarded as derived characters in Sciuri- dae by Vianey-Liaud. Whether or not the meato-cochlear bridge exists in Protosciurus jeffersoni was not mentioned by Emry and Thorington (1982).
It has been argued (Vianey-Liaud, 1985) that if the sciuromorphy of sciurids were transformed from the protrogomorphy of North American rodents, there is no record of such a morphological transition. She ad- mitted that if the sciuromorphous sciurids and the protrogomorphous aplodontids probably originated from primitive protro- gomorphous rodents, it might be possible to find a protrogomorphous “‘squirrel.” It seems that the sciuravids and paramyids have ear regions so primitive that they could be related to almost anything. It is impossible to trace relationships with later rodents by looking at these primitive characters. Reithroparamys is not sciuromorphous, but its auditory region displays conditions of typical sciurids, sug- gesting a divergence of at least some North American protrogomorphs toward the sci- urid-aplodontid rodents. If Lavocat and Par- ent (1985) are correct, one can simply call Reithroparamys a “pro-sciurid”’ or a protro- gomorphous “‘squirrel’’ as used by Vianey- Liaud (1985) because it has both a meato- cochlear bridge and the internal carotid artery crossing the fenestra rotunda, although the bony tube for the stapedial artery has not yet completely developed. This may lend
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support to the assumption of Black and Sut- ton (1984) that sciurids certainly appear to be North American in origin.
The systematic position of Gliridae is also controversial. Glirids have been considered to be related to the muroid-dipodoid clade by some authors (e.g., Wahlert, 1978; 1985a, 1985b), based on zygomatic structure. Other authors (Wood, 1980a; Dawson and Krish- talka, 1984; Flynn et al., 1985; Vianey-Liaud, 1985) believed that muroids and glirids ac- quired these features independently and that glirids are not myomorphous. According to Vianey-Liaud, the ““myomorphy”’ of glirids is only “‘pseudo-myomorphy,” derived from a primitive protrogomorphy, in contrast to an ancestral state of histricomorphy for mu- roids. Auditory features provided by Lavocat and Parent (1985) and Bugge (1985) support the grouping of glirids and sciurids, although such a relationship is more dubious. This group is supported in the present analysis (figs. 4-7).
Interestingly, Reithroparamyinae has been grouped with Gliroidea based on the “‘gli- roid” tooth pattern (Hartenberger, 1985). It was also shown that the glirids could have originated from the middle Eocene European Microparamys (Hartenberger, 1971); the lat- ter was included in the subfamily Reithro- paramyinae (Korth, 1984). In addition, rei- throparamyines and glirids share the condition of the proximal internal carotid ar- tery crossing the fenestra rotunda. This may hint at a special relationship of reithropara- myines and glirids. Moreover, because glirids likely acquired their ““myomorphy” from a protrogomorphous condition, it is possible that Reithroparamys represents the ancen- stral morphotype of glirids, retaining protro- gomorphy but sharing some derived char- acters with glirids.
SOME OTHER SELECTED GROUPS OF RODENTS
CASTORIDAE. The castorids are placed among the Sciuromorpha in Simpson’s (1945) classification. Hartenberger (1985) main- tained a similar view, considering the cas- torids and sciurids a monophyletic assem-
1990
blage on the basis of sciuromorphy. A close relationship between castorids and sciurids has been rejected by others (e.g., Schaub, 1953; Wood, 1955). The affinities of casto- rids to other rodents still remain problematic (Wood, 1959; Wahlert, 1977; Bugge, 1985; Lavocat and Parent, 1985). Bugge empha- sized the difference of the cephalic arterial system between castorids and sciurids and believed that a close relationship of these two groups is improbable. As demonstrated by Bugge, castorids retain the medial distal in- ternal carotid artery but lose the stapedial artery while sciurids lose the promontory ar- tery but retain the stapedial artery. Luckett and Hartenberger (1985) argued, citing Wah- lert (1977), that the stapedial artery does oc- cur in some fossil castoroids. However, the existence of the stapedial artery is a primitive character in rodents. In addition, the pathway of the stapedial artery in the castoroid Eu- typomys (Wahlert, 1977) is primitive, cours- ing along the ventral rim of the fenestra ro- tunda, instead of across the fenestra rotunda as in sciurids. Therefore, if castorids share any affinity to sciurids and aplodontids, as suggested by Wilson (1949), they must have diverged well before the origin of reithropara- myines. In other words, castorids and sci- urids, if related in some way, must have a very distant relationship. The presence of the medial distal internal carotid artery in cas- torids may turn out to be a clue for castorid phylogeny. It is more likely that castorids are derived from an ancestral stock giving rise to muroids than to sciurids. Nonetheless, the uncertain position of Castoridae is well re- flected in figures 4-7.
CTENODACTYLOIDEA. The living Ctenodac- tylidae have remained until recently a group of uncertain position relative to other rodents (Simpson, 1945; Dawson et al., 1984; Li et al., 1989). On the other hand, Luckett (1980, 1985) and George (1985) suggested a sister- group relationship of recent ctenodactylids with histricognathi. This relationship is strongly supported by auditory features as shown in figures 4—7, although this grouping may go too far by combining Ctenodactylidae with Caviomorpha as a sister group of Thryonomyidae (figs. 4, 5, and 7). However, I would like to see this as a sign for a more general scheme that Ctenodactylidae are
MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 27
closely related to Hystricognathi as shown in figure 6. As an alternative view, Flynn et al. (1986) suggested that Ctenodactyloidea plus Pedetidae are the sister group of Hystrico- gnathi.
Problems arise when early ctenodactyloids are included. Fossil ctenodactyloids have been extensively described from the Tertiary of Asia (Shevyreva, 1972; Dawson, 1977; Sahni and Srivastava, 1977; Hartenberger, 1977, 1980, 1982; Hussain et al., 1978; Li et al., 1979; Dasheveg, 1982; de Bruijn et al., 1982; Dawson et al., 1984). It has been noted that the early ctenodactyloids are the rodents most similar to the eurymylid Heomys, the mam- mal which is closest to the direct ancestor to the rodents (Li, 1977; Gingerich and Gun- nell, 1979; Chaline and Mein, 1979; Harten- berger, 1980; Dawson et al., 1984; Dawson and Krishtalka, 1984; Korth, 1984; Harten- berger, 1985; Li et al., 1987; Li et al., 1989).
Cocomys, the best preserved ctenodacty- loid, has almost all those primitive character conditions in its ear region, except for the stapedius muscle which was probably en- closed within the tympanic cavity and the facial nerve that is partially ossified (Li et al., 1989). Although theridomorphs also have primitive ear regions, they are not so prim- itive as Cocomys. For example, therido- morphs (Adelomys, for instance) have an in- flated bulla (Lavocat, 1967: pl. 2), whereas Cocomys has only a small elliptical bulla which is completely confined to the ventral side of the skull. Because the bulla is very poorly developed, it is not impossible that the stapedius muscle in Cocomys was partly exposed at the posterior end of the tympanic cavity. Moreover, Cocomys possesses some primitive characters that are either present in the genus alone or retained from ancestral eutherian condition but never occur in other rodents (Li et al. 1989). Among these are two auditory features: large epitympanic wing of petrosal and large pyriform fenestra. Coco- mys has perhaps the most primitive auditory region among rodents. When Cocomys (fam- ily Cocomyidae) and Ctenodactylidae are placed in the same superfamily Ctenodacty- loidea, it becomes difficult to discuss the re- lationship of the superfamily to other ro- dents, because it contains the most primitive as well as some of more derived rodents. Re-
28 AMERICAN MUSEUM NOVITATES
cently, Flynn et al. (1986) excluded Cocomys from Ctenodactyloidea because it is protro- gomorphous and lacks hypolophids. Li et al. (1989) considered that Cocomys shares only a few cranial features (deep pterygoid fossa and palatal process of palatine extending more anteriorly) with later ctenodactylids, such as Tataromys and Ctenodactylus. They there- fore conclude (ibid.) that this may be taken as evidence to support Wilson’s assumption that “recent Ctenodactylidae are still rather much of an incertae sedis.”
In considering the primitiveness of the Co- comys and the close relationship of the Ctenodactylidae with the Hystricognathi, an early dichotomy in rodent phylogeny be- tween Asiatic cocomyids and North Ameri- can-European paramyids (Hartenberger, 1980; Luckett and Hartenberger, 1985) ap- pears to gain support. The cocomyids may have given rise to the later ctenodactyloids in the Asiatic area, from which the Cteno- dactylidae and the Hystricognathi were likely descended. The cocomyid stock also may have given rise to the ischyromyids, the basal stock for some later North American and Eu- ropean groups. Such a dichotomy emerges in figures 4-7, which can be compared to that of Luckett and Hartenberger (1985: fig. 2). Cocomys itself is probably too derived to be the direct ancestor of ischyromyids in at least one respect, the relatively larger infraorbital foramen. Vianey-Liaud (1985) termed this ““pre-hystricomorphy” and believed that my- omorphy is derived from this condition.
ANOMALURIDAE AND PEDETIDAE. These two families are grouped together in this analysis. This is consistent with the result derived from analysis of the arterial pattern (Bugge, 1974b, 1985; George, 1981). Affinities of pedetids and anomalurids are neither supported nor contradicted by fetal membrane data (Luck- ett, 1985). Hartenberger (1980) suggested a possible relationship of Pedetidae and Anomaluridae with Hystricognathi, but re- cently (1985) he provided a theridomyid- anomalurid grouping and placed Pedetidae as incertae sedis. Flynn et al. (1986) placed Pedetidae with Ctenodactyloidea. A close re- lationship of anomalurids and pedetids with Phiomorpha was suggested by Lavocat and Parent (1985), but Wood (1985) held that the
NO. 2972
cheek-tooth patterns of anomalurids and pedetids have nothing in common and that there is no valid basis for assuming any re- lationships between the anomalurids and pedetids on the one hand and the Hystricog- nathi on the other. George (1985) distin- guished a sciuromorph-myomorph clade and a ctenodactylid-hystricognath clade, and sug- gested that anomalurids and pedetids do not associate readily with either of them. Alter- natively, Jaeger (1988) tentatively considered Anomaluridae as a sister group of the Ther- idomyidae. Auditory features support an anomalurid-pedetid sister group, but its re- lationships with other groups vary consid- erably (figs. 4—7).
MuRrRoIpEa. A close relationship of Muroi- dea and Dipodoidea has been suggested on various grounds (Klingener, 1964; Bugge, 1971a; Emry, 1981; Vianey-Liaud, 1985; Flynn et al., 1985; Hartenberger, 1985; Luck- ett, 1985; Sarich, 1985). Due to their prim- itiveness, auditory features contribute little to the discussion of this proposed relation- ship. It has been argued that muroids may be derived from North American sciuravids (Wood, 1959; Korth, 1984). According to Luckett and Hartenberger (1985), however, the unique pattern of apparent “‘absence”’ of the medial internal carotid artery, and the occurrence of a promontory artery in sci- uravids (Wahlert, 1974), would seem to con- tradict an ancestral-descendant relationship between sciuravids and muroids. The prem- ise of this statement is that the classical con- sideration of the internal carotid system is correct (see above). If the internal carotid ar- tery is but a single vessel and if its lateral placement is primitive in rodents, as as- sumed in this paper, then the ancestral-de- scendant relationship between sciuravids and muroids cannot be ruled out by this partic- ular character.
An alternative relationship is one between ctenodactyloids and muroids (Vianey-Liaud, 1985; Flynn et al., 1985). This is based on the observation that early muroids display a change of the infraorbital region from the hystricomorphous pattern to the typical my- omorphous condition (Vianey-Liaud, 1979, 1985). Hystricomorphy (or pre-hystricomor- phy) characterizes early ctenodactyloids.
1990
CONCLUSIONS
Some of the points discussed above are long-standing problems in rodent systemat- ics. Features from the auditory region of R. delicatissimus may offer data to be incorpo- rated in future discussion of rodent phylo- genetics. The salient systematic conclusions may be summarized as follows.
1. ‘““Franimorpha” is not a monophyletic group because no derived character supports such a group. The incipient hystricognathy of “‘Franimorpha” is ambiguous and not widely accepted. Even if this condition exists, it is probably not unique to “‘Franimorpha.” On the other hand, reithroparamyines pos- sess some shared derived characters with sci- urids and aplodontids, such as the meato- cochlear bridge and the internal carotid artery over the fenestra rotundum. In addi- tion, Protoptycus acquired hystricomorphy independently and Cylindrodontidae have an ear region that is derived in a different di- rection. These diverse morphologies indicate that these “franimorph” groups are more likely on different phylogenetic lines. Con- sequently, reithroparamyines can hardly be ancestral to hystricognaths nor can they be included in Hystricognathi.
2. Reithroparamyines are protrogomor- phous rodents that have several derived fea- tures compared to paramyids and other is- chyromyids and share only primitive characters with the latter. Little evidence sup- ports the placement of Reithroparamyinae, as the ancestral stock, in Paramyidae or Is- chyromyidae. Instead, reithroparamyines share derived characters with Sciuridae, Aplodontidae, and Gliridae. Although a re- vised classification is not attempted because the auditory features are only part of the total evidence, these features do reveal certain spe- cialization in reithroparamyines. It is not im- possible that reithroparamyines are included in a clade consisting of more derived groups such as Sciuridae and Aplodontidae, instead of in a grade of primitive protrogomorphs.
3. Finally, it is herein suggested that rei- throparamyines be separated from ischyro- myids and recognized as a family Reithro- paramyidae (Patterson and Wood, 1982), including only the genera and most of the
MENG: AUDITORY REGION OF REITHROPARAMYS DELICATISSIMUS 29
species recognized by Korth (1984). This family may be defined as protrogomorphous rodents with some derived features: P3 re- duced and P4 molariform; incisors more lat- erally compressed, with the anterior surface flat; masseteric fossa bounded by ridges and terminating more anteriorly on the side of the mandible; snout short and tapered; pre- maxillaries extended to or beyond the pos- terior margin of nasals; zygoma progressively moved forward until it is in line with anterior margin of P4; postglenoid foramen reduced; bulla co-ossified with skull, with internal sep- ta formed; bony tube for stapedial artery and facial nerve partially formed; stapedial artery crossing fenestra rotunda; meato-cochlear bridge developed; epitympanic recess ex- panded in the ectotympanic.
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