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The cranial morphology of an early Eocenedidymoconid (Mammalia, Insectivora)Jin Meng a b , Ting Suyin a c & Judith A. Schiebout ca Institute of Vertebrate Paleontology and Paleoanthropology, Academia Sinica,Beijing, PRCb Department of Vertebrate Paleontology, American Museum of Natural History, NewYork, New York, 10024c LSU Museum of Geoscience, Louisiana State University, Baton Rouge, Louisiana,70803

Available online: 24 Aug 2010

To cite this article: Jin Meng, Ting Suyin & Judith A. Schiebout (1995): The cranial morphology of an early Eocenedidymoconid (Mammalia, Insectivora), Journal of Vertebrate Paleontology, 14:4, 534-551

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Journal of Vertebrate Paleontology 14(4):534-551, December 1994 1994 by the Society of Vertebrate Paleontology

THE CRANIAL MORPHOLOGY OF AN EARLY EOCENE DIDYMOCONID(MAMMALIA, INSECTIVORA)

JIN MENGI.2, TING SUYIN'3, and JUDITH A. SCHIEBOUT3I Institute of Vertebrate Paleontology and Paleoanthropology, Academia Sinica, Beijing, PRC

2Department of Vertebrate Paleontology, American Museum of Natural History, New York, New York 100243LSU Museum of Geoscience, Louisiana State University, Baton Rouge, Louisiana 70803

ABSTRACT-An early Eocene didymoconid skull with perfect basicranial region from Hubei Provinceof China is described. Important cranial features include a small orbit, a broad temporal region, amaxilla-frontal contact in the orbit, absence of the jugal-lacrimal contact, a broad dorsal process ofthe alisphenoid, an extensive squamosal, absence of the postglenoid foramen, a transversely elongateglenoid fossa, a fully ossified bulla, an external auditory canal floored by the squamosal and mastoidof the petrosal, a greatly expanded mastoid of the petrosal, and a large foramen for the epitympanicsinus in the tympanic roof. In addition, the internal carotid artery may be reduced, although thestapedial artery is well developed. Analyses ofthe cranial features reveal that this skull is already highlyspecialized. The analyses also indicate that the North American genus Wyolestes and the Asiaticdidymoconids form a paraphyletic group and that derivation of both Wyolestes and didymoconidsfrom a Paleocene Yantanglestes-like mesonychid is not supportable. Instead, on the basis of cranialevidence, a didymoconid-insectivore relationship is suggested.

INTRODUCTIONDidymoconids are a specialized group of eutherian

mammals that lived in Asia from the late Paleoceneto late Oligocene. Specimens of didymoconids werefirst collected by the Central Asiatic Expedition of theAmerican Museum of Natural History during theI920s. Didymoconids were then assigned to the OrderCarnivora (Matthew and Granger, 1924), mainly onthe basis of the dental morphology of the first-de-scribed genus Didymoconus, from the middle Oligo-cene Hsanda Gol Formation ofthe Mongolian People'sRepublic. Since then, didymoconids have been occa-sionally reported from central Asia (Gromova, 1960;Tang and Van, 1976; Zhai, 1978 ; Li etaI., 1979; Zheng,1979; Huang, 1982; Meng, 1990a). All true didymo-conids have a reduced dentition (3.1.3 .2/2.1.3.2), inwhich the canine is carnivore-like but the cheek teethresemble those of several other mammalian groups.For instance, resemblance to Mesonychidae and toLeptictidae was recognized in the earliest study ofdidymoconids (Matthew and Granger, 1924). In itsdetails, however, the tooth pattern of didymoconidsdoes not compare well with that of any other knowneutherian group, and therefore the systematic positionof didymoconids has long been a problem. Previously,the family Didymoconidae has been related to morethan ten mammalian groups, including leptictids, cre-odonts, carnivores, and even lagomorphs (Gingerich,1981; Meng, 1992). A recent hypothesis ofWang (1976)and Gingerich (1981) favors a didymoconid-rneson-ychid relationship. Furthermore, Gingerich (1981)places the North American genus Wyolestes in the Di-dymoconidae, and presumes the derivation of didy-moconids from a primitive Yantanglestes-like meson-ychid in the Paleocene of South China.

534

Cranial anatomy of didymoconids was first knownfrom the Oligocene species Didymoconus rostratus(=Tshelkaria rostrata, Gromova, 1960) and, later, fromD. colgatei (Mellett, 1966). Meng (1992) presents moredetailed cranial morphologies and a systematic studyofdidymoconids. The skull reported in this paper rep-resents the earliest basi cranium of a didymoconid andtherefore provides additional important informationon didymoconid evolution and phylogeny. The spec-imen was collected from an early Eocene locality, Da-jian, at the west rim ofXichuan Basin, Junxian County,Hubei Province of China. The basin lies east of theQin Ling range and straddles the boundary betweenHubei and Henan provinces. The beds that yielded thespecimen are mainly light red and white mudstoneswith some thin beds ofpurple-red or green marls. Oth-er mammalian taxa in the same fauna include Rhom-bomylus sp., Advenimus hupeiensis, Gobiatherium sp.,Asiocoryphodon conicus, A . lophodontus, Mantiodonci.flerowi, Coryphodon flerowi, and cf. Heptodon sp.

We focus primarily on a description of the specimen,analysis ofa few important characters, and a briefdis-cussion of relationships of didymoconids, with partic-ular reference to mesonychids and insectivores. In thedescription, we follow the anatomical terminology pro-vided by McDowell (1958), MacPhee (1981), and No-vacek (1986). A general review of the previous studieson didymoconids is provided by Gingerich (1981) andMeng (1992) and will not be repeated herein.

ABBREVIATrONSInstitutional-IVPP, Institute of Vertebrate Pale-

ontology and Paleoanthropology, Beijing, China.Anatomical-AI., alisphenoid; AI.-Ect. S., alisphen-

oid-ectotympanic suture; Alis. Ca ., alisphenoid canal;

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MENG ET AL.-CRANIUM OF AN EARLY EOCENE DIDYMOCONID 535

Al.-Sq. S., alisphenoid-squamosal suture; As (Ect. Cr.),ectopterygoid crest of alisphenoid; B., bulla; B-occ.,basioccipital; B-sph., basisphenoid; E. A. M., externalauditory meatus; Et. F. , ethmoid foramen; F. A. Pal.,anterior palatine foramen; F. Epi. S., foramen of epi-tympanic sinus; F. Fr., frontal foramen; F. Int., foram-ina presumably for internal carotid nerve and/or ar-tery; F. M. L., middle lacerate foramen; F. M., foramenmagnum; F. M. Pal., middle palatine foramen; F. si.M., fossa for stapedius muscle; F. I. C. A. , foramen ofinternal carotid artery (middle lacerate foramen); F.0 ., foramen ovale; F.? , foramen ofuncertain function;Vas . F., vascular foramina; Fen. R., fenestra rotunda;Fr., frontal; Fr.-Pa. S., frontal-parietal suture; Gl. F.,glenoid fossa; Gr. Eus., groove ofeustachian tube; Gr.Int., groove for internal carotid nerve and/or artery;?Gr., groove with uncertain function; Hy. F., hypo-glossal foramen; In ., incus; J . F., jugular foramen; La.,lacrimal; La. F., lacrimal foramen; M 1, first upper mo-lar (alveolus); M2, second upper molar (alveolus); M.B. W., medial bullarwall; Mal. , malleus; Mas., mastoidofpetrosal; Max., maxilla; Max.-Fr. S., maxilla-frontalsuture; Na., nasal; Oc., occipital; Oc . c., occipital con-dyle; Oc.-Mas. S., occipital-mastoid suture; Op. F. ,optic foramen; Oph, F. , ophthalmic foramen; P3, thirdupper premolar; P4, fourth upper premolar; P. pal. F.,posterior palatine foramen; P. gl. Pr., postglenoid pro-cess ; P. Pro Fr., postorbital process of frontal; Pa ., pa-rietal; Pa.-AI.S., parietal-alisphenoid suture; Pa.-Sq. S.,parietal-squamosal suture; Pal., palatine; PaL-AI. S.,palatine-alisphenoid suture; Pal -Max. S., palatine-maxillary suture; Prom., promontorium; Prs., presphe-noid; Pt. (Ent. Cr.), entopterygoid crest (of pterygoid);Sag. Cr., sagittal crest; S-occ., supraoccipital; S-occ.Exp., dorsal exposure of supraoccipital; Sph. or. F.,sphenorbital fissure ; Sph. pal. F., sphenopalatine fo-ramen; Sq., squamosal; Sq.-AI. S., squamosal-alisphe-noid suture; Sq.-Mas. S., squamosal-alisphenoid su-ture; St ., stapes; St . A. c., stapedial artery canal; Styl.F., stylomastoid foramen; Supr. F. , supraorbital fo-ramen; Z. ProMax., zygomatic process of maxilla.

SYSTEMATIC PALEONTOLOGYOrder INSECfIVORA

Family DIDYMOCONIDAE Kretzoi, 1943Genus HUNANICTIS Li et aI., 1979

?HUNANICTIS sp.MateriaI-A partial skull with well-preserved basi-

cranium, but broken rostrum and teeth (IVPP-V5788).Horizon and Locality - Late early Eocene or early

middle Eocene, Yuhuangding Formation, north of Da-jian village, 4 km northeast of Xijiadian of JunxianCounty, about 20 km northwest of Junxian town, Hu-bei Province, China (IVPP Field Number, 76006).

Comments-Most of the dentition is broken and theidentification of this specimen is primarily based onits cranial morphology, in comparison with other skullsof didymoconids, particularly that of D. berkeyi de-

scribed by Zhai (1978) and Meng (1992). Although D.berkeyi is a late Oligocene didymoconid, its basic cra-nial structure is closely comparable to IVPP-V5788 .However, because subgroups of the Didymoconidaeare differentiated mainly by dental features , we cannotconfidently identify this specimen at the species levelor even at the genus level. IVPP-V5788 may actuallyrepresent a new species or a new genus. We tentativelyassign IVPP-V5788 to Hunanictis because of its smallsize, narrow M2, and small protocone on P3 .

DESCRIPTIONIVPP-V5788 is a small didymoconid skull with most

ofits teeth, the rostrum, and zygomatic arches missing.The rest ofthe skull , particularly the basicranial regionand middle ear, is well preserved. The most distinctiveaspect ofthe skull is its triangular shape, with a narroworbital region and a narrow snout, but a very broadbasicranium characterized by a well-developed bulla,a transversely elongate glenoid fossa, and an inflatedmastoid of the petrosal. Both bullae are perfectly pre-served. The left one was opened during preparation toexamine the middle ear. In dorsal view, the broadestportion ofthe skull is across the squamosal at the levelof the zygomatic processes (measured at 35 mm),whereas the narrowest is immediately posterior to thesupraorbital process (measured at 9 mm). In lateralview, the skull is rather flat, with a small orbit, but anextensive temporal fossa.

Dentition-IVPP-V5788 probably had the samedental formula for the upper tooth row as other generaof the family: 3. 1. 3. 2. The incisors and canines arenot preserved because the rostrum ofthe skull anteriorto P2 is broken. The large root of the left canine isvisible embedded in the maxilla, extending posteriorlyto a position above P3. The cross section ofthe canineabove P2 is oval, measuring 4.3 mm by 2.5 mm. Theroot ofthe canine consists of two layers, a thin opaqueouter layer and a semitransparent inner layer, with asmall oval pulp cavity in the center.

Only one root ofthe left P2 is preserved. It is difficultto ascertain whether it represents a single-rooted toothor the posterior root of a double-rooted tooth. Weinterpret it as a single-rooted tooth with a small crown.P3 is immediately behind P2 and its crown is badlydamaged. Still, it is clear that this tooth is transverselycompressed (3.5 mm long and 2.2 mm broad). Theparacone and metacone are seemingly present but theprotocone, if any, must have been very small. Thecrown of P4 , which is damaged, is transversely wide(2.9 mm long and 3.9 mm broad), and is more or lessY-shaped, indicating presence of the protocone on thelingual side and possibly the paracone and the meta-cone on the buccal side of the tooth. Although thereis no direct evidence from this specimen to prove thatthis tooth is the last premolar rather than a molar,comparison with other didymoconids suggests that thisis the best interpretation. The molars are not preserved,but the alveoli indicate the presence of only two . Mea-

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536 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 14, NO .4, 1994

-:-,....--,,4 --Fr.

1II-~-'-'1a_-F. Fr.

-H!\r-- - - Na.

tH~---=--~f---Sag. Cr.

Pa .---+~-

Max.- --...r......-

Max.-Fr.

Fr.- Pa. S.-~h"'-.---{;:;";

Sq.

FIGURE 1. Stereophotograph (top) and stippling drawing (bottom) of dorsal view of ?Hunanictis sp. (IVPP-V5788). Scale= 5 mm.

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MENG ET AL.-CRANIUM OF AN EARLY EOCENE DIDYMOCONID 537

-

Z. Pro Max. Sph . pal. F.

Oph, F.P. Pr o Fr .

F.

Max. Pal.

As (Eel. Cr.) E. A. M. F.

FIGURE 2. Stereophotograph (top) and stippling drawing (bottom) of left view of?Hunanictis sp . (IVPP-V5788). Scale = 5mm.

surement ofthe alveoli shows that M2 is slightly short-er but wider than M I. The alveoli also shows that thereare two separate roots on the buccal side of M I, re-flecting a separate paracone and metacone, whereas thebuccal side of M2 has only one alveolus, a sign of areduced metacone.

Nasals and Nasal Cavity (Figs. I , 2)-The posteriorportions ofboth nasals are preserved . They are narrowand rectangular, showing no lateral expansion. Theyare asymmetrical, with the right being slightly wider

and longer than the left . At the point where the maxilla-frontal suture joins the nasals, the posterior parts ofthe nasals taper backward, intruding into the anteriorborder ofthe frontals. The nasals and the frontals havea narrow contact on the roof of the skull.

The nasal cavity appears large and deep in relationto the size of the skull. In anterior cross-sectional viewthrough the broken rostrum, the nasal cavity is bound-ed by the nasals dorsally, by the maxilla laterally, andby the palatine ventrally. Medial to the nasal-maxillary

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538 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 14, NO .4, 1994

contact, the lateral margin of the nasal bone bendsventrally and descends downward into the dorsolateralwall of the nasal cavity. The lower part of the nasalcavity is divided bilaterally by a th in , Y-shaped vomer,which extends dorsally to the middle of the nasal cav-ity. Some of the turbinates can be seen on both sidesofthe vomer, but no turbinates are visible in the upperpart ofthe nasal cavity. In right lateral view ofthe face,where the maxilla is badly damaged, turbinates com-posed of very thin bony septa are visible in the nasalcavity between and above P4 and M 1.

Maxilla (Figs. 1-3)-Little of the right maxilla ispreserved, but most ofthe left one is present, with onlyits rostral portion anterior to P2 and the zygomaticprocess missing. In dorsal view, the maxilla broadlycontacts the frontal, in contrast to a narrow nasal anda very small facial exposure of the lacrimal. The max-illa-frontal suture starts at the triple junction of themaxilla, nasal, and frontal, and extends transverselyto the lacrimal tubercle at the anterior rim of the orbit.

In lateral view, the maxilla is the major componentof the facial region. It is penetrated by a short infra-orbital canal that opens anteriorly at the level betweenP3 and P4. The canal is nearly circular, about 1.2 mmin diameter, and continues anteriorly into a broadtrough. Lateral to the infraorbital canal and on thezygomatic base ofthe maxilla, there is a gently concavebut fairly rough area, wh ich is probably for muscleattachment. However, this roughened area is not con-cave enough to permit discrimination of an antorbitalfossa as in some lipotyphlans. Anterodorsal to theroughened area, the maxilla forms the anteroventralborder of the orbit. Although most of the zygomaticportion of the maxilla is broken, what remains of thisbone is nevertheless informative. In dorsal view oftheventral border of the orbit, the maxilla above the in-fraorbital canal is quite thick, and it gradually becomesnarrow toward the zygomatic arch. A groove startingfrom the lacrimal foramen runs along the dorsal borderofthe orbit and fades out distally. The maxilla-lacrimalsuture is clearly seen medial to the groove. Althoughnot preserved, the jugal bone certainly did not extendto the facial region, much less to the lacrimal. Thecross-section at the broken base of the zygomatic pro-cess shows that the maxilla is transversely and un-evenly compressed, with its dorsal part thicker thanthe lower one , indicating that the zygomatic arch shouldbe quite slender.

In the orbital region, the maxilla forms the entire,concave floor ofthe orbit. Behind the posterior openingof the infraorbital canal, which is low in position, thefloor is gently tilted, with its anterior end slightly el-evated. Medially, the maxilla has a small process inthe orbital wall , which contacts the frontal therein andtherefore separates the lacrimal from the palatine. An-terolaterally, the maxilla-frontal suture is continuouswith the maxilla-lacrimal suture that runs along thedorsal rim of the infraorbital canal. Posteroventrally,the maxilla-frontal suture bifurcates into the maxilla-

palatal and the palatine-frontal sutures. The maxilla-palatal suture extends along the lateral border ofa slit-like recess into which open the sphenopalatine foramenand the dorsal openings of the palatine foramina.

In ventral view, most of the left palatal process ofthe maxilla is preserved. The surface of the palatalprocess is uneven and is pierced medial to P3 by twosmall anterior palatine foramina. Ofthe two foramina,the anteromedial one is larger than the posterolateralone, and each of them connects anteriorly to a sharp,narrow groove that engraves the palatal surface. Thepalatine-maxillary suture starts at the posteromedialside of the last molar. It runs along the lingual side ofthe tooth row in a course almost parallel to the midlineof the skull to a point between P4 and Ml and thenturns to the midline of the palate to meet the oppositemaxilla-palatine suture. The anterior portion of thesuture is irregular and coarsely interdigitated.

Palatine (Figs. 2, 3)-In the orbit, the palatine is asmall element, compared to the extensive orbital pro-cess of the frontal. It has a slender anterior process thatextends anterodorsally in the orbital wall to contactthe frontal dorsally and the maxilla anteroventrally.Posteriorly, the palatine has onl y a narrow contact withthe orbitosphenoid, because the frontal descends deep-ly in the orbit between most of the palatine and theorbitosphenoid. Within the anterior process ofthe pal-atine and on the medial border of the orbital floor,there exists the distinct slit-like recess already men-tioned.

In ventral view, only the left palatal process of thepalatine is complete. It is roughly rectangular, with itsanterolateral corner rounded. If the right palatal pro-cess were preserved, the two palatal processes wouldhave made a tongue-shaped plate. Like the palatal pro-cess ofthe maxilla, the surface ofthe left palatal processof the palatine is also uneven and pitted. Near theanterior border of the palatine at the level between P4and M 1, there is a small middle palatine foramen,which is continuous anteriorly with a short groove. Atits posterior border, the palatal process forms a bluntpostpalatine torus. A distinct posterior palatine fora-men pierces the postpalatine torus just posteromedialto the last molar, and opens anteriorly into a broadtrough on the palate. Since its ventral wall is incom-plete, this foramen is actually a notch. The position ofthe postpalatine foramen and the way it penetrates thepalatine is similar to the situation in hedgehogs butdifferent from that of leptictids. In the latter, the pos-terior palatine foramen opens ventrally. Below thechoanal orifice, the postpalatine torus bears a reverseW-shaped emargination, with a blunt central postpa-latine spine projecting posteriorly. The palatine con-tinues posteriorly as a narrow, long process, stretchingto the anterior base of the ectopterygoid process of thealisphenoid.

Lacrimal (Fig. 2)-The lacrimal bone is preservedonly on the left side of the skull. It is small, with amajor orbital plate and a minor facial process. The

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MENG ET AL.-CRANIUM OF AN EARLY EOCENE DIDYMOCONID 539

F. A. P:II.- - - - -f;-

Pa I. -M ax, S .---+77?j77'7~t:::r"\..,

F. M. P.t1 .--~~~@~~.Pal.---~~'if77*~

PaL-A I. S.- -----.

PI. (EnI. Cr.) - - ---,E'!f:--::--7\ '\]'

As (Eel. Cr.)_---,~

P. gl. Pr.:--~--:--

Sq.-Ma s. S._~.,.:,.:,.. :~

S\,YI. F.- - -t';+'-""":':'''''''''-''A\'

Mas.----~t-~

~""'--P3

--,,'~;

540 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL . 14, NO.4, 1994

facial process is actually a very narrow band, ofwhichthe medial part is represented by a fairly strong lac-rimal tubercle. The latter overhangs a large, circularlacrimal foramen that appears between the maxilla andthe lacrimal. Most ofthe lacrimal plate is on the medialside ofthe foramen. In the orbit, the lacrimal is bound-ed by the lacrimal-maxillary suture anteriorly and ven-trally, and by the lacrimal-frontal suture posteriorly.The lacrimal does not contribute to the dorsal wall atthe posterior opening of the infraorbital canal. Ventralto the lacrimal foramen, the lacrimal has a short, sharpprocess that extends laterally and terminates imme-diately at the ventrolateral side of the lacrimal fora-men. Medial to the tubercle, a vertical trough leadsfrom the lacrimal foramen dorsally to the anterior rimof the orbit, possibly indicating another lacrimal fo-ramen therein.

Frontal (Figs. 1, 2)-The left frontal bone is com-plete, whereas the right one is only partially preserved.Both bones are very short, with a maximum length of8.5 mm, only about one-third of the length of the pa-rietal. Its anterior border is at the level of the anterioropening of the infraorbital canal, and its posterior bor-der is immediately behind the postorbital processes.In dorsal view, the frontals are butterfly-shaped. An-teriorly, the two broad "wings" contact the maxillaebroadly and the nasals narrowly. The surfaces of thefrontals are flat and contain three small foramina, twoin the left and one in the right. The two foramina inthe left frontal are closely spaced, with the lateral onebeing much smaller than its neighbor. A blunt post-orbital process is developed in the frontal to rim theorbit dorsally. Posterior to the postorbital process, thefrontal-parietal suture runs transversely across the nar-rowest region of the skull roof.

In lateral view, the frontal extends more than half-way down into the orbit and occupies most of theorbital wall. It meets the lacrimal anteriorly, the max-illa and palatine ventrally, and the orbitosphenoid andalisphenoid posteriorly. The alisphenoid-frontal con-tact is much narrower than the orbitosphenoid-frontalcontact. In the orbital wall, the frontal has a ventralprocess intruding deeply between the palatine and theorbitosphenoid. The end ofthis process reaches almostto the bottom of the orbit medial to the slit-like recessin the palatine. Dorsally, the blunt supraorbital crestruns from the lacrimal tubercle and ends as the post-orbital process anterior to the frontal-parietal suture.Beneath the postorbital process, there is a well-defined,nearly circular supraorbital foramen. Near the frontal-orbitosphenoid suture, there are two other small fo-ramina. One of these is the oval, ventrally-directedethmoid foramen, and the other is the semilunar-shapedophthalmic foramen that opens posterolaterally. Thelatter lies anteroventral to the former and both arewithin the frontal.

Parietal (Figs. 1, 2)-The parietal bones are com-pletely preserved, although their surfaces are fractured.They measure about 25 mm long and are the longest

elements of the skull. The anterior portion of the pa-rietal bone marks the narrowest region ofthe preservedskull, forming a "neck" posterior to the frontal-parietalsuture. The parietals gradually widen posteriorly to thepoint where they are overlapped by the squamosals;then they taper back to the end ofthe skull. The sagittalcrest begins on the parietal near the frontal-parietalsuture and continues posteriorly to the end ofthe skull.It is initially weak but becomes more prominent pos-teriorly. The parietals are primarily confined to theskull roof, and therefore the parietal-alisphenoid sutureis partially visible even in dorsal view ofthe skull. Thewedge-shaped posterior parts of the parietals overlapthe dorsal exposure of the supraoccipital and dividethe latter into two triangular portions. In the broadtemporal region of the skull , there is a set of sevenvascular foramina ofdifferent sizes on each side of thesagittal crest. Of these foramina, the anterior two onthe left side and the anterior one on the right are withinthe parietal, whereas others are either at the parietal-supraoccipital suture or within the squamosal. Theseforamina are comparable with those in Eoryctes(Thewissen and Gingerich, 1989), although differencesexist. The foramina in IVPP-V5788 appear to followa curved line in their distribution within the parietaland the squamosal, whereas those in Eoryctes seem tobe scattered randomly within the parietal only. Theseforamina must have held arteries or veins, or both.They are apparently related to the enlarged temporalregion, to which considerable musculature must haveattached.

Squamosal (Figs. 1-5)-The squamosals are the larg-est bones of the skull. They form a major part of theexpanded temporal region and contribute largely to thebasicranium. They also form most of the lambdoidalcrest, while the supraoccipital contributes only to thecentral portion of the crest. In dorsal view, the squa-mosal is bounded anteromedially by the squamosal-parietal suture and posteromedially by the squamosal-supraoccipital suture. Postmortem displacement con-siderably widened the squamosal-parietal sutures. Thesurface ofthe squamosal is smooth and gently convex,and it slopes anteriorly down to the anterior edge ofthe glenoid fossa . The posteromedial comer of thesquamosal on each side of the skull is penetrated bythe scatter of vascular foramina just mentioned; themost lateral one of these foramina is the largest. Thisforamen opens posterolaterally and must have trans-mitted a major artery or vein. The posterior edge ofthe squamosal forms a sharp, arched lambdoidal crest.The squamosal stretches laterally and narrows to thezygomatic process that marks the lateral extremity ofthe skull . This process is well preserved on the rightside but is missing on the left. Judging from the crosssection through the zygomatic base of the squamosal,the zygomatic arch must have been slender.

In ventral view, the squamosal-alisphenoid suturecan be traced on the lateral side of the ectopterygoidprocess. This suture terminates posteriorly at the an-

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MENG ET AL.-CRANIUM OF AN EARLY EOCENE DIDYMOCONID 541

terolateral corner of the bulla, where the squamosal-ectotympanic suture continues backward to the sty-lomastoid foramen. Although large, the squamosal doesnot contribute to the bullar wall. On the anterolateralside ofthe bulla, the transversely elongate glenoid fossais developed within the squamosal. The anterior edgeofthe fossa is well defined and slightly arched forward.The articular surface is smooth and faces anteroven-trally, with a semicircular outline in lateral view. Theglenoid fossa measures 7 mm long transversely and 4mm broad anteroposteriorly. A broad postglenoid pro-cess , roughly triangular in posterior view, bounds theglenoid fossa posteriorly. A unique feature in this re-gion is that the postglenoid process extends posteriorly,to meet the inflated mastoid of the petrosal, and me-dially, to join the ectotympanic. This connection fullyfloors the external auditory canal. There is no post-glenoid foramen, although a small blind pit is at thesquamosal-mastoid suture.

In lateral view, the squamosal forms the dorsal, an-terior, and ventral walls ofthe external auditory canal,whereas the mastoid of the petrosal encloses the ex-ternal auditory canal posteriorly. On the right side ofthe skull, the external auditory meatus is complete,showing an oval outline (about 5 mm by 2.6 mm indiameter). Between the anterior wall of the externalauditory meatus and the glenoid fossa, the squamosalis hollowed. This space is exposed at the left squamosalwhere this region was broken. A similar condition isalso observed in other didymoconid skulls (Meng,1992).

Sphenoid (Fig. 2)-In ventral view, the presphenoidis exposed in the arched roof of the internal nares. Itis a narrow, pointed element, confined by the ptery-goids laterally and the basisphenoid posteriorly. An-terior to the tip of the presphenoid, there is a ridgeoverhanging the roof of the internal nares. Becausesutures in this region are unclear, it is impossible todetermine which bone contributes to that ridge. Thebasisphenoid is trapezoidal and is separated from thepresphenoid by a short, transversely-oriented suture.It is laterally bounded by the alisphenoid and the bulla,and posteriorly by the basioccipital. The basisphenoid-basioccipital suture is positioned at a level posteriorto the anterior edge of the bulla.

Alisphenoid (Figs. 2, 3, 5)-Both ali sphenoids arepreserved. The alisphenoid is a complex element thatcontributes to several regions of the skull. In lateralview, the alisphenoid has an extremely large dorsalprocess and forms a considerable part of the side wallof the braincase. The process is wide at its dorsal endand gradually becomes narrow at its ventral end. Thedorsal end of the process almost reaches the roof ofthe skull to meet the parietal, so that the straight pa-rietal-alisphenoid suture is visible even in dorsal viewofthe skull. Anteriorly from top to bottom, the alisphe-noid meets the frontal, orbitosphenoid, and palatine.Ofthe three, the frontal-alisphenoid contact is the nar-rowest, and the orbito-alisphenoid one is the widest.

A large sphenorbital fissure opens anteriorly betweenthe orbitosphenoid and the alisphenoid. At the posi-tion ofthe sphenorbital fissure , the anterior edge ofthealisphenoid curves posteriorly. The alisphenoid joinsthe palatine at the bottom inside the sphenorbital fis-sure. The palatine-alisphenoid suture takes a curvedcourse in front of the sphenorbital fissure and is con-tinuous posteriorly with the medial side ofthe ectopte-rygoid process on the ventral side of the skull. Poste-riorly, the alisphenoid has a broad contact with thesquamosal. Most of the alisphenoid-squamosal sutureis nearly vertical in the temporal region, while its ven-tral portion turns posteriorly at a right angle and cours-es along the lateral side of the ectopterygoid crest tothe tympanic bulla. Because it is overlapped posteri-orly by the squamosal, the alisphenoid may have con-tributed to the braincase more than is actually ob-served.

In ventral view, the anterior extremity ofthe alisphe-noid extends to the mid-point on the lateral side of theposterior process ofthe palatine. No suture is observedbetween the alisphenoid and the pterygoid. Lateral tothe ectopterygoid, the alisphenoid-squamosal suturecourses along the ridge that marks the border betweenthe side wall and the floor of the braincase in the tem-poral region. Anterior to the bulla lie a few structuresin the alisphenoid. Lateral to the hamular process ofthe pterygoid, the alisphenoid forms a broad, gentlyconvex area, on which the ectopterygoid process of thealisphenoid is developed. This process is transverselycompressed, with a broad base and a sharp tip thatpoints lateroventrally. The base of the ectopterygoidprocess is continuous posterodorsally, with a slopingarea immediately in front ofthe bulla. In this area thereis a large recess, containing two foramina: the small,anterior one is presumably the posterior opening ofthealisphenoid canal, and the large, posterior one is theforamen ovale. Medial to this recess, a shallow butbroad trough extends from the opening of the eusta-chian tube to the posterior base ofthe hamular process.Further medial to the trough, there are two small fo-ramina and an oval depression, which may be relatedto auditory features and will be described below.

Orbitosphenoid (Fig. 2)-The orbitosphenoid is anoval bone surrounded by the frontal anteriorly, thealisphenoid posteriorly, and the palatine anteroven-trally. Its center is penetrated by a small optic foramen,from which a groove leads anterodorsally. Two smallforamina anterodorsal to the orbitosphenoid are theethmoid foramen and the ophthalmic foramen, de-scribed above. Posteroventral to the optic foramen,there is a large sphenorbital fissure. In anterior viewof the sphenorbital fissure (invisible in the lateral viewof Fig. 2), there are three separate foramina in thefissure. The most anteromedial one is the anterior lac-erate foramen (sphenorbital foramen); the middle oneis the foramen rotundum; the most posterolateral oneis presumably the anterior opening of the alisphenoidcanal. It seems that the alisphenoid canal is quite long,

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ifour identification ofthe anterior and posterior open-ings of the canal is correct.

Pterygoid (Fig. 3)-The pterygoid is preserved oneach side of the skull, but with the hamular processbroken on the right. The pterygoid fuses with the ali-sphenoid but is still separate from the palatine and thepresphenoid. It contributes to the internal wall of theposterior nasal passage and joins the presphenoid onthe roof of the internal naris. The hamular process istransversely compressed and slightly leans to the in-ternal naris. The ectopterygoid process of the alisphe-noid lies lateral to the pterygoid hamulus.

Occipital (Figs. I, 3-5)-The right condylar regionis damaged; the occipital bone is otherwise perfectlypreserved. In occipital view, the supraoccipital abovethe foramen magnum is roughly pentangular in shape.It is very thick, with a slightly concave central area. Awell-developed dorsal process of the supraoccipital isexposed on the roof of the skull. The dorsal process isdivided by the parietal into two unequal triangularportions: the right one is substantially larger than theleft one . Below the supraoccipital, the exoccipital isonly a narrow band on the lateral side of the foramenmagnum, contrasting sharply with the broad mastoidofthe petrosal. The occipital-mastoid suture meandersfrom the lambdoidal crest downward to the lateral sideof the occipital condyle. On two sides of the suture,the thick occipital and the inflated mastoid are ele-vated, so that a broad valley is formed along the courseof the suture. The occipital condyle is small whereasthe oval foramen magnum is fairly large.

In ventral view, the basioccipital is bounded by thebasisphenoid anteriorly and by the bulla and the mas -toid of the petrosal laterally. The basioccipital gradu-ally broadens towards the posterior. The surface of thebasioccipital bears a weak longitudinal keel. The oc-cipital condyle is flat and has an anterior border thatcurves slightly posteriorly. Immediately anterior to thecondyle there is a single large hypoglossal foramen.The large jugular foramen rests anterolateral to thehypoglossal foramen and between the occipital and thepetrosal.

Mastoid of the Petrosal (Figs. 2-5)-As in other di-dymoconids, the mastoid of the petrosal in IVPP-V5788 is very prominent and is therefore describedseparately from the rest of the petrosal. In the basi-cranial region , the mastoid forms a roughly semicir-cular area, bounded by the squamosal anterolaterallyand the bulla anteromedially. The arched edge of thissemicircular area is ridged and forms the posteriorborder ofthe basicranium. The squamosal-mastoid su-ture behind the postglenoid process is transverselystraight and parallel to the long axis of the glenoidfossa . The suture between the mastoid and the bullais irregular. It starts from the triple junction of thesquamosal, the mastoid and the ectotympanic in frontof the stylomastoid foramen, runs posteromediallyalong the anteromedial border of the stylomastoid fo-ramen, and terminates at the medial side of the jugular

foramen. Therefore, the mastoid, although greatly ex-panded, does not intrude into the bullar wall. The ec-totympanic extends only very narrowly into the medialwall of the stylomastoid foramen, and the foramen isprimarily enclosed by the mastoid ofthe petrosal. Lat-eral to the stylomastoid foramen, the surface of themastoid is concave, whereas medial to the foramen, itis convex. Externally, the bullar wall gradually mergeswith the convex part ofthe mastoid, but internally, thetympanic cavity does not extend into the convex partof the mastoid. There is a tiny foramen of uncertainfunction at the medial margin of the left mastoid, butthere is no corresponding foramen in the right.

In occipital view, the mastoid is extremely inflated,forming a nearly vertical, arched surface. The mastoidon each side of the skull occupies more than a thirdof the total width across the occipital region. Dorsally,it meets the squamosal to form a sharp lambdoidalcrest and expands medially to compress the exoccip-ital.

Tympanic Bulla (Figs. 3, 5)-The bulla is fully os-sified and is bean-shaped, with its long axis inclinedanteromedially. Its lateral side is slightly flattened,whereas the medial part is smoothly rounded. Its an-terior edge aligns with the level of the transverse axisof the glenoid fossa, so that the anterior one-third ofthe bulla is medial to the glenoid fossa. The bulla com-pletely encloses the tympanic cavity and is boundedby the alisphenoid anteriorly, the squamosal laterally,the mastoid posteriorly, and the basisphenoid and ba-sioccipital medially. Because the suture between thebulla and the surrounding bones can be clearly traced,it is certain that none of the latter elements makes anycontribution to the formation of the bulla. No sutureis detectable on the ventral surface of the bulla; how-ever, at the anterolateral corner inside the opened leftbulla, a suture runs along the medial base of the tym-panic ring and turns medially at the anterior foot ofthe tympanic ring to the opening of the epitympanicsinus. This structure, however, is not visible in ventralview of the tympanic region ; therefore, it is not illus-trated in Figure 5. We interpret this suture as the en-totympanic-ectotympanic suture and therefore con-sider the bulla to be bipartite. The ectotympanic issmaller than the entotympanic and forms only the lat-eral wall of the bulla. The lateral wall inclines at anangle about 60 to the frontal plane of the skull, cor-responding to that of the tympanic ring. In contrast,the entotympanic is the major element of the bulla,forming the medial and ventral bullar walls.

In front of the bulla, that is immediately posteriorto the the foramen ovale but separated from it by abony lamina, is a large opening for the eustachian tube.Medial to this opening is a small recess containing twoforamina. These two foramina connect posteriorly witha distinct groove on the medial tympanic roof withinthe tympanic cavity. The positions ofthese two foram-ina indicate that they probably conveyed the internalcarotid nerve and/or the internal carotid artery, al-

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MENG ET AL.-CRANIUM OF AN EARLY EOCENE DlDYMOCONlD 543

-

Sq.-Mas . S. B. Oc. C. .I. F. P. gI. Pr.

FIGURE 4. Stereophotograph (top) and stippling drawing (bottom) of occipital view of?Hunanictis sp , (IVPP-V5788). Scale= 5 mm.

though they appear quite small for an artery. At themost medial side and in front of the anteromedial apexof the bulla, there is an oval, shallow depression. An-terior to the depression, a tiny foramen of uncertainfunction penetrates the alisphenoid. The oval depres-sion itselfcontinues posteriorly as a foramen that com-municates with the braincase. This foramen is iden-tified in its position as the middle lacerate foramen. Itis largely blocked in ventral view by the bulla. On themedial side of the bulla, a broad groove starting fromthe jugular foramen extends anteriorly to the oval de-pression. This groove may also have contained a majorblood vessel (see Discussion). At the posteromedialcorner of the bulla, the carotid foramen occurs an-teroventral to the large jugular foramen, and the path-way for the internal carotid artery is sandwiched be-tween the bullar wall ventrally and the petrosal dorsally.

Tympanic Cavity (Fig. 5)-AII osseous structures inthe inflated tympanic cavity, including three delicateear ossicles in their original position, are nearly per-fectly preserved. On the medial-most side of the tym-panic cavity, the medial wall ofthe entotympanic turnslaterally to overlap the ventromedial surface of thepromontorium and to floor the posteromedial part ofthe fossa for the stapedius muscle. The promontoriumis the most prominent landmark in the tympanic cav-ity. Its anterior half is bullet-like and points antero-laterally. The posterior halfof the promontorium bendsslightly laterally so that the promontorium is some-what saddleshaped. At the posterior end of the pro-montorium a very large , circular fenestra rotunda opensposteroventrolaterally, through which the basal part ofthe osseous spiral lamina of the cochlea can be seen .Along the posterior edge ofthe promontorium the bony

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544 JO URNAL OF VE R TEBRATE PALEONTOLOGY, VOL. 14. N O. 4. 1994

F. O.Alis . Ca.

Gr .

F.

F? - - -+'-.#

F. M . L.

Prom.------\i'++_'_'_?G r.

M . B. W.-------l'--->.rt'~

.I. F.

By. F.

S.Mal.

Pro

In.

Styl. F.

FIGURE 5. Stereophotograph (to p) and stippling drawing (bo tto m) of the left bas icranial and audito ry regions of?Hunanictissp. (lVPP-V5788) . The manubrium of the malleus was removed in the stereophotograph and was rest ored in the d rawing.Scale = 3 mm.

tube for the internal carotid artery, confined by th epetrosal dorsally and the entoty m panic ventra lly, runslaterally from the carotid foramen to the ventral sideof th e fenestra ovalis and then gives rise to the tube

for the stapedial artery and the groove presumably forthe promontory artery. The stapedial tube is ent ire lyem bedde d in the petrosal. After passing through th estapedial foramen ofth e stapes , the stapedial tube joins

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MENG ET AL.-CRANIUM OF AN EARLY EOCENE DIDYMOCONID 545

the petrosal at the dorsolateral rim ofthe fenestra oval-is, on the medial side of the incus, and can not betraced further in the tympanic cavity. The narrow butvery distinct promontory groove extends forward, firston the ventromedial surface of the promontorium,and then on the anterior tympanic roof formed by abroad wing of the petrosal. It exits the tympanic cavityat the anteromedial corner and terminates at the pos-terior side of the small recess containing the two smallforamina mentioned above. This groove may conveyeither the internal carotid artery or the internal carotidnerve, or both, although it seems too narrow to containa functioning blood vessel (see Discussion).

The fenestra ovalis is located at the lateral side ofthe promontorium. It is a large, elongate opening, andis enclosed fully by the footplate ofthe stapes. The longaxis of the fenestra ovalis is slightly inclined with re-spect to the horizontal plane of the skull. Posterior tothe promontorium, the fossa for the stapedius muscleis deeply socketed at the posterior end of the tympaniccavity and is partially floored by the entotympanic.There is no sign of the facial nerve in the tympaniccavity. The facial nerve was obviously enclosed in abony tube that runs within the petrosal dorsal to thestapes and laterally to the fossa for the stapedius mus-cle. It exited the tympanic cavity through the well-defined stylomastoid foramen.

The epitympanic recess containing the incudo-rnal-leolar articulation is very large and deep. The recess isformed medially and principally by the petrosal andlaterally by the squamosal. The head of the malleusand the body of the incus are entirely enclosed withinthis recess . Lateral to the epitympanic recess is the largeinternal opening of the external auditory canal. Thetympanic ring, not visible in the illustration, is a horse-shoe-shaped ridge on the inner surface of the lateralbullar wall. The ring has a maximum diameter of 7mm and inclines at an angle ofabout 60. The epitym-panic recess occurs just at the open portion of thetympanic ring.

Another prominent feature is a large, roughly ovalopening anterolateral to the promontorium. This open-ing, similar in position to a piriform fenestra, is theforamen by which the tympanic cavity communicateswith the epitympanic sinus. The epitympanic sinus liesdorsal to the tympanic cavity, and is seen through theforamen filled with matrix. The foramen has a narrowposterior "tail" between the promontorium and thethe epitympanic recess , and a broad body that reachesto the anterior wall of the tympanic cavity. All edgesof the foramen are well-defined and smoothly curved.Because of this very large epitympanic foramen, theanterolateral tympanic roof is largely hollow. There-fore , this area was probably not the area for attachmentofthe tensor tympani muscle, suggesting that the tensortympani muscle might have either been lost or greatlyreduced. A reduced tensor tympani muscle, if present,may have originated in a small fossa laterodorsal tothe promontorium.

Ear Ossicles (Fig. 5)-Three auditory ossicles arepreserved in their original position, except that themanubrium of the malleus may be slightly displaced.The malleus is the largest bone of the three. The headof the malleus is slightly convex and about 1.3 mm inwidth measured transversely. It lies anterior to theincus in the anterior part of the epitympanic recess .The malleus has a convex articular facet for the incus,underneath which is the narrow neck. A round processextending directly forward from the neck probably isthe reduced anterior process. Opposite to the anteriorprocess is the muscular process, which is round andabout the same size as the anterior process. The ma-nubrium of the malleus is well-preserved. It is verythin and about 3.5 mm in length. In the stereophotosof the tympanic cavity, the manubrium is removed.When it is restored, it looks as illustrated in the drawing(Fig. 5). Both anterior and posterior surfaces of themanubrium are concave along its length, whereas itsventral side, that attaches to the tympanic membranein life, is flat. The ventral side widens toward the distalend and its tip slightly bends ventrally.

The incus is roughly spoonshaped and is smallerthan the malleus. The articular surface of the malleusis slightly concave. The short crus was embedded dor-sally in the fossa incudis ofthe epitympanic recess andlittle can be seen in ventral view. The long crus, mea-suring about 0.6 mm, is directed posteriorly and hasa processus lenticularis at its tip that articulates withthe stapes.

The stapes is stirrup-like, with a large stapedial fo-ramen. It lies roughly in a horizontal plane, with thestapedial foramen directed dorsoventrally. The headof the stapes is narrow and is articulated with the evennarrower processus lenticularis of the incus. The pos-terior crura are about 1 mm in diameter. The foot plateis invisible, but judging from the shape of the fenestraovalis, it must be elongate.

ANATOMICAL DISCUSSIONAlthough it is the earliest known cranium of a di-

dymoconid, IVPP-V5788 already displays many spe-cialized features that are commonly seen in later forms.Several cranial features that are of special significancein didymoconid anatomy and are particularly well-preserved in IVPP-V5788 are discussed below. A morethorough study of this kind has been attempted byMeng (unpubl. data).

Auditory Meatus - Klaauw (1931) distinguished twotypes of external auditory meatus: 1) false external au-ditory meatus; and 2) true bony cylindrical auditorymeatus. According to Klaauw, a true bony cylindricalexternal meatus is one in which the external auditorytube or canal is an extension ofthe element that formsthe entire bulla or forms at least the lateral part of thebulla. A false external auditory meatus is recognizedin cases where the posttympanic process ofthe petrosalcontacts the postglenoid process beneath the true ex-

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ternal auditory meatus. The posttympanic process ofthe petrosal and the postglenoid process, however, donot participate in the formation of the bulla, but in-stead form a passage lateral to the recess ofthe externalauditory meatus, such as in Smilodon (Cope, 1880).

The external auditory canal in IVPP-V5788 is formedby a broad squamosal extension that develops fromthe postglenoid process and contacts the mastoid pos-teriorly. The same structure is present in advanceddidymoconids, except that, in the advanced forms, thesquamosal completely fuses to the ectotympanic me-dially (Meng, 1992). Accordingly, the external auditorymeatus of didymoconids, by Klaauw's definition, is afalse external auditory meatus. This feature is unlikethat of Smilodon, in which the floor of the externalauditory canal is incomplete and is formed primarilyby an anterior process of the mastoid of the petrosal.A similar condition has not been reported in any othermammals known to us and is most likely a didymo-conid autapomorphy.

Foramen of the Tympanic Sinus-Initially, this fo-ramen was thought to be similar in position to thepiriform (pyriform) fenestra of some other mammals(McDowell, 1958; MacPhee, 1981) , which is a residualgap resulting from slow development of bones thatcontribute to the tympanic roof with respect to thedevelopment of the whole basicranium. Through thepiriform fenestra, the tympanic cavity communicateswith the braincase. The piriform fenestra has been sug-gested to be a trait characteristic of lipotyphlan insec-tivores (McKenna et aI., 1984) . Whether the openingin IVPP-V5788 communicates with the braincase isuncertain, but in other didymoconids this foramen leadsinto a large epitympanic sinus, a space between thebraincase and the tympanic cavity, and the epitym-panic sinus is separated from the braincase by the pe-trosal and parietal (Meng, 1992). We assume that asimilar constitution ofthe epitympanic sinus is presentin IVPP-V5788.

Reduction of the Postglenoid Foramen - Reductionof the postglenoid foramen is not rare in therians. Itoccurs in hyracoids, tubulidentates, perissodactyls, si-renians, and some carnivores (Novacek, 1986). Thisfeature may have evolved more than once in mam-mals. Commonly, the postglenoid foramen is reducedwithout modification of the postglenoid process. Re-duction of the postglenoid foramen in didymoconidsseems to have resulted from the development of thefalse external auditory canal. Because the bony laminathat stretches from the postglenoid process to join themastoid of the petrosal posteriorly forms the floor ofthe external auditory meatus, the postglenoid foramen,if present, would have penetrated the floor and ap-peared within the canal.

Orbital Process of the Maxilla - In regard to thebony mosaic of the orbital region, attention has beenfocused on the relationships of the maxilla, palatineand lacrimal. Their relative sizes and positions havebeen discussed by many workers (Gregory, 1920; Mul-

ler , 1934 ; Butler, 1956; Van Valen, 1965; Cartmill,1975; Novacek, 1980). The general conclusion is thata large orbital process of the palatine that contacts thefrontal and lacrimal in the orbital wall , as in didelphidmarsupials , is primitive; or in other words, the maxillacontributes little to the orbital wall and is completelyexcluded from the frontal because of the palatine-lac-rimal contact. In contrast, a large process of the max-illa, protruding into the orbital wall , contacting thefrontal and displacing the palatine in the orbital wall ,is widely believed to be a derived condition. The lattercondition, associated with a greatly reduced palatinein the orbital region, is regarded as an important fea-ture typical ofLipotyphla (McDowell, 1958; Novacek,1980 , 1986; Butler, 1988 ; MacPhee and Novacek,1993).

An affinity between leptictids and lipotyphlan in-sectivores was suggested by Novacek (1986). One ofthe supporting characters for this relationship is that,in leptictids, the maxillary intrudes into the orbit andnearly touches the frontal therein. The leptictid con-dition is certainly far from that of the lipotyphlans. Indidymoconids, however, the maxilla intrudes the orbitand has considerable contact with the frontal , and thepalatine is excluded from contacting the lacrimal. Inlater didymoconids, particularly in Didymoconus, theorbital process of the maxilla becomes more extensive(Meng, 1992). In this particular feature, didymoconids,even the early forms such as that represented by IVPP-V5788, have a condition much closer to that of thelipotyphlan insectivores than do leptictids.

J ugal-The transition of the jugal in mammals hasbeen discussed by several authors (Hogben, 1919; Mul-ler, 1934; Butler, 1956; McDowell, 1958; Novacek,1980). In general, the jugal in therians has undergonea gradual reduction. A large and high jugal that is usu -ally transversely compressed and that contacts the lac-rimal anteriorly, is a primitive condition in therians.In addition, the jugal may primitively have also hada wide contact anteriorly with the maxilla and poster-odorsally with the squamosal. It may even extend pos-teriorly into the glenoid fossa as in marsupials. In con-trast, the derived conditions of the jugal includereduction in size, both in length and height, and lossof contact with the lacrimal.

Although the jugal and the zygomatic process ofthemaxilla in IVPP-V5788 were broken, it is clear thatthe jugal neither extended to the facial region nor con-tacted the lacrimal. Many didymoconid specimens withbroken zygomatic arches display an identical condi-tion. In known didymoconids, the jugal is preservedonly in D . rostratus (Gromova , 1960; Meng, 1992). Asin some insectivores such as Erinaceidae and Talpidae,in which a jugal persists, the jugal in D. rostratus isreduced to a slender element that is confined to thecentral portion ofthe zygomatic arch. The anterior andposterior bases of the zygomatic arch are developedexclusively from the maxilla and the squamosal, re-spectively. The lower border ofthe orbit is thus formed

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entirely by the zygomatic process of the maxilla, atypical lipotyphlan condition as outlined by Butler(1988). Reduction of the jugal is here regarded as oneofthe features that suggest a didymoconid-insectivorerelationship.

Alisphenoid - The alisphenoid is small in primitivemammals such as monotremes, marsupials, and Man-is. In those forms, the alisphenoid is either confinedto the ventral side of the skull or remains low in theside wall ofthe braincase. In general, a relatively smallalisphenoid in the orbit is a primitive condition inmammals. In contrast, the alisphenoid with an ex-panded dorsal process, such as that of leptictids, isbelieved to be derived in mammals (Novacek, 1986).The alisphenoid of IVPP-V5788 unquestionably dis-plays a derived condition, since it is even more exten-sive than that of leptictids.

Internal Carotid Artery-The internal carotid sys-tem has been used to reconstruct phylogenies ofmam-mals at various levels by many authors (e.g., McKenna,1966; Lavocat, 1967; Bugge, 1974, 1978; Szalay, 1972,1975; Lavocat and Parent, 1985; Cartmill andMacPhee, 1980; MacPhee, 1981; Wible, 1986, 1987;Wible and Novacek, 1988; Meng, 1990b). It was orig-inally believed that the internal carotid artery in euth-erians primitively consisted of two main trunks; themedial trunk ran medial to the tympanic cavity andthe lateral one coursed along the ventral surface of thepromontorium (Matthew, 1909; Van Valen, 1965 ,1966; McKenna, 1966; Szalay, 1975; Archibald, 1977;Parent, 1980). The stapedial artery branches off thelateral trunk. A recent view is that the internal carotidartery in living mammals has only one major trunkthat may vary in position in the auditory region, de-pending on the development of the artery and the sur-rounding bony elements (Presley, 1979; Cartmill andMacPhee, 1980 ; MacPhee, 1981; Wible, 1984, 1986).

We accept the one-trunk hypothesis, for it seemsbetter substantiated. Nonetheless, we encountered somedifficulties in interpreting the internal carotid systemin IVPP-V5788 using this hypothesis. In IVPP-V5788a stapedial artery is surely present in the auditory re-gion, as it is in all other didymoconid specimens. Un-like the other didymoconids, such as D. berkeyi andD. colgatei, in which adequate evidence of the prom-ontory artery is not present, IVPP-V5788 exhibits anarrow but distinct groove, on the ventromedial sur-face ofthe promontorium, that is continuous anteriorlywith two tiny foramina immediately anterior to thebulla. In addition, IVPP-V5788, as well as other di-dymoconids, possesses a middle lacerate foramen. Itis unusual in mammals to see the middle lacerate fo-ramen and two other small foramina co-existing sideby side in front ofthe bulla. Ifthe one-trunk hypothesisfor the internal carotid artery is true, the groove on theventromedial surface of the promontorium in IVPP-V5788 may convey the internal carotid nerve and ar-tery , or only the nerve, as in some other mammals(Conroy and Wible, 1978; Wible, 1986). In front of

the bulla, the two small foramina may serve as thepathways for the internal carotid artery and the nerveseparately, or only for branches of the internal carotidnerve, since the latter may have more than one branch,as in dogs (Miller et al., 1964) . Judging from the sizeof the foramina and the groove, the internal carotidartery, if it existed in IVPP-V5788, must have beenvery small, and it was probably lost in advanced di-dymoconids. It follows that the middle lacerate fora-men transmits only the ascending pharyngeal artery ora vein, as in some other mammals (Wible, 1983; Story,1951).

Alternatively, the internal carotid artery in IVPP-V5788 may have taken a course medial to the bullarwall, entering the braincase through the middle lacerateforamen, while the stapedial artery alone traveled intothe tympanic cavity. There is at least one reason forthis. As described above, a distinct oval depressionoccurs at the anteromedial side of the bulla. The mid-dle lacerate foramen opens into it and the groove me-dial to the bullar wall terminates at it . Such a depres-sion can be made only by a vessel that forms a loopin front of the bulla and enters the braincase backwardthrough the middle lacerate foramen. A vessel thatmakes a loop at that particular position is reported insome carnivores, and that vessel happens to be aninternal carotid artery (Story, 1951; Miller et al., 1964;Hunt, 1974; Bugge, 1978). This hypothesis implies thatthe internal carotid artery and the stapedial artery hadtheir bifurcation medial to the bulla, an uncommonand therefore derived condition in eutherians (Wible,1987). However, if this pattern is correct and if theinternal carotid nerve always runs along with the in-ternal carotid artery, it is difficult to interpret the grooveon the ventral surface of the promontorium in IVPP-V5788. The tympanic nerve, a branch of the glosso-pharyngeal nerve (Miller et al. , 1964), might be a can-didate to occupy that groove. But the tympanic nerveusually has several terminal branches spreading overthe promontorium, a pattern highly different from thatofIVPP-V5788. Moreover, branches of the tympanicnerve do not run through the braincase via foraminalike those in IVPP-V5788. In any case, we do notassume that the internal carotid artery and the nerverun in separate courses, given the fact that in extanteutherian mammals the internal carotid nerve alwaysruns along with the internal carotid artery (Miller etal ., 1964; Conroy and Wible, 1978; MacPhee, 1981).The presence in IVPP-V5788 of the middle lacerateforamen on the one hand and the groove associatedwith two terminal foramina on the other suggests thatthe two-trunk model should not be overlooked.

PHYLOGENETIC DISCUSSIONDidymoconid relationships have long been a prob-

lem , which is fully reflected by the fact that this familyhas been related to about a dozen mammalian groups,including Leptictidae, Deltatheriinae, Palaeoryctidae,

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Creodonta, Mesonychidae, Hapalodectidae, Insectiv-ora, Macroscelidea, Lagomorpha, Carnivora, Anagal-idae, and Zalambdalestidae (Gingerich, 1981; Meng,1992). In recent studies, a didymoconid-mesonychidrelationship seems to be favored, and didymoconidsare thought to be derivable from an Asian Paleocenemesonychid, Yantanglestes conexus (Wang, 1976;Gingerich, 1981), which represents the earliest and mostprimitive form among the many mesonychids reportedfrom Asia (Chow et al., 1973; Chow and Qi, 1978;Dashzeveg, 1976; Van and Tang, 1976; Wang 1976;Ideker and Van, 1980). Furthermore, Gingerich (1981,1982) places the North American genus Wyolestes, onwhich a subfamily Wyolestinae has been established,in the Didymoconidae and suggests that, when bothDidymoconidae and Wyolestinae are traced back totheir earliest representatives, i.e., Archaeoryctes andWyolestes, respectively, they most closely resemble thePaleocene Yantanglestes. However, Yantanglestes co-nexus is known from only a few isolated and poorlypreserved teeth. Gingerich's classification was chal-lenged by others (Van Valen, 1988; Meng, 1992). WhileVan Valen does not provide any specific reason for hisdisagreement that didymoconids are related to Yan-tanglestes, Meng argues that the features presumablysupporting a didymoconid-mesonychid relationshipare not convincingly evident and that no unique de-rived feature has been found to suggest the monophylyofdidymoconids, if Wyolestes is included. The follow-ing discussion reflects Meng's original opinion, ofwhichMeng will take full responsibility, for no consensusabout didymoconid relationships has been reachedamong the authors of this paper.

According to Gingerich (1981 :528), didymoconids(including Wyolestes) share a number ofdental featureswith mesonychids. These include "molariform P4/p4,reduced M3/m3 (lost in didymoconines), nearly sym-metrical upper molars without an exaggerated meta-crista, and lower molars with a prominent anteropos-teriorly oriented hypoconid crest on the talonidoccluding against a similarly oriented centrocrista onthe upper molars. The trigonids of the lower molarsare simple and high ; the talonids have a very reducedentoconid and are open medially rather than basined."Among these features, however, p4 is not molariformin primitive didymoconids such as Archaeoryctes andArdynictis, although M4 is submolariform in theseforms. The lower fourth premolar is molariform onlyin the advanced genera of didymoconids. P4/p4 inWyolestes, little known when the genus was described(Gingerich, 1981), have later been shown to be non-molariform (Gingerich, 1982; Novacek et al., 1987;Novacek et al., 1991). The slightly reduced M3/m3 inmesonychids and Wyolestes is trivial compared withits absence in didymoconids. In this regard, many othermammalian groups display a closer resemblance to thedidymoconid condition than do mesonychids. For ex-ample, absence of M3/m3 is present in several Paleo-cene insectivores from Asia (Szalay and McKenna,1971; McKenna et al., 1984; Zhai and Wang, pers .

comm.). The hypoconid crest does not seem prominenton the lower molars of didymoconids. In fact, the tal-onids on the lower molars of didymoconids are muchshorter than those of mesonychids, such as Yantan-glestes . Simple and high trigonids can be found in manyCretaceous and early Tertiary mammals. A reducedentoconid on the talonid is found in Deltatherium (pers.obs .), which is regarded as the ancestor of didymo-conids by Gromova (1960), and possibly may occurin primitive pantodonts (Van Valen, 1988).

On the other hand, Gingerich (1981) recognizes afew features in which didymoconids differ from me-sonychids. These include less bulbous upper and lowercheek teeth, the protocone well separated from theparacone and metacone, and a prominent paraconidin primitive didymoconids. These features, which maypossibly distinguish didymoconids from mesonychids,do not necessarily separate didymoconids from othermammals. The first and third features are obviouslyprimitive therian states and can be found in many earlymammals. The second feature does not apply to theprimitive didymoconids such as Archaeoryctes, inwhich the protocone is not well separated from theparacone and metacone (Zheng , 1979; Meng, 1992).The weak point in the didymoconid-mesonychid hy-pothesis is that there is no uniquely derived feature forthe family Didymoconidae, in which Wyolestes is in-cluded. This strongly suggests that the Didymoconi-dae, if Wyolestes is included, is a paraphyletic group.

Cranial features from IVPP-V5788 and other di-dymoconid skulls (Meng, 1992) provide other traitsfor the further exploration of didymoconid relation-ships. Unfortunately, cranial specimens of early me-sonychids such as Yantanglestes and Dissacus are few,which prohibits any comparison. However, cranialspecimens of some advanced mesonychids, such asHarpagolestes and Mesonyx (Matthew, 1909; Szalayand Gould, 1966), allow us to make some comparisonbetween didymoconids and mesonychids. These ad-vanced mesonychids possess many primitive therianconditions, which are assumed to be present in theirearly relatives; otherwise, reversals must have hap-pened. Mesonychids have a large, posteriorly widenednasal bone, a narrow maxilla-frontal contact, a longinfraorbital canal, a large jugal contacting the lacrimalat the facial region, a large facial exposure of the lac-rimal, the maxilla separated from the frontal in theorbit, a small mastoid exposure, a poorly ossified bulla,and a tympanic cavity lacking any specialization (Meng,1992). We did not discover any significant cranial sim-ilarity between didymoconids and mesonychids.

Well-preserved material of Wyolestes from Baja Cal-ifornia (Novacek et al., 1987; Novacek et al., 1991)adds no additional features uniting Wyolestes and di-dymoconids. Instead, Wyolestes differs from didy-moconids in having a long and narrow infraorbitalcanal, the jugal contacting the lacrimal at the facialregion, a narrow glenoid fossa and postglenoid process,and probably a non-expanded ear region (Meng, pers .obs.). On the basis of the cranial evidence, we believe

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that Wyolestes is not a didymoconid and that the Di-dymoconidae are not related to the Mesonychidae. Thefamily Didymoconidae, excluding Wyolestes , can bedefined by several dental and cranial synapomorphies(Meng, 1992). The systematic position of Wyolestesremains undetermined. Novacek et al. (1991) recog-nize Wyolestes merely as Eutheria incertae sedis , butsuggest a possible relationship with other early Tertiarymammals, such as hyaenodontids. This genus is alsopossibly derived from some earlier North Americanmammals, such as Cimolestes (Meng, 1992).

Cranial traits suggest a didymoconid-insectivoranrelationship. In an extensive treatment of leptictids,Novacek (1986) elevated the rank of insectivores to asuperorder, which includes the extinct Leptictida andthe extant Lipotyphla. Cranial characters cited by No-vacek to diagnose this superorder include: 1) broadmaxillary-frontal contact in the facial region; 2) infra-orbital canal relatively short; 3) maxilla intruding intoorbit, nearly or extensively contacting frontal; 4) largecommon recess for separate sphenopalatine and dorsalpalatine foramina; 5) lacrimal confined to orbit or an-torbital rim; 6) well-defined Glaserian fissure in lateralroofand anterior wall oftympanic cavity; 7) prominentpetrosal ridge or crest on the medial promontorium;8) large, ossified mastoid tubercle (incorporates tym-panohyal) with distinct ventral fossa, medially pro-jecting and nearly contacting promontorium; 9) ante-rior border of ventral occipital condyle with strongsigmoid outline. Our study shows that IVPP-V5788and other didymoconids possess at least the first fivecharacters. Characters associated with the auditory re-gion (6-8) are not comparable with those of didymo-conids, because the latter display a much more derivedear. When the bulla is well developed and fused withthe surrounding elements, features such as the Glas-erian fissure, the mastoid tubercle, and the petrosalcrest on the medial promontorium are difficult to iden-tify. Even in some lipotyphlans with a specialized ear,such as golden moles, these features are not readilydetectable. Among the nine features, only the last failsto apply to didymoconids.

Within a broadly defined Superorder Insectivora theLipotyphla are further defined by the following cranialfeatures: a mobile snout or proboscis; a large or-bital process of the maxilla that displaces the palatine,and a reduced or absent jugal (Butler, 1988). Thesefeatures are not always obvious in fossil taxa (MacPheeand Novacek, 1993). Didymoconids are similar to li-potyphlans in having a reduced jugal, maxilla-frontalcontact in the orbit, and probably a protruded snout,because an extension of the premaxilla in front of theincisors is present in didymoconids (Gromova, 1960;Meng, 1992). The last two features are more lipotyph-Ian-like in didymoconids than in other early insecti-vores such as leptictids. Therefore, we believe that di-dymoconids are closely related to insectivores basedon the cranial evidence, and that they should be placedin the Insectivora as defined by Novacek. Althoughthe position ofDidymoconidae in that superorder needs

to be determined, they appear to represent a specializedlineage branching off the insectivoran stem early in thehistory of the group.

ACKNOWLEDGMENTSWe are greatly indebted to Professor Li Chuankuei,

Institute of Vertebrate Paleontology and Paleoanthro-pology (IVPP), Beijing, who led the field group at Da-jian in 1984. We are also grateful to Mr. Xie Shuhuaand Mr. Zheng Xianhe of IVPP, for their assistanceduring the field work. We also thank Mr. Zhai Renjieand Mr. Huang Xueshi ofIVPP for access to specimensunder their care. We extend our special thanks to Dr.Malcolm C. McKenna, American Museum ofNaturalHistory, New York, for his encouragement and supportof this research and comments on one of the drafts.We are grateful to Dr. R. D. E. MacPhee and an anon-ymous reviewer for instructive comments and sugges-tions. For valuable discussion and comments on theearly version of this paper, we thank John R. Wible,University of Louisville; Kenneth D. Rose, JohnsHopkins University; Robert Presley, University Col-lege, Cardiff; and Earl Manning, Museum of Geosci-ence, Louisiana State University. Kerry M. Lyle, De-partment of Geology and Geophysics, LSU, skillfullytook the stereophotos. Ruth Hubert of LSU helped toimprove the English. During the course of this re-search, Meng was supported by a faculty fellowshipfrom Columbia University, the Frick Laboratory En-dowment Fund, American Museum of Natural His-tory, and a post-doctoral fellowship from the Smith-sonian Institution. The LSU Museum of Geoscienceand Museum of Geoscience Associates provided sup-port for Ting.

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R eceived 22 Ma y 1992; accepted 23 Ju/y 1993.

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