Apternodus gregoryi Schlaikjer, 1933

ASHER, ROBERT J., McKENNA, MALCOLM C., EMRY, ROBERT J., TABRUM, ALAN R. & KRON, DONALD G., 2002, Morphology And Relationships Of Apternodus And Other Extinct, Zalambdodont, Placental Mammals, Bulletin of the American Museum of Natural History 2002 (273), pp. 1-117 : 37-48

Apternodus gregoryi Schlaikjer, 1933

TYPE SPECIMEN: MCZ 17685, complete skull (fig. 21) with left C, P3­M2, right C, P2­M2, broken M3, missing premaxillae and anterior dentition, associated mandibles with left p3­m3, right i1, and p3­m3.

REFERRED SPECIMEN: UW 13508, skull (fig. 22) with intact but distorted basicranium, palate with left I1 root, C­M3, roots for right I1–2, plus right C­M3, associated with mandibles (fig. 23) including broken left i1, i3­ c, p3­m3, and broken right i1, i3­m3. Also associated with proximal and distal fragments of the femur and os coxae (fig. 24), a fragment of the proximal ulna, five fragmentary caudal vertebrae, and several indeterminate fragments of ribs and/or longbones.

TEMPORAL AND GEOGRAPHIC DISTRIBUTION: Chadronian (late Eocene) to Orellan (early Oligocene) of eastern Wyoming (Dilts Ranch 10; Torrington 38).

DIAGNOSIS: The rostrum of Apternodus gregoryi shows small diastemata between P2 and P3 and on either side of the canine. The external auditory meatus is very narrow and defined posteriorly by a ventrally projecting posttympanic process. Anteriorly, this notch is defined by a ventrally elongate lower lip of the entoglenoid process. The rostral tympanic process of the petrosal is flat. The lacrimal foramen of A. gregoryi is posteriorly continuous with the anterior orbit. That is, unlike A. brevirostris , iliffensis , mediaevus , and baladontus (but similar to A. major ), the superior margin of the infraorbital canal is set apart from the lacrimal foramen posteriorly, leaving this region without a ridge of bone defining the posterior aspect of the foramen (figs. 21, 22). Protocones on the upper molars are reduced, smaller than their corresponding parastyles. The cingulid and posterior cusp of p3 are reduced.

REMARKS: The only specimen known of A. gregoryi besides the type is UW 13508. This

(top) and lateral (bottom) views.

(top), lateral (middle), and lingual (bottom) views.

specimen consists of a nearly complete skull (fig. 22) and lower dentition (fig. 23) from both sides, as well some postcranial elements (fig. 24). The skull is quite similar to MCZ 17685, although the ventral lambdoid plates of UW 13508 are broken. The large size of their breakage scars indicates that the lambdoid plates were ventrally extensive. What remains of the external auditory meatus is sharply concave, as in the A. gregoryi type specimen (fig. 21).

Postcranially, the femur and partial os coxae of UW 13508 appear small for an animal with a skull of this size. However, the ratio of anteroposterior skull length to indices of proximal femur and acetabular size (fig. 25) is similar to that of extant groups such as talpids and soricids ; hence, we assume that the partial femur, os coxae, and skull represent the same individual. In fact, prior to preparation, the distal femur of UW 13508 was embedded in matrix adjacent to the basicranium, making clear at least that the two specimens were deposited in close proximity, although in an anatomically unexpected position. The dentition of the associated skull is fully erupted, but shows minimal wear. Furthermore, the sphenoccipital synchondrosis of the skull is not completely fused (fig. 22), consistent with its association with the juvenile represented by the partial femur .

The distal femur preserves only the epiphysis. Proximally, the femur also appears to be detached from the epiphysis, although in this case the epiphysis is missing. Perhaps as a consequence, the femoral head lacks any sign of a fovea capitis. If what remains of the proximal femur (missing the epiphysis) accurately reflects the shape of the femoral head, then it is dorsally flattened and not separated from the greater trochanter by a deep dorsal concavity, in contrast to the morphology seen in most other insectivoran­grade taxa (e.g., Echinosorex ; see MacPhee, 1994: 131). Extant animals with a similar femoral morphology include Didelphis , Orycteropus , Tenrec ( MacPhee, 1994: 130–131) , and soricids. To varying degrees, these taxa show arboreal, fossorial, and terrestrial locomotor patterns and do not present a simple locomotor analogue to Apternodus .

Schlaikjer’s (1933) study of Apternodus gregoryi was considerably more detailed than that of Matthew (1910), but contained the following points that we cannot confirm.

First, Schlaikjer (1933: 10 and fig. 4) thought that there was some evidence that the mastoid extended onto the occiput to contact the exoccipital and supraoccipital. However, the mastoid­occipital suture actually lies on the lateral­facing lambdoid plate, as noted by Matthew and later by Schlaikjer himself (1934: 6). There is no visible exoccipital­supraoccipital suture on skulls of A. brevirostris or A. gregoryi . The entire occiput is composed of fused parts of the occipital bone.

The squamosal was believed to extend back on the lambdoid plate to what in reality is the mastoid­occipital suture. Schlaikjer’s (1933: fig. 3) figure of the lateral view of the skull omitted the squamosal­mastoid suture. The latter was corrected by Schlaikjer (1934) and McDowell (1958: 165, fig. 23).

On the lambdoid plate, Schlaikjer’s (1933: fig. 3, top) identification of the lateral exposure of the occipital as a part of the mastoid bone led him to depict a faint ‘‘mastoid­pa­ rietal’’ suture on the dorsal view of the skull. A suture between the occipital and the parietals was assumed to run along the lambdoid crests. A similar interpretation was given by McDowell (1958: fig. 23A). However, in available material of A. gregoryi , dorsal exposures of dermal cranial bones are fused and sutures cannot be accurately distinguished.

As noted above, piriform fenestrae were incorrectly shown as absent by Schlaikjer (1933, 1934) in both A. gregoryi and A. brevirostris . The type of A. gregoryi shows a relatively undistorted piriform fenestra (fig. 21). This region is less well­preserved in UW 13508 (fig. 22), but also appears to exhibit an incompletely ossified tympanic roof.

Apternodus iliffensis Galbreath, 1953

TYPE SPECIMEN: KU 9112, left maxillary fragment with P3­M3, associated left dentary with p4­m3 (fig. 26).

REFERRED SPECIMENS: USNM 455680, skull (fig. 27) with left P3­M3, fragmentary right I2 and canine root, associated left (fig. 28) and still articulated right posterior man­ dibles with m1­m3 and m2­m3, respectively; DMNH 1747, skull (fig. 29) with left C­M2 and right canine; FMNH PM34512, rostrum with right and left P2, left P3 and partial P4, lingual roots of M1­M2, alveoli for right and left canines, associated with brain endocast, right lambdoid plate, and petrosal; and TMM 40492­9, rostrum with left P4­M2, right P4­ M1, M3, partial P3, preserving pterygoid and orbitotemporal regions (fig. 30).

TEMPORAL AND GEOGRAPHIC DISTRIBUTION: Chadronian (late Eocene) of northeastern Colorado (Iliff 24, Fremont Butte 20) and central Wyoming (Beaver Divide 5); and late Duchesnean (late middle Eocene) of west Texas (Red Mound 32).

DIAGNOSIS: The type and referred specimens of Apternodus iliffensis are smaller than other species of Apternodus . They have reduced upper molar protocones that are continuous with the molar lingual cingula and a small M3. The anterior dentition of USNM 455680 and DMNH 1747 is premolariform, not bulbous; a diastema is absent between P2 and P3; and minute diastemata are present on either side of the canine. The lacrimal foramen is set off from the orbit by a prominent crest, formed by the proximity of the infra­ orbital canal. Medial to the promontory, the rostral tympanic process of the petrosal is flat. The external auditory meatus is weakly concave ventrally, and the ventral margin of the lambdoid plate does not extend far below the glenoid region, similar to the condition in A. baladontus .

REMARKS: Cranially, this species is the smallest within Apternodus , even though its cheek teeth are relatively larger than those of A. mediaevus and A. baladontus . Two nearly complete skulls are now known for A. iliffensis: USNM 455680 (figs. 27, 28) and DMNH 1747 (fig. 29). The lower dentition anterior to p4 remains unknown.

As indicated by DMNH 1747 and USNM 455680, the upper anterior dentition of A. iliffensis closely resembles that of A. brevirostris and A. gregoryi . That is, the P2, canine, and I2 are small, premolariform, non­bulbous teeth. I2 has one root, P2 and canine two. I2 has a small but distinct posterior cusp, connected to the apex of the tooth by a crest.

The basicranium is mediolaterally narrow­ er than that of other species. However, struc­ tures of the posterior braincase (i.e., the lambdoid plate, temporal fossa, and sagittal and nuchal crests) are at least as robust as those of other taxa.

USNM 455680 displays a large, undistort­ ed piriform fenestra anterior to the petrosal, bounded medially by a strut of alisphenoid, which defines the lateral border of the anterior carotid foramen. The smaller vidian foramen is present anteromedial to the anterior carotid foramen. The petrosal retains a subtle but distinct groove extending from the apex of the promontory, medial to the vestibular foramen, anteromedially toward the anterior carotid foramen. In life this groove was associated with the internal carotid artery and contradicts McDowell’s (1958: 168) interpretation that the internal carotid artery of Apternodus had a medial course adjacent to the petrosal­basisphenoid suture.

Referring the west Texas specimen (TMM 40492­9; fig. 30) to A. iliffensis makes this species the most geographically widespread within Apternodus . Novacek (1976b) tentatively referred this specimen to ‘‘ A. cf. bre­ virostris ’’. However, TMM 40492­9 contrasts with A. brevirostris , and is similar to the iliffensis type specimen, in having reduced molar protocones, a mesiodistally elongate P3, and a small M3. In fact, the M3 of TMM 40492­9 is considerably smaller than that of any other Apternodus specimen. It further differs from most other Apternodus individuals in showing a particularly elongate P3. Hence, at present we regard this specimen as a member of A. iliffensis ; however, recovery of additional material from west Texas may justify the recognition of a new species.

In his 1978 paper describing a second ap­ ternodontid specimen (FMNH PM34512) from White River Formation exposures north of Iliff, Colorado, Galbreath made several confusing statements. First, Galbreath (1978: 302) stated that ‘‘there should not be any doubt that the new specimen [i.e., FMNH PM34512] is an apternodontid but not an apternotid.’’ However, Galbreath’s intended meaning for the term ‘‘apternotid’’ is unclear. He may have been of the opinion that his new specimen should not be included in the genus Apternodus , in which case we would disagree for reasons given below.

Galbreath (1978: 301) indicated that ‘‘oth­ er details not mentioned in the original description of A. iliffensis acquire significance now with the discovery of a second apternodontid from the same locality’’, and described some aspects of the A. iliffensis infraorbital canal, presumably based on FMNH PM34512. However, on the next page he argued that ‘‘the dissimilar details of the infraorbital canal suggests that the two [i.e., KU 9112 and FMNH PM34512] would not be in the same specific taxon’’, in contrast to his previous implication that both specimens illuminate the anatomy of A. iliffensis .

Galbreath (1978: 299) stated that the ‘‘preorbital ridge so characteristic on the maxillaries of A. brevirostris and A. gregoryi ’’ is missing on FMNH PM34512. However, a ‘‘preorbital ridge’’ has not been explicitly mentioned by other authors, and Galbreath does not illustrate or further elaborate on the identity of this character. There is some variation in the distance between the superior margin of the infraorbital canal to the lacrimal foramen; in A. brevirostris this distance is shorter than that in A. gregoryi and defines the border between the lacrimal foramen and orbit with a ridge. However, this feature serves to distinguish the two species, not characterize them. Furthermore, a ridge posterior to the lacrimal foramen is present in FMNH PM34512. Hence, we regard this character as a misinterpretation by Galbreath of breakage along the lateral margin of the maxilla in FMNH PM34512.

The fragmentary teeth of FMNH PM34512, consisting of an intact P2, P3, partial P4, and the lingual remnants of M1­M2, are identical in size and appearance to the corresponding parts of P3­M 2 in KU 9112. Most importantly, both specimens lack protocones and are from temporally and geo­ graphically adjacent localities, and can easily be accommodated in the single species, Apternodus iliffensis .

Krishtalka and Setoguchi (1977) described upper and lower dental remains of ‘‘ Apternodus sp. cf. A. iliffensis ’’ from Badwater locality 20, now considered to be early Duchesnean in age. One of these specimens. CM 29012 (fig. 31), is a right maxillary fragment with P4­M1, not M1–2 as stated by Krishtalka and Setoguchi (1977), with weak protocones that presumably led these authors to associate the Badwater specimens with A. iliffensis . However, the protocones of CM 29012 are actually larger than those of the A. iliffensis type. Other potentially useful anatomical regions (e.g., P3 and M3) are missing. Hence, this specimen and others from Badwater locality 20 are not sufficiently diagnostic for identification at the species level. Nevertheless, the Badwater locality 20 Apternodus specimens, and similar material described by Storer (1995) from the approximately contemporaneous Lac Pelletier Low­ er Fauna of Saskatchewan, document the first occurrence of this genus in the middle Eocene of North America. The small ‘‘ Apternodus sp. ’’ described by Tong (1997) from China may be similar or even older in age.

Ostrander (1987) referred several isolated upper and lower teeth from the middle Chadronian Raben Ranch Local Fauna of Nebraska to A. iliffensis . However, Ostrander did not provide illustrations of these specimens, and the mesiodistal and buccolingual dimensions he reports are broadly consistent with those of several Apternodus species. In our experience, isolated upper or lower molars cannot be assigned with confidence to a single species of Apternodus .