Apternodus major, ASHER & McKENNA & EMRY & TABRUM & KRON, 2002

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 : 48-54

Apternodus major , new species

TYPE SPECIMEN: UW 11046 , nearly complete skull (fig. 32) preserving base of left I1, C, P3­M3, right I1, C­M3, missing posterior palate and pterygoid region, articulated right dentary with broken i1, c, broken p3, p4­m3, and associated left dentary (fig. 33) with p4­m3 ; associated ulna, proximal and distal fragments of right humerus (fig. 34), fragments of scapulae, cervical vertebrae, and ribs.

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

REFERRED SPECIMENS: UW 10981, rostrum associated with partial right dentary with p3­ m3, broken i1, i3, c; UW 10984, fragmentary skull with left M1­M3, right M1­M2; UW 11291, partial rostrum with left P2­P4, bro­ ken M1­M2; UW 11292, fragmentary skull with right M1­M3, associated left maxilla with P3­M2; UW 11295, fragmentary rostrum with left P3­P4, articulated posterior left dentary, associated with anterior left den­ tary containing p4­m1; and UW 11296, rostrum with articulated left and right dentaries, with complete dentition except for I/i1 and P/p2.

ETYMOLOGY: The name ‘‘ major ’’ was coined originally by Kron (1978) and refers to the large size of this species.

TEMPORAL AND GEOGRAPHIC DISTRIBUTION: Chadronian (late Eocene) of eastern Wyoming (Dilts Ranch 10).

DIAGNOSIS: This species is the largest yet known in the genus Apternodus . It is anatomically distinctive by virtue of the laterally flared anterior lambdoid plates and the enlarged, bony torus present along the ventral margin of the external auditory meatus. Unlike A. iliffensis , the external auditory meatus is ventrally concave, but not to the extent seen in A. gregoryi (fig. 21). A. major also has a prominent, anteromedially running rostral tympanic process of the petrosal, a laterally prominent maxillary rudiment of the zygoma, and a lacrimal foramen continuous with the anterior orbit. Diastemata between the upper incisors and adjacent to P3 are lacking. The anterior dentition of A. major is large, but premolariform, resembling that of A. gregoryi . The large, two­rooted, premolariform P2 of A. major is similar in size to P3 and much larger than the P2 of A. brevirostris . Protocones on the upper molars are reduced and barely distinguishable from the lingual cingulum.

REMARKS: Several specimens collected and first described by Kron (1978) are well­preserved, but none is as complete as the type. This specimen (UW 11046) is a particularly large and robust individual, showing extreme development of the anteriorly flared lambdoid plates, a laterally expansive maxillary zygoma, and distinct fossae in the maxillae above each canine. A. major is found nowhere else except at the Dilts Ranch area examined by Kron (1978), which has also yielded other species of Apternodus (UW 13508 A. gregoryi and AMNH 74952 possibly A. brevirostris ) but none of Oligoryctes . In addition to the details summarized in the diagnosis, the morphology of the type skull is as follows.

The maxilla adjacent to the anterior infraorbital canal is damaged on both sides. However, it can be seen on the left side to project far anterolaterally (fig. 32), providing extensive surface area for the attachment of anterior facial muscles. Following Butler (1956), the muscles that originate from this area include levator labii superioris, levator alae nasi, and zygomaticus. Whidden (MS in progress) has recently confirmed Butler’s assessment for the origin of these muscles in insectivoran­grade taxa such as Solenodon . A. major possesses a relatively huge area for the attachment of anterior snout­muscles, somewhat more elaborate than that of Solenodon , which supports an elongate rod of cartilage that stretches anteriorly from the external nares. Hence, the rostral morphology of A. major , and that of Apternodus generally, appears compatible with an anteriorly elongate, cartilaginous extension of the proboscis (as reconstructed anterior to the elongate upper incisor in the frontispiece of this monograph).

The dorsal braincase of A. major is rugose (fig. 32). Due in part to its laterally extensive anterior lambdoid plate, A. major has more surface area for the attachment of the temporalis muscle than other species of Apternodus . The robust temporalis musculature of A. major is also reflected in the pronounced muscle scar on the lateral aspect of the mandibular coronoid process (fig. 33).

As in other species of Apternodus , A. major shows prominent piriform fenestrae in the tympanic roof anterior to the pars cochlearis of the petrosal. Anteromedial to the prom­ ontory, on the lateral margin of the basisphenoid, a distinct vidian foramen is evident. The anterior carotid foramen appears to be confluent with the piriform fenestra (fig. 32). Anterior and slightly ventral to the caudal tympanic process of the right petrosal, UW 11046 preserves fragments of what might be a broken ectotympanic or tympanohyal (fig. 32).

The posterior palate and pterygoid region are not preserved in UW 11046, but from other, more fragmentary specimens (e.g., UW 10981, UW 10984, UW 11292) they can be inferred to be similar to those of other species of Apternodus . The jaw joint, well preserved in the type, also resembles that of other Apternodus , as described above for A. baladontus .

On both sides, the scapulae are unremarkable, with concave glenoid fossae for articulation with the proximal humeri. The neural arches of several vertebrae, probably thoracic, are partially exposed. Spinous processes of these vertebrae were either very small or not preserved in this specimen.

UW 11046 is one of the few Apternodus specimens with associated postcrania. These include a left ulna and both proximal and distal left humeral fragments (fig. 34). Both elements resemble those of Solenodon , but are smaller. The proximal humerus shows a broad greater tuberosity and well­defined fossa for the biceps tendon. The distal humerus shows an entepicondylar foramen, prominent medial epicondyle, well­defined trochlea and capitulum, and a small fossa posterior to the capitulum that is continuous with the supinator crest. The proximal ulna differs from that of Solenodon in having a smaller, more gracile olecranon process.

Distal fragments of both scapulae, a clavicle, vertebrae, and ribs are also preserved together in an associated block of matrix. The right scapula appears to be articulated with the proximal end of the right clavicle.