Amphicticeps makhchinus Wang, McKenna, and Dashzeveg, 2005

WANG, XIAOMING, McKENNA, MALCOLM C. & DASHZEVEG, DEMBERELYIN, 2005, Amphicticeps and Amphicynodon (Arctoidea, Carnivora) from Hsanda Gol Formation, Central Mongolia and Phylogeny of Basal Arctoids with Comments on Zoogeography, American Museum Novitates 3483, pp. 1-58 : 25-27

Amphicticeps makhchinus Wang, McKenna, and Dashzeveg , new species

Figure 9 ; Table 3

HOLOTYPE: MAE 93–213 ( AMNH cast 129862), right maxillary fragment with P4– M1, partial P3, and alveolus of M2. Collected by James M. Clark on 16 August 1993.

TYPE LOCALITY: MAE 93–213 was found in the Tatal Gol (Ulaan Khongil or ‘‘Grand Canyon’’) locality, 458179500N, 1018379160E, Tsagan Nor Basin, eastern Valley of Lakes, Obor­Khangay Province, Mongolian People’s Republic.

GEOLOGY AND AGE: MAE 93–213 was collected from the main exposure of the Tatal Gol locality, below the level of the lava, in the Tatal Member of the Hsanda Gol Formation, early Oligocene.

REFERRED SPECIMENS: Holotype only.

DIAGNOSIS: As the largest and possibly the most derived species of the genus, Amphicticeps makhchinus is distinguished from the other two species of the genus, A. shackelfordi and A. dorog , in its larger size, a broadened P3 with an extra lingual root, a low and lingually expanded P4 protocone crest, a slightly more reduced M1 parastyle, an enlarged M1 metaconule, and a more expanded M1 lingual cingulum.

ETYMOLOGY: Mongolian: makhchinus , meat eater, carnivore.

DESCRIPTION: Amphicticeps makhchinus is the least known of the three Hsanda Gol species of the genus. We are limited to two and a half teeth on the fragmentary right maxillary of the holotype. The maxillary clearly shows a shortened infraorbital canal, implying a shortened rostrum. Attached to this maxillary fragment is the anterior­most part of the jugal. The well­delineated jugalmaxillary suture indicates that the anterior jugal process stops at the antorbital rim and is probably not in contact with the lacrimal or frontal as in other species referred to this genus.

Only the posterior half of the P3 is preserved, which has a well­developed cingulum. The P3 has a significantly broadened lingual side and appears to have an extra lingual (third) root, in contrast to the doublerooted condition in other species of Amphicticeps . The P4 is typical of the genus, with a complete cingulum surrounding the entire tooth. The anterolabial corner of the cingulum is the strongest, but it does not elevate to form a parastyle. Like that of other species of Amphicticeps , the P4 protocone is composed of a raised lingual cingulum. However, the protocone is more expanded toward the lingual side than in the other species of the genus. As in the other two species of Amphicticeps , there is a low crest on the labial side of the protocone. The broad­based paracone has an anterior ridge leading up to the cingulum.

Overall proportions of the M1 have an anteroposteriorly broadened appearance for a basal ursoid. The parastyle is large and rises above the paracone, but does not reach to the same degree of expansion as seen in A. shackelfordi and is more similar to that of A. dorog . Likewise, the cingulum adjacent to the metacone shows no sign of reduction as in A. shackelfordi . Consequently, the angle between the labial borders of the P4 and M1 remains a relatively large 1248, 158 greater than in A. shackelfordi but almost identical to that in A. dorog . A distinct pre­ and postprotocrista are present, the latter being slightly more posteriorly directed than in A. shackelfordi and A. dorog . There is no protoconule (paraconule) and the metaconule is only indicated by a vague platform (probably suffered from some wear) slightly raised above the surrounding areas. The M1 internal cingulum is broad and thick, much more expanded than in A. shackelfordi . An anterior spur of this cingulum is present near the base of the preprotocrista. M2 is missing. Its partial roots, however, indicate a transversely broadened M2 whose lingual border is more internal than that in the M1. Its labial border is flush with that of M1, similar to that in A. dorog but in contrast to a lingually shifted M 2 in A. shackelfordi .

COMPARISON: Amphicticeps makhchinus is the largest species of the genus so far known. It is 16% larger than A. dorog and 32% larger than A. shackelfordi (based on measurements of P4 labial length). It is 62% larger than Amphicynodon teilhardi . Besides its large size, A. makhchinus is also the most hypocarnivorous species in the genus. Dental features that indicate such hypocarnivory include an enlarged but low­crowned P4 protocone, a reduction of M1 parastyle, expansion of M1 lingual cingulum, a reduced angle between lingual borders of P4 and M1, and an enlarged M2.

Dental morphology of Amphicticeps makhchinus is reminiscent of certain ursids, particularly a basal ursid such as Cephalogale , so far known mostly in the Oligo­ Miocene of Eurasia and North America. In particular, the French early Oligocene Quercy fissure fills produced some of the most primitive forms (e.g., Cephalogale minor ). Similarities between A. makhchinus and Cephalogale include an enlarged grinding part of the dentition (M1–2) at the expense of the shearing part (P4). More specifically, A. makhchinus has a low, shelflike P4 protocone and a quadrate outline on M1, features often seen in Cephalogale . However, structural details of these features tend to argue against a true homology in the hypocarnivorous dentitions shared between A. makhchinus and Cephalogale . For example, the P4 protocone in all ursids (including Cephalogale ) is formed by a swollen lingual cingulum, often in the form of a crest instead of a conical cusp, that has receded far back from the anterior border of the tooth, in contrast to an essentially conical protocone located on the anterolingual corner of P 4 in A. makhchinus . Another derived character for the Ursidae is a posteriorly oriented postprotocrista of M1. This is a highly consistent feature among all known ursids. Such a condition is lacking in A. makhchinus (although wear in this region in the holotype of A. makhchinus renders our observation less certain). Finally, all ursoids, including the commonly acknowledged basal ursoids such as Amphicynodon , have a highly reduced parastyle and lingual cingulum on M1, in sharp contrast to a still relatively prominent parastyle in A. makhchinus .

Conversely, everything about Amphicticeps makhchinus is consistent with other species of Amphicticeps , despite its modest deviations toward the direction of hypocarnivory. Our inclination to assign it to Amphicticeps is further helped by the transitional nature of A. dorog between A. shackelfordi and A. makhchinus —in just about every aspect of its dental morphology A. dorog bridges the gap between the extremes in A. shackelfordi and A. makhchinus . In the final analysis, given what we know, it is easily conceivable that a series of three endemic species of Amphicticeps form a clade in the early Oligocene of central Asia.

Figures

Fig. 9. Amphicticeps makhchinus, n.sp, MAE 93–213, holotype, photographs of a polyester cast (AMNH 129862). A, occlusal view of upper teeth, stereophotos; B, lateral view of upper teeth and maxillary; and C, lingual view of upper teeth and maxillary. Scale 5 5 mm.