Arctotitan honghoensis Wang, 1978

Mihlbachler, Matthew C., 2008, Species Taxonomy, Phylogeny, and Biogeography of the Brontotheriidae (Mammalia: Perissodactyla), Bulletin of the American Museum of Natural History 311 (1), pp. 1-475 : 359-361

Arctotitan honghoensis Wang, 1978

HOLOTYPE: IVPP V3201 , a skull fragment consisting of the premaxillomaxillary rostrum with right I1–P2 and left I1–I2, P1– P3.

TYPE LOCALITY: Yinpocun locality, near Xian (loc. 65009), Shaanxi Province, Lantian District, Hongho Formation.

AGE: Middle Eocene ( Russell and Zhai, 1987).

DETERMINATION: Nomen dubium. This species could be a synonym of Gnathotitan berkeyi .

DESCRIPTION

IVPP V3201 is the anterior portion of a premaxillomaxillary rostrum of an extremely large brontothere (fig. 185). The dorsolateral surface of the rostrum is intact from its anterior edge to a point about above the P1, indicating that the lateral nasal incision extended to a point posterior to the P1. The rostrum itself is very broad and shallow. The premaxillomaxillary sutures are clearly visible on both sides of the specimen. The nasal processes of the premaxillae diverge posterolaterally from the midline symphysis and extend posteriorly to a point above the P1s.

For the most part, the teeth are only moderately worn. The incisors form a moderately arched row and compare well with most hornless brontothere species in their subcaniniform shape. Though moderately worn, each incisor has a short lingual heel and a poorly defined labial cingulum. The incisor crowns progress in size laterally; the I1 is the smallest and the I3 is the largest incisor. There is a sizeable diastema between the median incisors and a rather long postcanine diastema.

The morphology of the P1 crown is obscured by wear, although it appears to have been relatively complex. The labial side of the P1 crown forms a curved ectoloph. The anterior portion of the P1 ectoloph contains a large paracone. A distinct metacone could have been present on P1, and there is certainly room for a metacone to have been present on the posterior portion of the ectoloph. The lingual side of the crown contains a broad, obliquely shaped heel. Any distinct cusps (protocone) or crest that might have been present on this lingual heel have been obliterated by wear.

The crown of P2 is rhomboidal with parallel anterior and posterior margins that form oblique angles with the lingual and labial sides. The P3 is nearly rectangular. The parastyles of P2 and P3 are slightly angled labially; otherwise, the ectolophs of these premolars are straight. Labial ribs curve in a posterodorsal direction across the labial face of the paracone on each tooth. The lingual paracone rib of the P3 is somewhat narrower than that of P2. Labial metacone ribs are not seen on these teeth.

There are heavily worn protocones on both P2 and P3. On each tooth the protocone is positioned on the anterior portion of the broad lingual heel. There is certainly room for a hypocone to have been present on P2, but if it was present it has been erased by wear. On P3 the hypocone is represented by a small circular exposure of dentin on the posterolingual corner of the crown that is well separated from the protocone. On both of these premolars, there is an indistinct preprotocrista. There is no evidence of a lingual crest on either the P2 or P3 of IVPP V3201, although wear may have erased it. The P2 and P3 have distinctly continuous anterior and posterior cingula. The labial and lingual cingula are relatively weak and discontinuous; however, the labial cingulum below the metacone of P3 is relatively strong.

REMARKS

Arctotitan honghoensis Wang 1978 is based on the rostral fragment (IVPP V3201) of an enormous brontothere. This specimen is one of just three fossil mammal specimens (all perissodactyls) from the Hongho Formation ( Wang, 1978; Russell and Zhai, 1987). The remaining specimens, identified as cf. Deperetella and Breviodon , suggest a middle Eocene age ( Russell and Zhai, 1987).

Wang (1978) and Russell and Zhai (1987) note that Arctotitan honghoensis is larger than any early or middle Eocene brontothere, and that it compares in size only with late Eocene brontotheres. However, at least one middle Eocene (Irdinmanhan) species, Gnathotitan berkeyi , is of a similar size. IVPP V3201 differs from other very large brontotheres, such as Embolotherium and Protembolotherium by its plesiomorphic incisor morphology and poorly developed premolar hypocones. Aktautitan hippopotamopus has large incisors like IVPP V3201, although I1 is substantially more spherical. Moreover, the premolars of Aktautitan have slightly narrower lingual heels that completely lack hypocones, the rostrum seems deeper and the postcanine diastema is much shorter ( Mihlbachler et al., 2004a). The broad and shallow rostrum, the relatively long postcanine diastema, and the low relief of the lingual features of the premolars of IVPP V3201 closely resemble Rhinotitan . Rhinotitan kaiseni retains large subcaniniform incisors, like IVPP V3201. However, the P1 of Rhinotitan kaiseni is narrower and simpler. Moreover, Rhinotitan kaiseni is the smaller species of Rhinotitan , and none of the known specimens approaches IVPP V 3201 in size. On the other hand, the larger specimens of Rhinotitan andrewsi approach IVPP V 3201 in size, and P1 is equally as advanced. However, the incisors of Rhinotitan andrewsi are distinctly more globular in shape. Therefore, IVPP V3201 is consistent with neither species of Rhinotitan .

Gnathotitan berkeyi is essentially the same size as IVPP V3201; premolar measurements differ by no more than a few millimeters. The upper incisors of G. berkeyi are unknown, although the lower incisors of that species are large and subcaniniform, and are thus consistent with the upper incisors of IVPP V3201. Some of the apparent differences between G. berkeyi and IVPP V3201 can be attributed to taphonomic deformation or they are traits found to have intraspecific polymorphic tendencies. For instance, the narrow symphyseal region of the G. berkeyi mandible (AMNH 20106) is inconsistent with the broad rostrum of IVPP V3201; however, that mandible is transversely crushed. The limited sets of upper premolars of G. berkeyi either lack hypocones (AMNH 141231), or have poorly developed hypocones that are not well separated from the protocones (AMNH 20121). IVPP V3201 seems to have better developed hypocones that are well separated from the premolars. However, premolar hypocone development is highly variable in many brontothere species, and the taxonomic significance of this difference is dubious. Finally, the lingual heel of the P1 of IVPP V3201 is wider, broader, and more fully developed than the P1 of any specimen of G. berkeyi ; however some species, such as Protitanotherium emarginatum show notable variation in the P1. Because IVPP V3201 does not offer conclusive evidence for a unique species, Arctotitan honghoensis is a nomen dubium, but it is possibly synonymous with Gnathotitan berkeyi .