Brachydiastematherium transylvanicum Böckh and Maty, 1876

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 : 203-205

Brachydiastematherium transylvanicum Böckh and Maty, 1876

HOLOTYPE: Anterior part of a lower jaw with several incisors, right c, p2–p3 and left c–m1.

TYPE LOCALITY: Andrásháza (Seibenbürgen), Hungary (Transylvania), eastern Hungary, about 150 miles northeast of Belgrade.

AGE: Middle Eocene (?).

DIAGNOSIS: Brachydiastematherium transylvanicum is a large brontothere with a mandible very similar to Metatitan . Features of the holotype jaw are a robust mandibular symphysis that extends behind the p4, intermediately sized incisors that are semispatulate and subcaniniform that form an arched row anterior to the canines. A postcanine diastema is absent. A metaconid is absent on p2, but present on p3 and p4. The p2 trigonid and talonid have similar lengths. The labial notch in the crown of p2 is broad.

Brachydiastematherium transylvanicum can be distinguished from Metatitan relictus and M. primus by its larger more spatulate incisors with distinct lingual cingulids and a much broader labial p2 notch. B. resembles M. khaitshinus in these respects; however, the p4 of B. transylvanicum has an autapomorphic crest extending posteriorly from the middle of the cristid obliqua.

DESCRIPTION

MANDIBLE AND LOWER DENTITION: The holotype of Brachydiastematherium transylvanicum consists of the anterior portion of a mandible with an incomplete set of lower teeth. The following description is based primarily on cast material in the AMNH (108188) and the figure provided by Osborn (1929a), reprinted here as fig. 98. The angle of the inferior margin of the symphysis was relatively shallow. The posterior margin of the symphysis is incomplete ; however, what is left indicates that it originally extended behind the talonid of p4. The alveolar incisor surface is incomplete and all but one of the incisors (left i3) is completely detached. However, the preserved portion of the symphysis seems to indicate that the incisor row arched slightly anterior to the canines. The incisors are of moderate size. One of these incisors is semispatulate, while the other is slightly more conical. Each of the incisors has a distinct beaded lingual cingulid. The canines are somewhat large and have prominent lingual cingulids.

Brachydiastematherium transylvanicum lacks a postcanine diastema. The p1 has a single cusp and a broad talonid heel. The trigonid of the p2 is only slightly longer than the talonid; the p3 trigonid is about the same length as the talonid, and the p4 trigonid is shorter than the talonid. The paralophid of p2 is slightly angled lingually, creating a small but distinct lingual notch. The p2 protolophid is straight but deflected slightly lingually. The trigonids of the p3 and p4 are more molariform with paralophids and protolophids that arch fully lingually, creating nearly molariform basins. The talonids of p2, p3, and p4 are well developed with long cristids obliqua, long hypolophids, and broad, valleylike talonid basins. Lingual premolar cingulids are absent. The labial cingulid of the p2 is relatively faint and discontinuous around the base of the protoconid and hypoconid. However, the labial cingulids of p3 and p4 are thin, but they are distinct and continuous. Finally, the p4 has an unusual crest of enamel extending posteriorly from the middle of the cristid obliqua. This crest is also present on p3, but it is shorter and less distinct. The left m1 is the only preserved lower molar. It is heavily worn and lacks any features worthy of mention.

REMARKS

Böckh (1876) described the first undoubt- ed brontothere from eastern Europe, Brachydiastematherium transylvanicum . Presently, no other material is directly referable to this species. This fossil is considered to be middle Eocene (Uintan equivalent) by Lucas and Schoch (1989a), although this assessment is based on the opinion, expressed by Lucas (1983a) and Lucas and Schoch (1989a), that B. transylvanicum is a senior synonym of North American Uintan (Uinta C) genus, Diplacodon . However, B. transylvanicum differs from Diplacodon elatus in significant ways. For instance, there is no postcanine diastema; the p3 metaconid is essentially molariform, and the talonids of the premolars are generally much broader. Brachydiastematherium is similar to Metatitan in these respects.

In an analysis of Asian brontothere relationships, Mihlbachler et al. (2004a) found Brachydiastematherium to share close phylogenetic affinity with Metatitan and considered the possibility that Metatitan is actually a junior synonym of Brachydiastematherium . In particular, B. transylvanicum is very similar to M. khaitshinus . Both species share moderately sized semispatulate incisors and a broad labial notch on the p2. M. relictus and M. primus have smaller more globular incisors and a much narrower labial notch on the p2. However, the unusual enamel crest extending posteriorly from the cristid obliqua of the p4 of B. transylvanicum is not seen in M. khaitshinus . The taxonomic significance of this unusual crest is uncertain. It could be an autapomorphic character of B. transylvanicum , although it is also possible that it is an anomalous characteristic of the individual represented by the only known specimen. Presently I consider it to be an autapomorphy of B. transylvanicum . If my interpretation of this character is correct, it validates B. transylvanicum , although better material is clearly needed.

When considering the taxonomic identity of Brachydiastematherium transylvanicum , one must also consider other brontothere fossils found in Europe. Nikolov and Heissig (1985) described several brontothere teeth from the Black Sea Coast of Bulgaria, representing the second find of European brontotheres. The incomplete nature of European brontothere fossils limits comparison of the material described by Nikolov and Heissig (1985) with the holotype of B. transylvanicum . It is nonetheless possible that they represent more material of the same species. However, the upper premolars of these specimens are most consistent with Sivatitanops birmanicum , a species otherwise known only from Pondaung sandstones of Myanmar, southeast Asia. A partial skull from Pondaung deposits that is possibly Sivatitanops birmanicum is also similar to the jaw holotype of B. transylvanicum in lacking a postcanine diastema, but having incisors that are somewhat larger and less globular than other brontotheres (e.g., Metatitan ) that lack postcanine diastemata. It is therefore possible that S. birmanicum is a junior synonym of B. transylvanicum or that these species are closely allied (along with Metatitan ?). (For more discussion of the possible synonymy of these taxa see remarks under S. birmanicum .)