Desmatotitan tukhumensis Granger and Gregory, 1943

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 : 33-35

Desmatotitan tukhumensis Granger and Gregory, 1943

HOLOTYPE: AMNH 21606 , a partial mandible with right i1–c, p2–m3, left i1, i3, and c.

TYPE LOCALITY: Ulan Shireh Formation, four miles north of Tukhum Lamasery, Inner Mongolia, China.

AGE: Middle Eocene (Irdinmanhan land mammal ‘‘age’’).

DIAGNOSIS: Desmatotitan tukhumensis is an intermediate-sized brontothere with three pairs of large incisors that strongly arch anterior to the canines. The i1 and i2 are semispatulate while the i3 is subcaniniform. The posterior margin of the symphysis extends to the p3 metaconid. There is both a postcanine diastema and a short p1–p2 diastema. The p1 is anchored obliquely in the jaw. Metaconids are absent on p2–p3 but present on p4. The molar ribs are weak, the lingual enamel is thinner than the labial enamel, and m3 is more elongate than those of Palaeosyops and Bunobrontops .

Desmatotitan tukhumensis is one of a few brontotheres with a p1–p2 diastema. Among these, D. tukhumensis is most similar in size and morphology to Mesatirhinus junius . It shares with M. junius a semispatulate i1 and i2. However, the premolars of D. tukhumensis are broader than those of M. junius , the crescents of the lower molars are more strongly rounded, and the p1 is anchored obliquely in the jaw.

DESCRIPTION

MANDIBLE AND LOWER DENTITION: The holotype jaw of Desmatotitan tukhumensis (AMNH 21606) includes a symphysis and right ramus with a complete set of teeth that are not heavily worn (fig. 11). The ramus is very thin and the roots of the cheek teeth are exposed, possibly reflecting poor health near the time of death. The left side of the symphysis is not preserved but it has been reconstructed with plaster. Judging by the intact right side, the proportions of the symphysis as it is reconstructed is accurate. The angle of the inferior margin of the symphysis is somewhat less then 45 °. The symphysis extended about to the middle of the p3.

The lower dental formula (3-1-4-3) is unreduced. The incisor row forms a strong arch anterior to the canines. Overall, the incisors are large and are all roughly of similar size. The crowns of the i1 and i2 are semispatulate and i3 is more subcaniniform and mesiodistally elongate but with a slightly shorter crown. The apices of both i1s are worn off. The distal apex of i2 forms a dull rounded tip. A distinct lingual cingulid is present on each incisor. The lingual cingulid of the i3 is less distinct than those of i1 and i2. The canine is both large in diameter and in height.

There are no diastemata between any of the incisors or canines. However, there is a relatively long postcanine diastema. On the alveolar surface, between the canine and p2, an elongate p1 alveolus extends into the mandible in a posteroventral direction, indicating that the root of p1 was anchored obliquely in the mandible. The position of the p1 alveolus indicates a short p1–p2 diastema.

The p2 trigonid is nearly twice as long as the talonid. Moreover, the trigonids of p3 and p4 are somewhat longer than their talonids. The lingual side of the p2 is incomplete although the trigonid and talonid of that tooth appear to have been similar in width. However, the talonids of p3 and p4 are distinctly wider than the trigonids. The paralophid of p2 curves slightly lingually, creating a small lingual trigonid basin. The protolophid is not curved although it is slightly lingually angled. The paralophid and protolophid of p3 have orientations similar to those of p2, but the protolophid is more curved and longer in p3, resulting in a shallow but broad lingual trigonid notch. The paralophid and protolophid of the p4 each arch fully lingually, creating a nearly molariform trigonid notch. The p2 and p3 lack metaconids, while p4 has a large lingually positioned metaconid. The p2 talonid has a short but distinct cristid obliqua and hypolophid; the lingual side of the p2 talonid is a slightly convex sloped surface. The cristids obliqua and hypolophids of p3 and p4 are longer, creating broader and more nearly molariform talonid basins. The labial premolar cingulids are strong, but they are discontinuous around the proximal bases of the protoconids.

The basins of the molars are shallow, the labial ribs are very weak, and the lingual enamel is thinner than the labial enamel. The m3 is proportionately longer than those of Palaeosyops or Bunobrontops , and is similar in proportions to Sthenodectes incisivum , but it is relatively short in comparison to most other brontotheres. The double crescentshaped lophids of the lower molars, particularly that of m3, are very rounded in shape. The labial molar cingulids are distinct and continuous around the proximal bases of the cusps. There is an additional strong beaded cingulid on the lingual side of the m3 hypoconulid, although lingual cingulids are otherwise absent in the holotype of D. tukhumensis . The m2 shows the tendency for the protoconid and hypoconid to form rounded exposures of dentin in early stages of wear, a plesiomorphic wear pattern seen in basal brontotheres such as Palaeosyops and Eotitanops . In more progressive states of wear, characterized by the m1, this plesiomorphic wear pattern is not notable.

REMARKS

Granger and Gregory (1943) recognized AMNH 21606 as a distinct species and concluded that it was a close ally of Metatelmatherium Granger and Gregory (1938) . With only a single mandible, it is difficult to confirm Granger and Gregory’s conclusion. Characteristics of the molars, such as the weak labial ribs, and the thinner lingual enamel are different from Palaeosyops and Bunobrontops and suggest a close relationship with more advanced brontotheres. Among more advanced brontotheres, a p1– p2 diastema is seen in Dolichorhinus , Mesatirhinus , Telmatherium , and Metatelmatherium . Among these, only Metatelmatherium occurs in Asia. However, the incisors of Desmatotitan tukhumensis are decidedly less subcaniniform than those of Metatelmatherium . In this respect, D. tukhumensis more closely resembles Mesatirhinus . However, the relatively broad premolars and very rounded lower molar crescents are out of character with North American specimens of Mesatirhinus junius . In this respect D. tukhumensis seems to resemble Microtitan mongoliensis , although that species differs from D. tukhumensis in having a much more elongate m3, narrower premolars, much smaller body size, and absence of a p1–p2 diastema. For this reason Desmatotitan tukhumensis is accepted, although more material is clearly needed to generate a more thorough diagnosis of this species.