Dolichorhinus vallidens ( Cope, 1872 )

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 : 329-330

Dolichorhinus vallidens ( Cope, 1872)

LECTOTYPE: AMNH 5098 , a partial mandible (in two pieces) with right c (partial), p2, dp4, m1–m3, and left p4–m3.

TYPE LOCALITY: ‘‘Mammoth Buttes, Southwestern Wyoming, near headwaters of Bitter Creek’’; Adobe Town Member (Washakie B of Granger, 1909) of the Washakie Formation, Wyoming

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

DETERMINATION: Nomen dubium: holotype lacks diagnostic features.

DESCRIPTION

AMNH 5098 includes a pair of right and left mandible fragments that are most similar in size to Dolichorhinus hyognathus , Sphenocoelus uintensis , and Telmatherium validus (fig. 164). Dental dimensions are as follows: p2L 5 20.6, p2 W 5 12.0, p4L 5 22.8, p4 W 5 16.2, m1L 5 30.9, m1 W 5 19.8, m2L 5 35.6, m2 W 5 22.2, m3L 5 57.6, m3 W 5 23.6, p2–p4 5 64.6, m1–m3 5 122.45, p2–m3 5 188.0. The ramal fragments are dorsoventrally shallow, and the right ramus appears to be unusually short in comparison to Dolichorhinus hyognathus mandibles. However, the canine root has been plastered onto the specimen in an artificially posterior position. The erect orientation of the canine is artificial also. The crown of the canine is not preserved, but the diameter of the root indicates a canine of typical size. No bone anterior to the p1 alveolus is preserved. The p2 crown is held onto the ramus by plaster, however, remnants of in situ p2 roots are visible on the dorsolabial surface of the ramus.

A short diastema (, 7 mm) exists between the p1 alveolus and the p2 roots, although it has been incorrectly represented in previous figures. Cope’s (1884) figure of a left ramus (pl. 53, fig. 1) that was reprinted in Osborn (1929a: fig. 95: A2) is a composite drawing of the left ramal fragment and the mirror image of the anterior part of the right ramal fragment. In this figure, a hypothetical outline of the missing p1 is drawn right next to the p2 with no diastema between them. However, this drawing is inaccurate. The right ramus of the specimen clearly has a short diastema between p1 and p2. Another figure in Cope (1885: pl. 52, fig. 3; reprinted in Osborn 1929a: fig. 95: A1) correctly illustrates the p1–p2 diastema with the root of the p1 positioned several mm anterior to the p2. Other aspects of the mandible and lower dentition of AMNH 5098 are typical of other hornless middle Eocene brontotheres.

REMARKS

Cope (1872) originally referred this species to the genus Palaeosyops . He described three specimens from three individuals, AMNH 5097 (portions of an upper dentition), AMNH 5099 (more upper dental elements), and AMNH 5098 (right and left partial mandibular rami), but did not designate a holotype. However, Cope later stated that the ‘‘lower jaws may be regarded as typical of the species’’ ( Cope, 1884: 700). Osborn (1929a) concluded from this statement that Cope intended the mandible (AMNH 5098) to be the type, and therefore designated it as the lectotype. While the right ramus retains a dp4, the left ramus has a fully erupted and slightly worn p4. However, the degree of wear on the molars of these two jaw fragments is identical, so it remains probable that these pieces are from one individual.

The lectotype was reassigned to the genus Telmatherium by Osborn (1895) and then to Manteoceras by Hatcher (1895). Finally, Osborn (1929a) questionably referred this species to the genus Dolichorhinus , but he continued to recognize it as a valid species due to the artificially shortened lectotype jaw. The lectotype of Dolichorhinus vallidens has a distinct p1–p2 diastema, by which it can be distinguished from Sphenocoelus uintensis , a brontothere of similar size and dental morphology. The dimensions of the teeth that are preserved with the lectotype fall above the size ranges of Mesatirhinus megarhinus , Metarhinus fluviatilis , and M. abbotti . Metatelmatherium ultimum and Telmatherium validus are two other taxa that have short p1– p2 diastemata. The dental dimensions of the lectotype of D. vallidens fall below the lower size limit of M. ultimum . However, the fragmentary lectotype specimen (AMNH 5098) is morphologically consistent with, and falls within the dental dimensions of Telmatherium validus and Dolichorhinus hyognathus . Mader (1989) came to a similar conclusion. Therefore Dolichorhinus vallidens Cope is best considered a nomen dubium.