Metatelmatherium ultimum, (Osborn 1908)

Mader, Bryn J., 2008, A species level revision of Bridgerian and Uintan brontotheres (Mammalia, Perissodactyla) exclusive of Palaeosyops, Zootaxa 1837 (1), pp. 1-85 : 80-82

Species METATELMATHERIUM ultimum ( Osborn 1908)

= M. uintensis ( Douglass 1909)

Holotype. AMNH 2060 , a skull and lower jaw.

Referred specimens. AMNH 2004, CM 2339, CM 2388 (holotype of " Manteoceras " uintensis ), CM 11380. AMNH 2029, a badly crushed skull also appears to be referable to this species.

Diagnosis. Species of Metatelmatherium distinguished from the type species by the shorter mandibular symphysis and possibly by a relatively short and less wide posterior cusp on P1, relatively small P2, relatively thick cingulum on P3 and P4, relatively narrow occiput, and relatively narrow coronoid process.

Discussion. As noted in the preceding discussion for the genus Metatelmatherium, Osborn described the holotype skull ( Fig. 26 ) and lower jaw of M. ultimum (AMNH 2060) in 1908 and recognized it as a new species of Telmatherium . After examining this specimen, however, Granger and Gregory (1938) correctly concluded that Telmatherium ultimum Osborn was congeneric with the Asian brontothere Metatelmatherium cristatum .

In 1909 Douglass described the front part of a skull (CM 2388) from the Uinta Basin of Utah, which he identified as a new species of Manteoceras , M. uintensis ( Fig. 30A 1 and A 2 ). In 1929 Osborn upheld this identification, but Mader (1989) concluded that this skull is not referable to Telmatherium (= Manteoceras ) but is instead a specimen of Metatelmatherium . This conclusion was based on the skull's large canine, long diastema, and forwardly placed lateral incision of the external nares, all of which are diagnostic of Metatelmatherium (see Fig. 30 ). The zygomatic arches are imperfectly preserved in the type skull of " Manteoceras " uintensis , but may have had the flange on the underside of the jugal that is also diagnostic of Metatelmatherium .

Osborn (1929) listed the following characters, which he believed allied Douglass's specimen to Manteoceras : the presence of round-topped incisors; robust, recurved canines; twin convexities on the premolar ectolophs; a broad, subhypsodont M3 with large parastyle and mesostyle; and widely arched zygomata. Each one of these characters is also typical of Metatelmatherium , however, and I restate my opinion that the similarities to Metatelmatherium heavily outweigh any similarities to Telmatherium .

Most specimens of Metatelmatherium ultimum have the distinct flange on the base of the jugal that is apomorphic for the genus. As noted above, however, a skull (CM 11380) referable to Metatelmatherium from the Sand Wash Basin of Colorado lacks the characteristic flange but resembles specimens of Metatelmatherium ultimum in all other respects. Because of this morphologic difference and the earlier age of the Sand Wash specimen, it is possible that the specimen represents an entirely new species. Until the intraspecific variation of M. ultimum can be better documented, however, I choose to regard the Sand Wash specimen as representing M. ultimum and have included it in the list of referred specimens above. Even if the specimen should prove to be a distinct species, however, I find no basis for referring it to a new genus as suggested by Mihlbachler (2005).

Figures

FIGURE 1. Scatter plots showing length of M2 plotted against width of M2 for five brontothere genera. The solid diagonal line that crosses both graphs represents the points at which the length and width of the tooth are equal. Specimens that plot out above the line have teeth that are wider than long, while specimens that plot out below the line have teeth that are longer than wide. Specimens that plot out directly on the line have teeth in which the length is equal to the width and are thus square in shape. Black symbols, subfamily Dolichorhininae; white symbols, other brontothere genera (Palaeosyops and Telmatherium).

FIGURE 2. Skull of Mesatirhinus megarhinus in A, anterior; B, dorsal; C, ventral; and D, lateral view. After Osborn. 1929.

FIGURE 26. Holotype skull of Metatelmatherium ultimum (AMNH 2060) in A, anterior; B, ventral; C, dorsal; and D, lateral views. A, B, and C after Osborn, 1929; D after Osborn, 1908 and 1929.

FIGURE 30. Comparison between the holotype skull of Manteoceras uintensis (CM 2388) and a skull of Metatelmatherium ultimum. A1, holotype skull of Manteoceras uintensis in ventral view; A2, the same in lateral view; B, skull of Metatelmatherium ultimum (AMNH 2004, reversed). All figures after Osborn, 1929.