Eotitanotherium, Peterson 1914 c

Mader, Bryn J., 2008, A species level revision of Bridgerian and Uintan brontotheres (Mammalia, Perissodactyla) exclusive of Palaeosyops, Zootaxa 1837 (1), pp. 1-85 : 70-72

Genus EOTITANOTHERIUM Peterson 1914c

= Diploceras sensu Peterson 1914a

Age. Uintan.

Subage. Late Uintan.

Type species. E. osborni ( Peterson 1914a) .

Included species. Genus is monospecific.

Diagnosis. Large-sized (length P2 to M3 approximately 221–236 mm) diplacodont brontothere with short, angular horns that are elliptical in cross-section; gracile nasals that are distally rounded and downturned; and two well separated lingual cusps on P4. In general, the length of the cheek tooth series is longer than in most specimens of Pseudodiplacodon , but the skull is smaller in size.

Discussion. In 1914 O.A. Peterson described a new brontothere taxon, Diploceras osborni , based on the anterior part of a skull ( Fig. 24 ) and jaws with a large part of the postcranial skeleton (CM 2859; Peterson 1914a). Shortly later, Peterson (1914c) changed the generic name to Eotitanotherium when it came to his attention that the name Diploceras was occupied by a mollusk. In addition to the type specimen, Peterson (1914a) had also designated several paratypes for " Diploceras " osborni , which included a skull (CM 2858) that is referable to Pseudodiplacodon progressum (see Mader 2000).

Osborn (1929, p. 196) stated that Eotitanotherium is doubtfully separated from Diplacodon Marsh , but he provisionally (p. 435) recognized both as valid genera. Mader (1989; 2000) also noted the close similarity between Eotitanotherium and Diplacodon but refrained from synonymizing the genera because of the poor condition of Marsh's type of Diplacodon elatus (the type species of Diplacodon ), the relatively few specimens of Eotitanotherium that are known, and our poor understanding of Uintan horned brontotheres in general.

Eotitanotherium has two distinct lingual cusps on the third and fourth upper premolars. This morphology contrasts with specimens of Pseudodiplacodon , in which the lingual cusps on P3 and P4 are poorly separated. The premolars of Eotitanotherium are very similar, however, to the premolars of the type of Diplacodon elatus , in which the lingual cusps on P3 and P4 are quite distinct and well separated.

Mader (1989) reported that the type of Diplacodon elatus is 6 to 7% larger (based on tooth measurements) than the largest specimen of Pseudodiplacodon , while it is very similar in size to specimens of Eotitanothe- rium. Later, however, Mader (2000) noted that this observation is no longer entirely valid since there is one specimen of Pseudodiplacodon (FMNH P 14632) that falls within the size range of Eotitanotherium and the type of Diplacodon elatus .

It is very likely that Eotitanotherium is a junior synonym of Diplacodon , but the type of Diplacodon elatus is so poorly preserved that it is difficult to be certain of this. Furthermore, the specimens of Eotitanotherium that are available are so few and so fragmentary that it cannot be honestly said that the taxon has been well characterized. As a group, Uintan aged horned brontotheres are still rather poorly known, and I believe that there is at least one valid genus in addition to the three recognized here (see Discussion section for Protitanotherium ). I continue, therefore, to refrain from formally synonymizing Eotitanotherium and Diplacodon . For the purposes of the present paper I regard Eotitanotherium as a valid taxon but treat Diplacodon as a nomen dubium. When Uintan horned brontotheres are better understood, it may be possible to determine with certainty whether the type of Diplacodon elatus represents the same taxon as the type of Eotitanotherium osborni .

As noted with regard to the genus Pseudodiplacodon , above, Mihlbachler (2005) synonymized that taxon with Eotitanotherium and incorporated both into a single species, Diplacodon elatus . Although Mihlbachler asserted that there is a morphological continuum between specimens assigned by Mader (1989; 2000) to Pseudodiplacodon and Eotitanotherium , I believe that there are several important faults with his analysis. The morphology of the horn (as opposed to horn size) is distinctly different between the two forms and, despite the claim to the contrary, the differences in premolar morphology cited by Mader (2000; present paper) remain valid. Furthermore, the fact that Eotitanotherium is smaller in size, but has a longer tooth row than Pseudodiplacodon , suggests an adaptive difference and implies that they should be considered discreet taxa. For a more detailed consideration of these various arguments, see the Discussion section of the present paper for the genus Pseudodiplacodon .

Pilgrim (1925) described several brontothere remains from the Pondaung Formation of Burma, which he provisionally referred to the genus Eotitanotherium and named Eotitanotherium (?) lahrii . Colbert (1938) selected from among Pilgrim's cotypes a right maxilla fragment with two teeth intact (GSI C342) as a lectotype for the species. Colbert, however, tentatively recognized this taxon as a valid species of Metatelmatherium rather than of Eotitanotherium . As Colbert (1938) noted, the lectotype of this taxon is so imperfectly preserved that it is practically impossible to make conclusions about its taxonomic position and, in light of this difficulty, I believe that Eotitanotherium (?) lahrii should probably be regarded as a nomen dubium.

No other North American species of Eotitanotherium have been proposed since Peterson named the type species in 1914. Only three skulls are known (see referred specimens for E. osborni below) and none of these are complete. Thus the sample is insufficient for a detailed morphologic or statistical analysis and there is no basis for the recognition of more than a single species.

Figures

FIGURE 24. Holotype skull of Eotitanotherium osborni (CM 2859) in lateral view. After Peterson, 1914a.