Protitanotherium, Hatcher 1895

Mader, Bryn J., 2008, A species level revision of Bridgerian and Uintan brontotheres (Mammalia, Perissodactyla) exclusive of Palaeosyops, Zootaxa 1837 (1), pp. 1-85 : 63-66

Genus PROTITANOTHERIUM Hatcher 1895

Age. Uintan.

Subage. Late Uintan.

Type species. P. emarginatum ( Hatcher 1895) .

Included species. Genus is monospecific.

Diagnosis (based on holotype of type species only)–Large-sized (cheek tooth series measurements unknown) plesiomorphic diplacodont brontothere with laterally directed horns that are elliptical in cross-section, short, broad nasals, and a robust canine.

Discussion. In 1895 Hatcher described an unusual brontothere skull with small horns (YPM-PU 11242, see Fig. 21 ) that he provisionally referred to the genus Diplacodon Marsh (type species D. elatus ). Although Marsh (1875) had asserted that Diplacodon lacked horns, Hatcher pointed out that Marsh's claim was probably conjectural, because the type of Diplacodon elatus (YPM 11180) lacked the entire horn and nasal region. Hatcher argued that in all other respects his specimen appeared to be identical to Diplacodon elatus but larger in size. He therefore recognized the specimen as a new species of Diplacodon , which he named Diplacodon emarginatum . Hatcher proposed that the generic name Protitanotherium be applied to the specimen, however, if there should prove to be hornless forms with the same dental characters.

According to Hatcher (1895), the cranial vertex in the type of Protitanotherium emarginatum is "slightly concave antero-posteriorly? and is further characterized by the absence of a sagittal crest." Because only the anterior part of the type skull is preserved this statement may be conjectural, but it is possible that Hatcher observed the cranial vertex in the field. Hatcher (1895) stated that when the type of P. emarginatum was collected, the posterior part of the skull had already weathered out and was badly damaged but many of the pieces were fitted together. According to Hatcher, these fragments show some of the more important characters of the posterior region of the skull. If any portion of the posterior part of the skull was collected by Hatcher, however, none of it is preserved in the Princeton collection at Yale University.

Osborn (1929) recognized the genus Protitanotherium as valid and argued that it was distinct from Diplacodon regardless of whether Diplacodon had horns or not. Most subsequent authors have also recognized Protitanotherium as valid ( Simpson 1945; Mader 1989; 1998; McKenna & Bell 1997), although Lucas (1983) and Schoch and Lucas (1985) synonymized Protitanotherium with Diplacodon . Mader (1989; 2000) argued that Protitanotherium is valid and that Diplacodon is a possible senior synonym of Eotitanotherium (see also Osborn 1929, p. 196).

In 1908 Osborn described the lower jaw (AMNH 2501) of a very large brontothere from the Uinta Basin, which he referred to the genus Protitanotherium . Osborn noted several differences between this specimen and the jaw included in the type material of Protitanotherium emarginatum and concluded that it represented a new species of Protitanotherium , which he named P. superbum . Osborn (1908; 1929) never fully explained why he referred the specimen to Protitanotherium and admitted some doubt as to whether the generic assignment was correct (1929, p. 185).

The type of Protitanotherium emarginatum has large, robust, canine tusks; short, broadly rounded horns that are elliptical in cross-section at the base; and distinctive nasals that are short and distally squared. No other specimens with this exact combination of morphological characters has been recovered from the Uinta Formation or chronologically equivalent deposits. There are, however, two distinct Uintan aged forms that possess horns very similar to those of the type of P. emarginatum .

The first of these forms is represented by a single, nearly complete skull (CM 10200) with a widened, saddle-shaped cranial vertex and both horns and nasals that are very similar morphologically to those of the type of Protitanotherium emarginatum . Unlike the type of P. emarginatum , however, CM 10200 has a canine that is rather small in size.

The second form is represented by a series of skulls (AMNH 117163; TMM 41723-3, the holotype of " Sthenodectes " australis Wilson ; TMM 41747-106; and UFH V 81.3.1) from localities in Utah (Uinta Formation), Wyoming (Wiggins Formation), and Texas (Devil's Graveyard Formation). These specimens represent a single taxon that is similar to the type of Protitanotherium emarginatum in the shape of the horns and presence of a large canine tusk, but differing in the shape of the nasals, which are relatively longer and distally rounded. The nasals of this form are virtually identical to those of Pseudodiplacodon progressum and Eotitanotherium osborni .

Regardless of the generic identity of the second form, it is the most plesiomorphic horned brontothere known. The cranial vertex, although sufficiently widened to eliminate the sagittal crest, is very narrow and does not nearly approach the wide, saddle-shaped morphology characteristic of all other horned brontotheres (diplacodonts). Furthermore, the premolars are rather plesiomorphic, in that some specimens (TMM 41723-3) do not possess more than a single lingual cusp on P3 and P4 (as in Eotitanops and the outgroup perissodactyls Hyracotherium and Homogalax ) while other specimens (AMNH 117163) have two lingual cusps on these teeth as do all other horned brontotheres.

Given the incompleteness of the type specimen of Protitanotherium emarginatum , it is possible that either CM 10200 or the plesiomorphic series of skulls discussed above could be referable to the genus Protitanotherium . Both forms are similar to the type of P. emarginatum in some respects but exhibit important differences as well. I do not believe that both forms could be referable to Protitanotherium because the primitiveness of one form (the series of skulls) and the more highly derived state of the other (CM 10200) seem to preclude the possibility that they represent the same genus.

Thus, at least two brontotheres with short, broadly rounded horns existed during the Uintan Land Mammal Age. One of these is Protitanotherium and the other may be represented by either CM 10200 or the series of skulls listed above. Alternatively, the type of Protitanotherium emarginatum, CM 10200, and the series of skulls could all represent different genera, in which case there are three Uintan brontothere genera with similar horn morphology.

Because of the uncertainties involved, I am reluctant to assign generic names to either of the two forms discussed above and choose to restrict the name Protitanotherium to the type of P. emarginatum only. Mihlbachler (2005), however, has concluded that the name Protitanotherium should be applied to the plesiomorphic form that includes the holotype of " Sthenodectes " australis (TMM 41723-3).

Figures

FIGURE 21. Holotype skull (YPM-PU 11242) of Protitanotherium emarginatum in A, anterior; B, dorsal; and C, lateral views. A and C after Osborn, 1929; B after Hatcher, 1895.