Sphenocoelus uintensis, Osborn 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 : 44-45

Species SPHENOCOELUS uintensis Osborn 1895

= S. blairi ( Cook 1926)

= S. bridgeri ( Cook 1926)

= S. harundivorax ( Cook 1926)

Holotype. AMNH 1501 , the posterior part of a skull.

Referred specimens. DMNH 479, DMNH 484, DMNH 541, DMNH 507, DMNH 8000, and UCMP 81301.

Diagnosis. Plesiomorphic species of Sphenocoelus of moderately large to large size (length P1 to M3 approximately 190–214 mm, length P2 to M3 approximately 172–199 mm, length M1 to M3 approximately 114–128 mm, based on DMNH 507 and DMNH 541) with a prominent sagittal crest (plesiomorphic character state) and very large occipital condyles (presumably a derived condition). The horn and the anterior flange of the suborbital protuberance may be absent or weakly developed in specimens of this species.

Discussion. The massive size of the occipital condyles appears to be a synapomorphy diagnostic of this species, although the occipital condyles of the more derived species, S. angustidens , are unknown. The occipital condyles of S. intermedius and S. hyognathus are proportionately smaller and appear to be similar to those of other brontotheres and outgroup perissodactyls such as Hyrachyus .

Skulls referred by Cook to Tanyorhinus are more plesiomorphic than those that have been assigned to Dolichorhinus or Dolichorhinoides in that there is a narrow sagittal crest similar to that of other early brontotheres (e.g., Palaeosyops , Mesatirhinus , and Metarhinus ) and outgroup perissodactyls. In specimens referred to both Dolichorhinus and Dolichorhinoides the cranial vertex is widened, a derived condition paralleling the widened cranial vertex seen in the brontotheriine brontotheres (see Mader 1998). Skulls of Tanyorhinus compare very closely, however, with the partial type skull of Sphenocoelus uintensis , which is hyperdolichocephalic and has a narrow sagittal crest, and a skull (UCMP 81301) insightfully identified as S. uintensis by Bruce Hanson in the UCMP catalog.

Among these plesiomorphic specimens of Sphenocoelus , four species have been recognized: S. uintensis (the type species), S. blairi , S. bridgeri , and S. harundivorax . The last three were the three species originally assigned to the genus Tanyorhinus by Cook (1926).

Cook did not specify which of the three species that he recognized was the type species for Tanyorhinus and the type species was never formally fixed by any subsequent systematic reviser. The catalog of the Denver Museum of Natural History (the institution where Cook’s specimens are housed) identifies the type of T. blairi (the first species mentioned in Cook’s publication) as the holotype of the type species, but this cannot be recognized as an official determination. In their published catalog of type specimens in the Denver Museum, however, Woods and Stucky (1992) cite T. blairi as the type species of Tanyorhinus Cook and, by doing so, have effectively fixed it as such under Article 69 of the International Code of Zoological Nomenclature ( Ride et al. 1999).

West and Dawson (1975) argued that the three species of Tanyorhinus that Cook had recognized are biologically unnecessary because the differences that Cook cited between them could be attributed to sexual dimorphism and ontogenetic age. I concur with West and Dawson that only one species is probably represented, but note that the type of T. harundivorax (DMNH 552) is a partial lower jaw and is, thus, not directly comparable with at least one of the other Tanyorhinus types. Tanyorhinus bridgeri was based on a skull only (DMNH 479), while Tanyorhinus blairi was based on a skull (DMNH 541) and jaw (DMNH 542). The skull and jaw were not found in articulation, however, but rather a few feet apart and were only assumed to represent the same individual ( Cook 1926).