Sphenocoelus intermedius, (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 : 45-48

Species SPHENOCOELUS intermedius ( Osborn 1908)

= S. heterodon ( Douglass 1909)

= S. fluminalis ( Riggs 1912)

= S. superior ( Riggs 1912)

Holotype. AMNH 1837 , a skull and associated postcranials.

Referred specimens. CM 2340 (holotype of S. heterodon ) , FMNH P 12168 (holotype of S. superior ) , FMNH P 12193 , FMNH P 12200 , FMNH P 12205 (holotype of S. fluminalis ) , FMNH PM 3870 , LACM 128402 , UCMP 31845 , UCMP 31846 , YPM-PU 11241 .

Diagnosis. Derived species of Sphenocoelus , similar to S. hyognathus , but 10% to 15% smaller in size (length P1 to M3 approximately 173–191 mm, length P2 to M3 approximately 154–175 mm, length M1 to M3 approximately 101–112 mm). The anterior flange on the suborbital protuberance is usually small or absent in this species.

Discussion. In 1908 Osborn briefly described the type skull (AMNH 1837) of a small Sphenocoelus from the Uinta Basin of Utah, which he identified as a new species of Dolichorhinus , D. intermedius . Osborn distinguished “ Dolichorhinus ” intermedius from “ Dolichorhinus ” hyognathus , by its smaller size, less progressive premolars with subconic deuterocones (= protocones), less robust cingula, more pointed and less distally expanded nasals, and narrower infraorbital shelf. In 1929, Osborn speculated that “ Dolichorhinus ” intermedius was a dwarfed side branch of “ Dolichorhinus ” not directly related to “ Dolichorhinus ” hyognathus .

In 1909 Earl Douglass named two new species of “ Dolichorhinus ,” “ D.” heterodon (= Sphenocoelus intermedius in the present paper) and “ D.” longiceps (= S. hyognathus , see discussion for S. hyognathus , below). Douglass described the holotype skull of “ Dolichorhinus ” heterodon (CM 2340) in some detail, but he did not compare it to either “ Dolichorhinus ” hyognathus or “ Dolichorhinus ” intermedius . Osborn (1929, p. 416), however, who accepted “ Dolichorhinus ” heterodon as a valid species, distinguished it from “ D.” hyognathus by its smaller size and from “ D.” intermedius by its heavier cingula, well developed mesostyle on P4, and prominent parastyle on P3 and P4. Osborn stated that “ D.” heterodon might be a more progressive successor of “ D.” intermedius , but elsewhere (1929, p. 188) expressed doubt that it could be separated taxonomically from “ D.” intermedius .

My own impression is that the upper dentition of the holotype of “ Dolichorhinus ” heterodon falls within the morphological continuum of dentitions here referred to Sphenocoelus intermedius . Although the presence of a mesostyle on P4 is somewhat uncommon, this and other unusual premolar morphologies are occasionally found in brontothere genera. The holotype of “ Dolichorhinus ” fluminalis (FMNH P 12205, discussed below) also appears to have a mesostyle on P4 (observation based on a cast of the type) as does CM 11081, a specimen of Sphenocoelus hyognathus . The presence of a mesostyle on P4 is probably not a good diagnostic character for any brontothere species and appears to be an anomaly restricted to certain exceptional individuals.

In 1912 Elmer Riggs described several new brontothere taxa based on specimens collected from the Uinta Basin by the Field Museum expedition of 1910. Among the materials that Riggs described was the type of a new species of “ Dolichorhinus ,” “ D.” fluminalis . According to Riggs the primary diagnostic character of “ D.” fluminalis was that the internal nares were located further posteriorly (between the hamular processes) than in any other species. Otherwise Riggs regarded “ D.” fluminalis as being rather closely related to “ D.” intermedius , except that the skull of “ D.” fluminalis was longer and the molars much smaller. Riggs also noted that the skull of “ D.” fluminalis was more gracile than that of “ D.” cornutum and that the postorbital process of the jugal (incorrectly described as the “jugal process of the maxillaries” on page 34) was located posterior to the last molar rather than adjacent to it as in D. longiceps .

Peterson (1924) provisionally accepted “ Dolichorhinus ” fluminalis as valid, but argued that the internal nares are located very far back in all specimens of “ Dolichorhinus .” As Peterson noted, the true position of the internal nares was seldom observed by his contemporaries because of the inherent frailty of this region of the skull. The anatomy of the palatine and pterygoid regions of Sphenocoelus is rather unusual and a description of it is warranted here to help clarify Peterson’s observations on the narial morphology.

Figure 16 illustrates the palatine and pterygoid region in a well-preserved specimen of Sphenocoelus (in this case S. hyognathus , discussed below, based mostly on AMNH 13164). It will be noted from this illustration that adjacent to M3 there is a depression in the palate (marked “A”) that may represent the plesiomorphic position of the internal nares ( Peterson 1924). Osborn, (1929, p. 411 and fig. 347) described this depression as a secondary palatal plate formed by a backward and upward extension of the dorsal surface of the palatine. Behind the depression is an orifice (marked “B”) that represents the actual position of the internal nares, although these were evidently sealed off and non-functional. Extending posteriorly from this orifice to the basicranial region is an extension of the nasal septum that forms a thin flange of bone that projects ventrally between the pterygoids. This posterior narial flange (vomerine plate of Peterson 1924) is a common feature of brontothere skulls and has been identified in the majority of genera ( Mader 1998).

In well-preserved skulls of Sphenocoelus , thin-walled, bony pouches (shaded gray on the diagram) surround the vomerine plate on either side. Peterson described these pouch-like structures but, in the text of his paper, did not identify the bones from which they are formed. In his figure captions for plate XLV, fig. 3 (illustrating CM 11080) and plate XLVI, fig. 2 (illustrating CM 11081), however, he identified these pouches as the choanae, an identification that seems plausible. Posterior to the choanal pouches and near the termination of the vomerine plate at the basicranium are two large apertures (marked “C” on the diagram) that appear to have been the functional internal nares.

The palatine and pterygoid region of the holotype of “ Dolichorhinus ” fluminalis thus do not appear to be anatomically different from that of other specimens of Sphenocoelus (such as AMNH 1845 and AMNH 13164, both specimens of the large derived Sphenocoelus species , S. hyognathus ). Peterson was correct, therefore, in not accepting the position of the internal nares in S. fluminalis as a diagnostic character of the species.

The location of the postorbital process of the jugal posterior to the third molar is also not an appropriate diagnostic character for Sphenocoelus fluminalis because the position of the process in the holotype skull has been altered by taphonomic deformation. In the holotype the dorsal surface of the skull is shifted posteriorly while the ventral surface is shifted anteriorly. Thus, the lambdoidal crest is sharply angled backward and the nasals are retracted from their normal position over the premaxillary region. As a result of this deformation the postorbital process has been shifted backward so that it lies behind the third molar rather than adjacent to it.

It should be noted that the basilar length of the skull in the type of Sphenocoelus fluminalis is rather large (about 515 mm), and is only about 5% shorter than the smallest skull belonging to the larger derived Sphenocoelus size group (FMNH P 12175, approximately 541 mm). It is possible, therefore, that S. fluminalis might be a junior synonym of S. hyognathus rather than S. intermedius . Dental measurements for the type of S. fluminalis , however, all fall clearly within the size range of S. intermedius .

In addition to the holotype of “ Dolichorhinus ” fluminalis, Riggs (1912) also described a skull (FMNH P 12168) that he identified as a new species of Mesatirhinus , M. superior . Peterson (1924) and Osborn (1929), however, correctly recognized that this skull is actually a specimen of the hyperdolichocephalic brontothere here called Sphenocoelus . Peterson (1924) discussed the taxon in his review of species of “ Dolichorhinus ,” but did not state whether he accepted its validity. Osborn (1929) regarded “ Dolichorhinus ” superior as being transitional between Mesatirhinus and “ Dolichorhinus ,” but provisionally accepted it as a valid species of “ Dolichorhinus .” Osborn (1929, p. 405) listed a number of diagnostic characters for “ Dolichorhinus ” superior (quoted verbatim from Riggs 1912) that presumably distinguish it from all other species of “ Dolichorhinus ,” but only one of the characters (that the internal nares open opposite the anterior margin of the third upper molar) is unique. Each of the other characters cited by Osborn could be applied to almost any small specimen of derived Sphenocoelus .

Although the position of the internal nares would at first seem to be a valid diagnostic character, Peterson (1924) noted that the type skull is poorly preserved in this region. I agree with Peterson that, in the skull’s original state, the morphology of the palate and internal nares was probably quite similar to that of other specimens of Sphenocoelus . In the holotype of Sphenocoelus superior , the typical “secondary palatal plate” formed by the backward and upward extension of the palatines has been destroyed, causing the internal nares to appear to be further forward than they actually were.

Figures

FIGURE 16. Skull of Sphenocoelus hyognathus in ventral view illustrating the anatomy of the palatine and pterygoid regions. A, depression in palate marking probable original location of internal nares; B, actual location of internal nares (which are apparently closed off); C, apertures in pterygoid region at the termination of the nasal passage (functional internal nares). Choanal pouches shaded gray. Illustration based on AMNH 1851 (after Osborn, 1929) and AMNH 13164.