Sphenocoelus hyoganthus, (Osborn 1890)

Mader, Bryn J., 2008, A species level revision of Bridgerian and Uintan brontotheres (Mammalia, Perissodactyla) exclusive of Palaeosyops, Zootaxa 1837 (1), pp. 1-85 : 48-49

Species SPHENOCOELUS hyoganthus ( Osborn 1890)

= S. cornutum ( Osborn 1895, the type species of Dolichorhinus )

= S. longiceps ( Douglass 1909)

Holotype. YPM-PU 10273 , partial lower jaw.

Referred specimens. AMNH 1845 , AMNH 1850 , AMNH 1851 (holotype of S. cornutum ) , AMNH 1852 , AMNH 13164 , CM 2347 (holotype of S. longiceps ) , CM 11080 , FMNH P 12167 , FMNH P 12175 , FMNH P 12182 , FMNH PM 3873 , MCZ 17678 , UFH V-190.

Diagnosis. Derived species of Sphenocoelus similar to S. intermedius but 10% to 15% larger in size (length P1 to M3 approximately 203–216 mm, length P2 to M3 approximately 184–192 mm, length M1 to M3 approximately 119–131 mm).

Discussion. In 1890, Osborn described an extremely elongated lower jaw (YPM-PU 10273), which he provisionally referred to the genus Palaeosyops and named P. hyognathus . In 1929, Osborn recognized that this was actually the lower jaw of " Dolichorhinus " (i.e., Sphenocoelus ) and argued that it represented the same species as " Dolichorhinus " cornutum , the type species of Dolichorhinus . Thus, according to Osborn, the type species of Dolichorhinus , D. cornutum , would be invalid because it is a junior synonym of D. hyognathus .

Although I was previously unsure whether Osborn was correct ( Mader 1989), I now agree that both species are synonymous. The holotype jaw of Sphenocoelus hyognathus is imperfectly preserved but the dimensions that can be ascertained indicate that it belongs to the large-size group of derived Sphenocoelus , which includes the type skull of S. cornutum . In Sphenocoelus the length of p2 to m3 on the lower jaw should slightly exceed the length of the upper check tooth series. The length of p2 to m 3 in the type jaw of S. hyognathus is approximately 213 mm, which is much longer than the cheek tooth series in all specimens of S. intermedius but is within the size range of specimens belonging to the large-size group. Furthermore, the length of the type jaw of S. hyognathus from the mental symphysis to the back of the angle of the jaw is approximately 490 mm, which is longer than the entire basilar length of the skull in some specimens of S. intermedius . Even the longest skull of S. intermedius for which I have data is only 24 mm longer than the type jaw of S. hyognathus , which is much too short for the skull and jaw to articulate. Skulls belonging to the largesize group of Sphenocoelus , however, are generally of a sufficient length to have articulated with the jaws. Thus, the dimensions of the type lower jaw of S. hyognathus would seem to preclude the possibility that it represents the small species, S. intermedius . Although there is a slight possibility that the jaw could represent S. uintensis (based on size), the rarity of this species in the Uinta Formation and the chronologically equivalent strata in the Washakie Formation, makes this seem highly improbable.

As explained in the Discussion section for Sphenocoelus intermedius, Earl Douglass described the holotype skull of " Dolichorhinus " longiceps in 1909 along with the holotype skull of " Dolichorhinus " heterodon (= S. intermedius ). According to Douglass, the skull and teeth of " Dolichorhinus " longiceps are larger than those of " Dolichorhinus " heterodon , but morphologically the teeth of the two species are indistinguishable. Douglass (1909) distinguished " Dolichorhinus " longiceps from " Dolichorhinus " hyognathus by its broader skull, morphologically different suborbital process (= suborbital protuberance), more gracile postglenoid process, broader palate, and more narrowed cranium anterior to the crest of the occiput. Douglass did not compare " Dolichorhinus " longiceps to " Dolichorhinus " intermedius , the only other species of " Dolichorhinus " named up to that time.

Peterson (1924), however, expressed reservations about each of the characters that Douglass had used to establish " Dolichorhinus " longiceps . According to Peterson the wideness of both the skull and palate in the holotype of " Dolichorhinus " longiceps is probably the result of crushing and is not indicative of the original cranial proportions. Peterson also noted that the postglenoid process of the type is imperfectly preserved so that its shape cannot be accurately compared to that of " Dolichorhinus " cornutum . Peterson believed that the other characters listed by Douglass could be attributed to individual variation (including sexual dimorphism) within a single species (" Dolichorhinus " cornutum ) and need not imply a distinct taxon. In a footnote on page 408 of his 1924 paper Peterson stated that " Dolichorhinus " longiceps Douglass might be a female of " Dolichorhinus " cornutum Osborn. Nevertheless, Peterson provisionally recognized " Dolichorhinus " longiceps as valid, noting the low stratigraphic horizon (the lower part of Uinta horizon B) and the fact that no other " Dolichorhinus " skull with "large osseous knobs on the nasals" had been previously recorded from these lower beds.

In 1929, Osborn also recognized " Dolichorhinus " longiceps as valid and suggested that it was directly ancestral to " Dolichorhinus " hyognathus (p. 405). Osborn did, however, express some doubt concerning the species' validity. Although Osborn clearly stated (p. 406) that Douglass' (1909) recognition of " Dolichorhinus " longiceps as a distinct species was "fully confirmed" by additional specimens collected by Riggs, elsewhere (p. 189) he remarked that the status of " Dolichorhinus " longiceps as a distinct species is "somewhat doubtful." Osborn's ambivalence concerning this taxon is further suggested by his mention (1929, pp. 406– 407) of a specimen (AMNH 1852) that "appears to bridge over" the size differences between " Dolichorhinus " longiceps and " Dolichorhinus " cornutum (which Osborn synonymized with " Dolichorhinus " hyognathus ).

The holotype skull of “ Dolichorhinus ” longiceps is rather poorly preserved, but clearly belongs to the larger-sized derived Sphenocoelus group (i.e., S. hyognathus ) based on its dorsal dimensions. The basilar length of the skull, as well as most of the dentition, was too poorly preserved to measure. I was only able to obtain precise measurements for the length of the left premolar series, length of left M1, length of left P4, and the length and width of left P3. The relatively few measurements that are available for comparison between specimens probably explains why the type of “D.” longiceps tends to group with specimens of S. intermedius in some cluster analyses (e.g., in Fig. 1 , the type of “ D.” longiceps groups very near to the type of “D.” heterodon ), since individual tooth measurements can overlap between S. intermedius and S. hyognathus .

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).