Sphenocoelus uintensis Osborn, 1895

Mihlbachler, Matthew C., 2008, Species Taxonomy, Phylogeny, and Biogeography of the Brontotheriidae (Mammalia: Perissodactyla), Bulletin of the American Museum of Natural History 311 (1), pp. 1-475 : 49-59

Sphenocoelus uintensis Osborn, 1895

HOLOTYPE: AMNH 1501 , the posterior part of a skull.

TYPE LOCALITY: Northeastern Utah, Wagonhound Member (Uinta B1) of the Uinta Formation, Wyoming.

SYNONYMS: Tanyorhinus blairi Cook, 1926 and Tanyorhinus bridgeri Cook, 1926 .

AGE: Middle Eocene (early Uintan land mammal ‘‘age’’).

REFERRED SPECIMENS: (From the Wagonhound Member of the Uinta Formation of Utah) CMNH 2963 , a posterior part of a cranium (partially prepared); (from the Sand Wash Basin of Moffat County, Colorado) DMNH 479 (holotype of Tanyorhinus bridgeri ), a skull with left P3–P4, M2–M3 ; DMNH 484 , a right maxilla with C, P2–M2 ; DMNH 507 , an anterior portion of a skull with left P1–M3 ; DMNH 509 , an anterior portion of a skull with right P3–P4 (all partial) and left P2–P3 ; DMNH 517 , a right premaxillomaxillary fragment with isolated incisors and isolated P2–M1 ; DMNH 541 (holotype of Tanyorhinus blairi , in part), a skull with P1–P4, M3, and left P1–M3 ; DMNH 542 (holotype of Tanyorhinus blairi , in part), a mandible with right i1–m3, left i1– p4, and m2–m3 ; DMNH 2830 , a skull with right I2–I3, P1–M3, left P2–M3, and a complete mandible with complete dentition ; DMNH 14219 , a mandible with complete dentition ; DMNH 29411 , a skull with right P1–M3 and left C–M3; (from the Adobe Town Member [Washakie B of Granger, 1909] of the Washakie Formation , Wyoming) UCMP 81281, a partial mandible with right p3–p4 and left p2–m1; UCMP 81301 , a skull that is missing the premaxilla and zygomatic arches with right P3–M3 and left M1–M3 ; UCMP 81443 , a partial mandible with right i1–c, p2– p4, and left i1–p3 .

SPECIMENS REFERRED TO CF. SPHENO- COELUS: The following specimens are consistent with Sphenocoelus uintensis and probably belong to that species, but they lack sufficiently diagnostic characters to definitively refer them to S. uintensis : (from the Sand Wash Basin of Moffat County, Colorado) DMNH 504, a mandible with right and left p1–m3; DMNH 505, a partial mandible with right m1–m2, left m2–m3, isolated canine, and right p4; DMNH 506, left M1– M3 (very worn); DMNH 506a, right M3; DMNH 556, a mandible fragment with right p4–m2 and m3 (partial); DMNH 2584, a partial mandible with right p2–m3; (from the Adobe Town Member [Washakie B of Grang- er, 1909] of the Washakie Formation, Wyoming) UCMP 81294, a partial mandible with right p3–m3; UCMP 81299, a partial mandible with right p3–m3; UCMP 81366, a mandible fragment with left p4–m2; UCMP 81369, a left M1 or M2; UCMP 81370, a mandible fragment with left p3–m1; UCMP

81402, a right maxilla fragment with P1–P4; UCMP 81412, a left P2; UCMP 81413, left p2; UCMP 81414, a left P3 or P4; UCMP 81416, a right P4; UCMP 81422, a right M2 (partial), and M3; UCMP 81438, a right maxilla fragment with P1 (roots) and P2–P3; UCMP 81440, a partial mandible with broken incisors, right p2–p4, left m2 (partial) and m3; UCMP 81448, a partial mandible with left p3–m3; UCMP 81451, a partial mandible with right p2 (partial), p3–m2, and m3 (partial); UCMP 81462, an isolated incisor.

DIAGNOSIS: Sphenocoelus uintensis is an intermediate-sized hornless brontothere in which the frontal bone does not overlap or intrude into the nasal bone. The cranium of S. uintensis is highly dolichocephalic. The nasal incision extends as far back as the anterior margin of the M1. The nasal process is horizontal, unelevated, of relatively constant transverse width, narrow, with thin and relatively shallow lateral walls, and without a well-defined or strongly rounded distal margin. The orbits do not protrude laterally; they are positioned above the posterior part of M2 and the anterior part of M3 with the anterolateral root of M2 and the posterolateral root of M1 below the anterior orbital rim. There is a prominent infraorbital process on the jugal. The premaxillomaxillary rostrum deepens posteriorly and is not covered by bone dorsally. Other cranial characteristics include a short sagittal crest, thin and weakly curved zygomatic arches, a ventrally open and steeply posteriorly angled external auditory pseudomeatus, disproportionately wide occipital condyles, and large paired ventral sphenoidal fossae. The cranium is dorsally arched but more weakly so than Dolichorhinus .

Dentally, Sphenocoelus uintensis is characterized by large subcaniniform upper incisors, a postcanine diastema, a simple P1, a distinct P2 metacone, weak premolar preprotocristae, and with short crests extending posteriorly from the premolar protocones. Premolar hypocones are absent. The molars of S. uintensis have tall, lingually angled ectolophs with weak labial ribs, and thinned lingual ectoloph enamel with wedge-shaped paracones and metacones. A cingular parastyle shelf and an anterolingual cingular cusp are absent. Central molar fossae are present in some specimens but absent in others. S. uintensis molars occasionally retain vestigial paraconules, but all traces of a metaloph are lost. The lower dentition of S. uintensis includes large semispatulate incisors that are all of a similar size, a postcanine diastema, no p1–p2 diastema, an elongate p2 trigonid, a metaconid on p4 but not on p2 and p3, shallow molar basins, and a slender m3.

Sphenocoelus uintensis is most similar to Mesatirhinus junius , but it is clearly distinct from that species due to its larger size, more dolichocephalic proportions, and paired ventral sphenoidal fossae. Likewise, paired sphenoidal fossae, an unspecialized premaxillomaxillary rostrum, and a sagittal crest distinctly set it apart from Dolichorhinus hyognathus .

DESCRIPTION

SKULL: The holotype of Sphenocoelus uintensis (AMNH 1501) is the posterior part of a skull from the Uinta Basin that has suffered minor shearing distortion but is otherwise in good condition (fig. 17). Other specimens referable to S. uintensis have been recovered from the Sand Wash Basin of Colorado (e.g., Cook, 1926) and from a collection of the Washakie Basin of Wyoming made by McKenna and Kent in 1954. Among these are several complete (or nearly complete) skulls and a variety of cranial fragments and mandibles. The following description of the skull and upper dentition is based upon the holotype and five other skulls, UCMP 81301 (fig. 18b) DMNH 479 (fig. 18a), DMNH 541 (not shown), DMNH 2830 (figs. 18c–d, 19a), and DMNH 29411 (not shown). Other specimens provide additional information on variation.

Sphenocoelus uintensis is an intermediate-sized brontothere (table 3) whose skull is generally similar to Mesatirhinus junius but notably more dolichocephalic. Hornlike nasal or frontonasal protuberances are not seen in S. uintensis . The frontonasal suture, most clearly visible in UCMP 81310 (not shown), recedes posteromedially, but near the midline the direction of the suture is acutely redirect- ed anteriorly. The frontal does not overlap the nasal or protrude into the nasal as in Telmatherium validus .

The nasal incision is shallow and long and extends as far back as the anterior edge of M1, while the orbit is positioned above the posterior part of M2 and the anterior part of M3. The anterolateral root of the M2 and the posterolateral root of the M1 are situated directly below the anterior orbital rim.

The premaxillomaxillary rostrum lacks the specializations seen in Dolichorhinus . From a lateral view of the skull the dorsal edge of the premaxillomaxillary rostrum ascends posterodorsally. The posteriormost notch of the nasal incision is level with the top of the orbit. The nasal processes of the premaxillae

TABLE 3 Summary statistics for selected morphometric variables of Sphenocoelus uintensis See Methods for measurement definitions diverge laterally and the premaxillomaxillary cavity is open dorsally. A distinct remnant of the right premaxillomaxillary suture in UCMP 81310 indicates that the premaxilla does not contact the nasal bone.

The bones of the nasal process are not strongly fused together. The nasal processes are slightly shorter and somewhat narrower than the premaxillomaxillary rostrum. The transverse width of the nasal process is nearly constant throughout its length. In the least distorted skulls, such as DMNH 479, the nasal process extends horizontally from the skull, or it bends just slightly downward so that the dorsal surface of the nasal process is slightly convex. The lateral walls of the nasal process are relatively thin and dorsoventrally shallow, but they are of constant depth throughout the length of the nasal process. The anterior border of the nasal process is thin, roughened, and angled downward moderately.

The dorsal surface of holotype skull fragment (AMNH 1501) is essentially flat, although complete skulls indicate a more dorsally arched cranium. In the least distort- ed skulls (DMNH 29411, DMNH 479, and UCMP 81301), the dorsal surface of the skull above the orbits is flat or slightly concave. However, the dorsal surface of the posterior half of the skull is strongly convex. The highly elongated posterior half of the cranium tends to be slightly dorsally arched from a lateral view, but not as strongly as the skull of Dolichorhinus hyognathus . In the holotype and in other specimens, the parasagittal ridges merge to form a short sagittal crest.

The zygomatic arches are relatively thin, slender, and slightly bowed laterally. The jugal portion of the zygomatic is essentially horizontal, while the squamosal portion rises posteriorly at a moderate angle, thus giving the zygomatic arch a weakly curved shape. The jugal of Sphenocoelus uintensis has a large rounded infraorbital process. The infraorbital process resembles those of Mesatirhinus and Dolichorhinus , but it is larger and more distinct than that of Metarhinus .

The occipital can be adequately described from the holotype skull (AMNH 1501). The occiput is moderately tilted backward. From a dorsal view of the skull the nuchal crest is thin and concave. From the posterior view the nuchal crest is arched. The center of the occiput is not deeply recessed between the small occipital pillars. The occiput is narrow- er dorsally than it is ventrally. The occipital condyles of Sphenocoelus uintensis are disproportionately very large and almost as wide as the entire occiput.

Many aspects of the ventral surface of the skull of Sphenocoelus uintensis are most clearly revealed in UCMP 81310 (fig. 18b). The anterior rim of the posterior nares is positioned at the anterior margin of the M3. The anterior edge of the posterior nares is abrupt; that is, there is no horseshoe-shaped emargination, nor is there a bony palatal extension like that of Dolichorhinus . A short median process protrudes posteriorly from the midline of the anterior edge of the posterior nares, marking the posteriormost contact point of the vomer and palatal bones. The posterior narial canal is extremely long and continues into a large cavity in the sphenoid bone. The vomer is missing in UCMP 81310, but it would have originally formed a long, thin plate of bone that bisected the elongate posterior narial canal. Remnants of the elongate vomer can be seen in the form of a thin ridge of bone running along the dorsal roof of the posterior narial canal in UCMP 81301.

The posterior narial canal continues into a large vacuity in the sphenoid bone. Deep vacuities in the sphenoid can also be seen in the holotype ( AMNH 1501 ) and in all other skulls of Sphenocoelus uintensis . Osborn referred to these fossae as ‘‘pits’’ (Osborn, 1895) and ‘‘sphenoidal pits’’ ( Osborn, 1929a). In some specimens, such as UCMP 81310, the cavities formed by the ventral sphenoidal fossae are large, although they seem to have been narrower in others such as AMNH 1501 and DMNH 2830 . The basisphenoid is highly modified and forms a narrow septum that partitions the ventral sphenoidal fossae ; this is most clearly seen in AMNH 1501 (fig. 17). The partitioning basisphenoid is so thin that it is commonly not preserved .

In most of the specimens the ventral sphenoidal fossae form a continuous channel with the posterior narial canal. The thin partitioning basisphenoid would have connected with the elongate vomer to form a continuous partition of the posterior narial canal and sphenoidal fossae. The holotype (AMNH 1501) is somewhat unusual in this respect because the ventral sphenoid fossae appear to extend deeper into the ventral surface of the cranium, thus forming distinct pits. Apparently the depth and diameter of the ventral sphenoidal fossae are intraspecifically variable.

The external auditory pseudomeatus, formed by the mastoid and postglenoid processes of the squamosal bone, enters the skull at a strongly posteromedial angle, a condition shared with Dolichorhinus . Other aspects of the basicranium of Sphenocoelus uintensis are more typical. For instance, the external auditory pseudomeatus is not enclosed ventrally and the foramen ovale is widely separated from the foramen lacerum. One final peculiar aspect of S. uintensis is a distinct fossa on the ventral surface of the zygomatic process of the squamosal just lateral to the glenoid fossa (this is best seen on the holotype skull fragment, AMNH 1501).

UPPER DENTITION: Unfortunately, none of the skulls of Sphenocoelus uintensis has an intact set of upper incisors. In DMNH 2830 there are six incisor alveoli that form a semicircular arch anterior to the canines and are separated from the canines by a short I3–C diastema (fig. 19a). The two preserved incisors (right I2, I3) are large and though they are worn, they appear to have been subcaniniform (conular and lingually curved). The I3 is larger than the I2. Three isolated incisors are associated with DMNH 517, a maxilla that appears to be S. uintensis (fig. 19d, e). Although there is no way to know which crown is which, judging by comparison with the partial set of incisors in the skull of DMNH 2830, they appear to form a right incisor row, with the smallest incisor I1 and the largest I3. The crowns are subcaniniform with lingually curved crowns and narrow lingual heels. The I1 is the shortest incisor, while I2 and I3 are progressively taller and larger in diameter.

The canines are not well preserved in any of the skulls. One specimen, DMNH 29411, retains a left canine with a tall crown and a rounded cross section. Another specimen, DMNH 2830, includes an isolated canine of similar morphology. Despite the rather dolichocephalic proportions of the skull of S. uintensis , the postcanine diastemata of S. uintensis is very short in all specimens and is usually only a few millimeters long.

The most adequately preserved sets of cheek teeth are of DMNH 2830, DMNH 29411, and UCMP 81310. Figured are the complete and relatively unworn cheek teeth of DMNH 29411 (fig. 19b, c). The P1 is simple, with a single cusp and a low posterior heel. The P2 is slightly oblique in outline due to a posterolingually angled anterior edge. The anterior and posterior sides of P3 and P4 are more nearly parallel. The labial side of P2 is strongly rounded, while those of P3 and P4 are progressively flatter. The parastyle of P2 arches slightly lingually, while the metastyle is nearly straight. The metacone of P2 is positioned slightly lingually with respect to the protocone. The parastyle and metastyle of P3 are straight, while those of P4 are angled slightly labially. Small labial paracone ribs are present on P2–P4 and become progressively shorter on more posterior premolars.

A very small but distinct preprotocrista connects the protocone with the lingual base of the paracone in P2. In P3 and P4 this crest becomes progressively smaller, so that it is barely perceptible in P4. A low crest descends the posterior slope of the protocone on P2– P4. This lingual crest is not present on more worn sets of dentition (e.g., DMNH 2830), although it is present on the P2, P3, and P4 of DMNH 29411. Yet in others (e.g., UCMP 81301) the lingual crest is not seen on P4. There are no hypocones on the premolars. The anterior and posterior cingula of the premolars are thick and most often stretch around the lingual side forming a continuous lingual cingulum, although it is occasionally discontinuous, as is the case in UCMP 81301. The labial premolar cingula of the P2 and P3 typically connect to the posterior ridge of the paracone rib, but the labial cingulum of P4 most often stretches across the base of the crown.

The molars are Sphenocoelus uintensis have tall, lingually angled ectolophs, weak labial ribs, thin lingual ectoloph enamel, and wedge-shaped lingual sides of the paracone and metacone in molars that are not heavily worn (e.g., M3). The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. Sphenocoelus uintensis molars lack an anterolingual cingular cusp. Very small paraconules are occasionally retained on the molars, although no evidence of a metaloph was found on any specimen. Among the specimens referred to Sphenocoelus uintensis , some (DMNH 507, DMNH 484, and DMNH 29411) have shallow but distinct central fossae in the molars. Howev- er, in other specimens (UCMP 81301, DMNH 2830, DMNH 541, and DMNH 517) they appear to be absent or very shallow. A M3 hypocone is generally absent, although a thick cingulum traces around the distolingual corner of the M3 crown. Cook (1926) reported a small ‘‘postero-internal’’ cusp on the M3 of DMNH 479, but I was unable to examine the ventral surface of that skull because it is part of a composite skeleton encased in glass on exhibit at the DMNH. The lingual molar cingula are typically discontinuous around the protocone and hypocone. Likewise, the labial molar cingula are thin and tend to be discontinuous around the mesostyles.

MANDIBLE AND LOWER DENTITION: One skull of Sphenocoelus uintensis is associated with a mandible (DMNH 2830) that retains a complete set of well-worn lower teeth (fig. 20a–b). Additionally, several mandibles, some with less worn dentition, are morphologically consistent with the mandible of DMNH 2830, but are not directly associated with diagnosable skulls. The following description of the mandible and lower dentition of S. uintensis is based primarily on DMNH 2830 and other specimens with well-preserved incisors and premolars (UCMP 81281 and UCMP 81443) (fig. 20c–e).

The horizontal ramus of S. uintensis is elongate and shallow, like that of Dolichorhinus . However, the ventral margin of the symphysis is steeper than Dolichorhinus hyognathus , and more similar to that of Mesatirhinus . The position of the posterior margin of the symphysis varies slightly but it usually extends to the posterior part of the p3. The three pairs of incisors are large and form an arched row that reaches anterior to the canines. The incisors increase in size laterally. There are small gaps between the incisors of DMNH 2830, but these gaps are probably a result of extensive wear. UCMP 81443 has a better set of incisors, though they are also heavily worn. In this specimen, there are no gaps between the incisors. Judging by the shape of the wear facets of UCMP 81443, the i1 and i2 were semispatulate, while the i3 was more subcaniniform. A distinct lingual cingulid can be seen on each incisor. Moreover, each incisor has a distinct labial cingulid.

The lower canines of DMNH 2830 are large although the canines of other specimens (UCMP 81443) are somewhat smaller. The canine is followed by a postcanine diastema that is generally shorter than the p2. However, the lower postcanine diastema is longer than the upper postcanine diastema. The p1 is a simple tooth with a single cusp and a short and narrow talonid. There is no p1–p2 diastema in DMNH 2839 or any other mandible referred to Sphenocoelus uintensis . This is an important diagnostic feature that helps to differentiate the mandibles of S. uintensis from those of Dolichorhinus hyognathus , a species with a distinct p1–p2 diastema.

The p2–p4 of DMNH 2830 and UCMP 81443 are moderately worn, but much of their morphology is still discernable. UCMP 81281 has unworn premolars. The p2 trigonid is much longer than the talonid, but the trigonid and talonid are of similar width. The talonid and trigonid of p3 are of comparable length but the trigonid is slightly narrower than the talonid. The trigonid of the p4 is both shorter and narrower than the talonid. The paralophid of the p2 is either straight (e.g., UCMP 81281) or slightly arched lingually (UCMP 81443), thus creating a small lingual notch in the trigonid. The p3 paralophid arches at a slightly more lingual angle than that of the p2. The p4 paralophid is strongly directed lingually, creating a broad lingual trigonid notch; that of p3 is angled somewhat lingually, and that of p4 arches fully lingually. A metaconid is present only on p4. The talonid of p2 has only a minor lingual notch and a short cristid obliqua and hypolophid. The p3 and p4 have longer cristids obliqua and hypolophids with much broader talonid basins.

The molars of Sphenocoelus uintensis are typical of brontotheriines, with relatively thin lingual enamel and an elongate m3. The m3 hypoconulid heel of DMNH 2830 is unusually narrow, but this is atypical. Other specimens have a broader m3 hypoconulid. Lingual cingulids are absent, while the labial cingulids of p2–m3 vary in distinctness (this is related to wear), and tend to be discontinuous around the protoconid. Occasionally the m3 cingulid traces around the distal end of the hypoconulid (e.g., DMNH 14219).

REMARKS

Osborn (1895) recognized that AMNH 1501 (fig. 17) represented a new species, Sphenocoelus uintensis , due primarily to the peculiar pair of ventral sphenoidal fossae. Because the specimen lacked teeth, Osborn (1895) was initially unsure what family of perissodactyls S. uintensis belonged to, but he ultimately ( Osborn, 1929a) concluded that S. uintensis was a brontothere largely because of the relative positions of the basicranial foramina, and the similarity of the glenoid facets to that of Dolichorhinus . Osborn (1929a) also noted similarities between AMNH 1501 and YPM PU10041, a braincase of a brontothere that is probably Mesatirhinus junius .

In retrospect, by the time Osborn’s (1929a) argument for Sphenocoelus uintensis as brontothere was published, Cook (1926) had described several complete brontothere skulls from the Sand Wash Basin of Moffat County, Colorado. However, Cook (1926) made no comparison of this material with Osborn’s holotype of S. uintensis , nor did he mention the conspicuous ventral sphenoidal fossae of these specimens. He erected a new genus and two new species, Tanyorhinus blairi (DMNH 541) and T. bridgeri (DMNH 479). These species were differentiated from each other by minor differences in size (6 %), and a variety of other characters that can be attributed to taphonomic distortion (e.g., more curved zygomatic arches) and other aspects of variation that do not warrant species distinctions (e.g., variable presence of M3 hypocone). The apparent difference in head orientation between these two supposed species suggested by Cook (1926) is artificial and can mostly be attributed to deformation in the more poorly preserved specimen (DMNH 541).

Mader (1998) considered Tanyorhinus blairi and T. bridgeri to be junior synonyms of Sphenocoelus uintensis , a revision that is upheld here. Mader (1989) also considered the genera Dolichorhinus Hatcher (1895) and Dolichorhinoides Granger and Gregory (1943) to be junior synonyms of the genus Sphenocoelus . However, Dolichorhinoides is actually a synonym of Epimanteoceras Granger and Gregory (1943) , a taxon that clearly differs from Sphenocoelus in numerous ways, including the absence of ventral sphenoidal fossae, the absence of a sagittal crest, the presence of small frontonasal protuberances, and significantly, more molarized premolars. Additionally, Dolichorhinus differs from Sphenocoelus in many significant ways, including the specialized rostrum, the bony palatal extension, the more strongly arched cranium, and the p1– p2 diastemata. Additionally, Sphenocoelus differs from Dolichorhinus in having large ventral sphenoidal fossa, a sagittal crest, and a steeper mandibular symphysis.

In overall appearance, the skulls of Sphenocoelus uintensis resemble those of Mesatirhinus junius , although M. junius lacks ventral sphenoidal fossae and is smaller than S. uintensis . Several brontotheres share large ventral sphenoidal fossae with Sphenocoelus including Protitan , Diplacodon , and Metatitan .