Protitanotherium emarginatum Hatcher, 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 : 157-167

Protitanotherium emarginatum Hatcher, 1895

HOLOTYPE: YPM-PU 11242, the anterior portion of a cranium with incisors and canines, and a partial mandible with right i1–p2, left i1–m2, and m3 (partial).

TYPE LOCALITY: Kennedy’s Hole, Myton Member (Uinta C) of the Uinta Formation, Uinta Basin, Utah, eight miles north of White River and 25 miles east of Ouray Agency.

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

SYNONYMS: Protitanotherium superbum Osborn, 1908a ; Sthenodectes australis Wilson, 1977 .

REFERRED SPECIMENS: (From the Myton member of the Uinta Formation of Utah) AMNH 2501 (holotype of Protitanotherium superbum ), a left maxilla fragment with M2– M3, and a partial mandible with right p1–m3 and left c–m3 ; YPM PU11213, a distal fragment of a nasal process; CMNH 2855 , a dorsal surface of the anterior portion of a cranium ; YPM 11146, an anterior portion of a mandible with right i1, i2 (partial), i3–c, p2–p3, p4 (partial), left i1, i2 (broken), and i3–p4; (from the Pruett Formation of the Agua Fria Area in Trans-Pecos Texas) TMM 41723-3 (holotype of Sthenodectes australis ), a skull with right C–M3 and left P1–M3; TMM 41723-6, a skull with right and left P1– M3, TMM 41747-106, a partial skull with right C–M3, left P1–M3, and isolated right I3.

DIAGNOSIS: Protitanotherium emarginatum is a large brontothere with small, elliptical frontonasal horns positioned low on the skull. The nasal incision is dorsoventrally shallow and extends as far back as the P4. The nasal process is nearly horizontal, unelevated, short and broad, with thickened sides, and with a thickened and imperfectly rounded distal edge with a distinct median notch. The lateral margins of the nasal process are not upturned. The orbits are positioned above M2. The premaxillomaxillary rostrum thickens posteriorly and it is not sealed by bone dorsally. Other cranial characteristics include a saddle-shaped cranium, separate parasagittal ridges that strongly constrict the dorsal surface of the cranium posteriorly, nearly straight zygomatic arches, and tube-shaped and mediolaterally angled external auditory pseudomeati. The posterior narial canal does not extend past the foramen ovale and large ventral sphenoidal fossae are absent.

Dentally, Protitanotherium emarginatum has three intermediate-sized subglobular upper incisors, and metacones on P1 and P2. Premolar hypocones are absent, although a short lingual crest can occasionally be seen extending from the protocones. The molars have tall lingually angled ectolophs with weak labial ribs, and thinned lingual ectoloph enamel with wedge-shaped paracones and metacones. A cingular parastyle shelf is absent. Central molar fossae are present, but anterolingual cingular cusps are absent. Paraconules and metalophs are absent. The lower dentition includes three intermediate-sized semispatulate incisors with an enlarged i2, a distinct postcanine diastema, a metaconid on p4 but not on p2 or p3, shallow molar basins, and a slender m3.

Protitanotherium emarginatum most closely resembles Protitan grangeri and Protitan minor , but it can be most easily distinguished from these species by the smaller, less subcaniniform incisors and tube-shaped external auditory pseudomeatus. Additionally, Protitanotherium emarginatum is distinct from Diplacodon elatus in its relatively smaller horns, dorsoventrally shallow nasal incision, less deeply saddle-shaped cranium, differently shaped nasal process, and less molarized premolars.

DESCRIPTION

SKULL: The holotype of Protitanotherium emarginatum consists of the preorbital portion of a skull (fig. 73) and a partial mandible (fig. 77). The skull has been slightly sheared. None of the previously published figures of this specimen ( Hatcher, 1895; Osborn, 1929a) clearly differentiates real bone from those parts that are reconstructed with plaster. A large fragment of maxilla and nasal bone is missing from the left face between the orbits and nasal incision. In the mandible the crowns of the left canine and left i3 are almost entirely reconstructed. The lingual half of the left m2 is plaster, as is the majority of the left m3.

In addition to the holotype, two nearly complete skulls from the Pruett Formation of Texas allow for a more thorough description of the skull of Protitanotherium emarginatum . TMM 41723-3 (fig. 74) is most significantly damaged in the region above the orbits ; essentially the frontonasal horns appear to have been forced back and down over the orbits, significantly distorting this region of the skull including the frontonasal protuberances. Neither the original figures of this specimen nor the initial description of Wilson (1977) reveals that the specimen is damaged in this way. Additionally, the left zygomatic arch is incomplete although a large central segment of it is reconstructed. The second skull, TMM 41723-6 (fig. 75), is less distort- ed, although the nasal bone is split and the left zygomatic arch is incomplete.

The horns of the holotype (fig. 73) are oval, widely separated on the skull, and they project in a dorsolateral direction. They are larger and more prominent than those of Protitan grangeri or P. minor , but they are smaller than those seen on most specimens of Diplacodon elatus . The horns of TMM 41723- 3 (fig. 74) are similar in size and shape although they seem to be directed somewhat more laterally. TMM 41723-6 almost completely lacks hornlike protuberances (fig. 75). The considerable size variation in the horns of P. emarginatum is not unlike other horned brontotheres (e.g., Diplacodon elatus ).

From a lateral view of the holotype skull it can be seen that the short horns are formed by both the frontal and nasal bones. This configuration of facial bones resembles that of Telmatherium validus , which has an anteriorly projecting process of the frontal bone overlapping the nasal bone. The nasal bone forms the proximal base of the frontonasal protuberance. Two thin sheets of roughened bone appear to overlap the upper surface of the nasal swelling. The upper layer extends to the peak of the protuberance and represents the frontal bone. The lower layer extends over the entire protuberance. It is not clear whether the lower layer is part of the frontal bone or it represents secondary growth of the nasal bone. Below the frontonasal protuberance, the nasal contacts the maxillary in a distinct arch.

The horns of the holotype skull are positioned above the preorbital portion of the face and they are not highly elevated above the skull as in Diplacodon elatus . In TMM 41723-3 the horns are positioned directly above the orbits, although this portion of the cranium is clearly distorted in that area and the horns have been displaced posteriorly. An earlier reconstruction of this specimen ( Wilson, 1977: fig. 2) places the horns anterior to the orbits.

Due to the fragmentary nature of the holotype specimen it is difficult to define the position of the posterior margin of the nasal or the position of the orbit with respect to the dentition. The length of the nasal incision is similar to Protitan minor . In TMM skulls, the

Fig. 73. The holotype skull of Protitanotherium emarginatum . (Division of Vertebrate Paleontology, YPM PU11242. º 2005 Peabody Museum of Natural History, Yale University, New Haven, Connecticut, USA. All rights reserved.) (A) Left view, (B) anterior view, (C) dorsal view, (D) lingual view of incisors and canines.

nasal incision extends to the P4 and the orbits are positioned more or less above the M2.

Unlike Diplacodon elatus , the nasal incision of Protitanotherium emarginatum is dorsoventrally shallow. The nasal process of the holotype is unelevated, straight, angled slightly downward, and it is shorter than the premaxillomaxillary rostrum. The sides of the nasal process are thickened. The thickened lateral edges extend downward, but they

(B) left view.

do not form deep vertical walls. The sides of the nasal process are not upturned as in D. elatus . From a dorsal view the nasal bones are strongly fused but the nasal suture is still visible. The nasal process is as wide as the premaxillomaxillary rostrum although its width is slightly constricted proximally. The distal margin nasal process is nearly flat, although it is somewhat concave medially. The anterior edge of the nasal process of TMM 41723-3 is more rounded and more strongly notched at the midline in comparison to the holotype. From the anterior view of the holotype, the anterior edge of the nasal process is relatively flat, thick, and mildly roughened. The anterior margin is not strongly deflected downward at the midline as in Protitan grangeri or D. elatus .

The premaxillomaxillary rostrum is best preserved in the holotype (fig. 73). From the lateral view the dorsolateral margin of the rostrum rises posterodorsally at a very shallow angle. The premaxillae are strongly fused at the symphysis. A premaxillomaxillary suture can be seen although it is indistinct. The premaxillae end before reaching the posterior notch of the nasal incision and they do not contact the nasal bone. From the anterior view the dorsolateral margins of the rostrum are laterally divergent and the rostral cavity is not sealed dorsally by bone.

Although the holotype is merely an anterior cranial fragment, Hatcher (1895) described the skull of Protitanotherium emarginatum as ‘‘slightly concave anteroposteriorly? and further characterized by the absence of a sagittal crest’’ ( Hatcher, 1895: 1085). Hatcher must have made these observations on parts of the holotype that have been lost. Hatcher (1895) indicates, ‘‘the posterior region had already weathered out and was badly injured, but many of the pieces have been fitted together and show some of the more important characters of this region of the skull’’ ( Hatcher, 1895: 1084–1085). The TMM specimens are consistent with Hatcher’s observations on these missing pieces. The dorsal surfaces of these skulls are concave from the orbits to the nuchal crest. The parasagittal ridges do not join to form a sagittal crest, although they strongly constrict the posterior portion of the dorsal surface.

The zygomatic arches of TMM 41723-3 are thick whereas those of TMM 41723-6 are thinner; this variation corresponds with the differing degrees of horn development and canine size in these specimens. In both specimens, the jugal portion of the zygomatic is straight from a dorsal view and from a lateral view it is horizontal. The squamosal portion of the zygomatic is very weakly angled posterodorsally and the zygomatic arch is almost completely straight from a lateral view. A posterior zygomatic processes is not seen in Protitanotherium emarginatum .

There is no discernable infraorbital process on the TMM specimens. Nonetheless, Wilson (1977) described the TMM specimens as ‘‘give[ing] the appearance of having an infraorbital protuberance, but if the effect of the crushing were removed, such a protuberance would probably not have been more prominent than in CMNH 2398’’ ( Wilson, 1977: 5). (CMNH 2398 is the holotype of Sthenodectes incisivum .) It is not clear what Wilson (1977) meant by this statement, since the lower portion of the orbit and suborbital region do not appear to be significantly distorted.

The occiputs of the TMM specimens are wide and strongly tilted backward. The dorsal portion is essentially the same width as the ventral portion and the occiput is not constricted in its middle. The nuchal crest is thin and is somewhat concave at the midline.

The anterior rim of the posterior nares is positioned between the protocones of the M3s in TMM 41723-3 and at the anterior margin of M 3 in TMM 41723-6. The posterior nares of both specimens are rimmed by a distinct U-shaped emargination. In TMM 41723-6 the posterior nares have been more fully cleared of sediment, revealing that the full width of the bony emargination that surrounds the posterior nares. In TMM 41723-6 the thin vomer can be seen bisecting the elongate posterior canal and joining with the sphenoid. The posterior narial canal, which is filled with sediment, extends somewhat into the sphenoid but not posterior to the foramen ovale. Large ventral sphenoidal fossae are not seen on this specimen. Other aspects of the basicranium, such as the widely separated foramen ovale and foramen lacerum, are typical for brontotheres. The mastoid process is short and curves anteroventrally, contacting the postglenoid process, thus, the external auditory pseudomeatus is closed ventrally and tube-shaped. This configuration differs from Protitan , but resembles all other horned brontotheres.

UPPER DENTITION: The upper incisors of the holotype are in good condition and lightly worn (fig. 73d). No other upper teeth are preserved on the holotype except for the poorly preserved canines. On the other hand, the TMM specimens lack incisors but have more complete cheek teeth, particularly TMM 41723-3 (fig. 76b, d).

Overall, the incisors of YPM-PU 11242 are significantly smaller than those of Protitan . The incisors increase in size laterally. The crowns of I1 and I2 are very short and blunt, although distinct lingual cingula can be seen on these teeth. The I3, on the other hand, is much larger and more subcaniniform than I1 or I2. The incisor row forms a very shallow arch that is positioned anterior to the canines. The I3 and canine are separated by a large diastema. Though the crowns of both canines are incomplete, it is apparent from the roots and the base of the right canine crown that the canines of this specimen were very large. The canines of TMM 41723-3 are of a similar size, although the left canine alveolus of TMM 41723-6 (fig. 75c) indicates a somewhat smaller canine. A distinct postcanine diastema is consistently present in these specimens, although its length is variable.

No cheek teeth are attached to the holotype skull, however, Hatcher (1895: pl. 38) and Osborn (1929a: fig. 317) each figured and described a left P1. The tooth was lost sometime after the fossil was molded, because the tooth is present on plaster casts found in numerous North American museums. Hatcher (1895) described it as ‘‘a very simple tooth fixed in the jaw by two roots, and consisting of a single cone with a posterior heel’’ ( Hatcher, 1895: 1086). Osborn described the P1 somewhat differently as a ‘‘bifanged tooth…with a simple protocone, a sessile or rudimentary posterior heel, and a posterointernal cingulum and concavity’’ ( Osborn, 1929a: 378). Inspection of a cast (AMNH 10385) reveals that Osborn’s (1929a) description is more accurate. There is both a paracone and a smaller metacone, although wear has virtually obliterated the metacone. However, Osborn’s identification of a protocone on the P1 is misleading. There is a small, posteriorly positioned lingual heel with a minor undulation of enamel on it, but it does not bear a distinct protocone. The P1 of TMM 41723-3 is smaller than the P2, but it is morphologically very similar, with two labial cusps of similar size, a relatively straight ectoloph, and a well-developed lingual heel. The lingual heel appears to have a small protocone and a small anteroposteriorly oriented lingual crest.

The P2–P4 of TMM 41723-3 are in good condition (fig. 76b). These teeth are closely pressed together. The P2 crown is slightly oblique, while the P3 and P4 crowns are progressively more rectangular. The anterior margins of P3 and P4 are worn away because of interstitial wear resulting in the formation of concave anterior margins. It is interesting that the posterior margins of P2 and P3 fit perfectly into the concave worn anterior margins of P3 and P4, but curiously, the posterior margins of P2 and P3 seem not to have been experienced significant interstitial wear. The ectoloph of P2 is straight, although in P3 and P4, the parastyles are somewhat labially directed. Distinct labial paracone ribs can be seen on P2–P4, though they become progressively smaller in more posterior premolars. The lingual heels of the P2–P4 are broad with nearly flat lingual edges. There are no distinct lingual cusps on the P2. Rather, a short lingual crest extends anteroposteriorly along the lingual side of the crown and is joined anteriorly by a small preprotocrista. Both P3 and P4 have a distinct protocone. The preprotocrista of P3 is faint, while the P4 has no preprotocrista. The protocones of P3 and P4 are followed posteriorly by very weak lingual crests. There are no hypocones on any of the premolars, thus the premolars of Protitanotherium emarginatum are significantly less molarized than are those of the contemporary species, Diplacodon elatus . The labial premolar cingula are weak. The lingual P2 cingulum is strong and continuous. The lingual cingulum of P2 is slightly discontinuous and it is more discontinuous on P4.

In addition to the molars of TMM 41723-3 (fig. 76d), a close up view of the M2 and M3 of AMNH 2501 is provided (fig. 76c). The molars of Protitanotherium emarginatum exhibit typical brontotheriine apomorphies including tall, lingually angled ectolophs, weak labial ribs, thin lingual ectoloph enamel, and wedge-shaped lingual sides of the paracone and metacone in relatively unworn molars. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. The following traits can be more distinctly seen in AMNH 2501. Shallow but distinct central molar fossae are present. The lingual portion of the anterior cingulum is thickened, but it does not form a distinct anterolingual cingular peak. All remnants of paraconules or metalophs are lost. There is no hypocone on the M3, but a portion of the posterior cingulum is thickened. The labial molar cingulum tends to be weak and discontinuous around the mesostyles.

MANDIBLE AND LOWER DENTITION: In the mandible of the holotype (fig. 77) the inferior margin of the symphysis is angled slightly less than 45 ° and it extends to the anterior margin of p4. The ascending ramus is not preserved, although in another specimen, AMNH 2501 (fig. 78), the coronoid process is long, slender, moderately curved, stands higher than the condyle, and is not generally different from other brontotheres.

The lower incisors are significantly smaller than those of Protitan grangeri . They are more similar in size to those of Diplacodon elatus , but they are not reduced to the essentially vestigial condition seen in more advanced horned brontotheres such as Duch-

Fig. 77. The holotype mandible of Protitanotherium emarginatum . (Division of Vertebrate Paleontology, YPM PU11242. º 2005 Peabody Museum of Natural History, Yale University, New Haven, Connecticut, USA. All rights reserved.). (A) Left view, (B) dorsal view, (C) right p1 and p2, (D) left premolars, (E) lingual view of incisors and canines, (F) labial view of incisors and canines.

esneodus uintensis . The lower incisors are only lightly worn. They are semispatulate in shape. The i2 is significantly larger than either the i1 or the i 3 in height and in labiolingual width. There are no labial cingulids and the lingual cingulids are faint. The incisors form only a slight arch anterior to the canines. The lower canines of both YPM-PU 11242 and AMNH 2501 are rather large. There is no precanine diastema. The postcanine diastema is asymmetrical in YPM PU11242. On the right side it is slightly longer than the p2, but on the left side it is a little shorter.

The right and left p1s of YPM-PU 11242 are bilaterally asymmetrical. The right p1 is a narrow single cusped tooth with an elongate posterior talonid heel. The left p1 is notably wider, particularly the posterior portion of the crown. The p2–p4 have relatively low slender crowns. The trigonids of p2 and p3 are less than twice as long as the talonids. The trigonid and talonid of p4 are of similar length. The talonids of p2–p4 are slightly wider than the trigonids. On p2 and p3 the paralophids and protolophids extend from the protoconid in a slightly lingual direction. There is only a small lingual trigonid notch on the p2, although the lingual trigonid notch of the p3 is large. The paralophid and protolophid of p4 arch 90 ° lingually, forming a nearly molariform trigonid basin. Metaconids are absent on p2 and p3 but they are present on p4. The cristids obliqua and hypolophids of p2 and p3 are well developed, but they are short and the lingual-talonid notch does not form a molariform basin. However, the cristid obliqua and hypolophid of p4 are much longer and create a more molariform talonid basin. Lingual premolar cingulids are absent, while labial premolar cingulids are absent (p2, p3) or very weak (p4).

The molars are typical, with thin lingual enamel, weak lingual ribs, and shallow talonid basins. The m3 of the holotype is incomplete but that of AMNH 2501 is elongate. The labial cingulids of the molars are much stronger in comparison to the premolars.

REMARKS

Hatcher (1895) named Protitanotherium emarginatum from a specimen (YPM-PU 11242) consisting of the anterior portion of a skull with short frontonasal horns and a partial mandible. Previously, Marsh (1875) had named another species of horned brontothere, Diplacodon elatus . When Hatcher (1895) described P. emarginatum the existence of a horn in D. elatus was in question. For this reason Hatcher (1895) cautiously referred his new species to the genus Diplacodon but suggested, ‘‘should future discoveries show that there are hornless forms with the same dental characters as Diplacodon , it will be necessary to establish for the present specimen (YPM-PU 11242) a new genus, which may be called Protitanotherium ’’ ( Hatcher, 1895: 1084). Although it turns out that Diplacodon elatus does have horns, Osborn (1929a) demonstrated several distinctions between Hatcher’s (1895) D. emarginatum and Marsh’s D. elatus . Therefore, he adopted Hatcher’s recommended genus name, Protitanotherium . Lucas and Schoch (1989a) felt that Protitanotherium was a junior synonym of Diplacodon , however Mader (1989; 1998) concluded that Protitanotherium was sufficiently different to warrant its distinct genus name. Mader’s conclusion is accepted here.

Several additional specimens from the Uinta Formation are referred to Protitanotherium emarginatum , including the holotype mandible (AMNH 2501) of P. superbum Osborn (1908a) . Osborn’s specific characters for P. superbum do not clearly diagnose a new species. These include: ‘‘Canines in males very robust; p1 double fanged (an erroneous description); post canine diastema abbreviat- ed; premolar series relatively abbreviated; p2 with very large talonid and crescentic protoconid; p3, p4 with talonid heavy and prominent, i.e., submolariform, but no entoconid; m3 with hypoconulid sharply constricted off at base’’ ( Osborn, 1908a). The mandible and cheek teeth of AMNH 2501 sufficiently resemble the holotype of P. emarginatum to refer it to that species. It distinctly differs from the contemporaneous horned species, Diplacodon elatus , particularly in the absence of a p3 metaconid. AMNH 2501 is somewhat larger than the holotype of P. emarginatum , but it falls within an acceptable body size range for that species. Other specimens, such as the nasal bone YPM PU11213 are larger than the holotype as well, suggesting that the type specimen is a rather small individual of the species. A pair of upper molars (left M2, M3) is also assigned to AMNH 2501 although Osborn (1908a) made no mention of these upper molars in the original description of P. superbum . Later, Osborn (1929a) assumed that these upper molars were part of the same specimen. Whether or not these upper molars are associated with the mandible (AMNH 2501) is questionable, but both, nonetheless, seem to belong to P. emarginatum .

A brontothere consistent with Protitanotherium emarginatum also occurs in the Whistler Squat local fauna of the Pruett Formation of the Big Bend–Trans-Pecos area, Texas, although until now that material has not formally been recognized as P. emarginatum . Wilson (1977) described this material as a new species, Sthenodectes australis . This species was based primarily on two nearly complete skulls. Wilson (1977) mistakenly referred this species to the genus Sthenodectes . According to Wilson (1977: 7) ‘‘the length of the tooth row and the approximate size of the teeth show that the partial skull is close to Sthenodectes .’’ And secondly ‘‘the very large molars in proportion to the skull length, the advanced condition of the premolars, and the very rudimentary horns…seems to best fit Sthenodectes ’’ ( Wilson, 1977: 8). Despite Wilson’s conclusion, it is clear that these skulls are far closer to Protitanotherium than Sthenodectes . It seems that Wilson (1977) failed to compare the material to P. emarginatum , largely because his comparisons were based on upper tooth measurements, elements lacking largely lacking in the holotype of P. emarginatum . Moreover, as with most work done on brontotheres after 1929, Wilson (1977) drew heavily from Osborn (1929a). Osborn (1929a: fig. 301) erroneously figured Sthenodectes incisivum as having a small horn. Actually, none of the S. incisivum specimens has a horn. Mader (1998) suggested that the skulls described by Wilson (1977) might be referable to Protitanotherium , but he made no formal revision. Indeed, the skulls of TMM 41723-3 and TMM 41723-6 are consistent with P. emarginatum . Differences between these skulls and the holotype of P. emarginatum , such as the morphology of P1 and variation in horn and canine size, are characters that are intraspecifically variable in other brontotheres. S. australis appears to be a junior synonym of P. emarginatum . Wilson (1977) referred several other specimens to S. australis , including some partial mandibles. These specimens could belong to P. emarginatum , but they lack sufficiently diagnostic morphology to make conclusive identifications.