Parabrontops gobiensis ( Osborn, 1925 )

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 : 280-298

Parabrontops gobiensis ( Osborn, 1925)

HOLOTYPE: AMNH 20354 , a severely distorted skull with right P1–P4 (broken but complete), M1–M3 (severely damaged), left P3–M1 (partial), and M2–M3.

TYPE LOCALITY: Urtyn Obo Formation, Urtyn Obo, Shara Murun Region, Inner Mongolia, China.

AGE: Late Eocene (Ulangochuian land mammal ‘‘age’’).

REFERRED SPECIMENS: (From the ‘‘Ulan Gochu’’ faunal zone [sensu Radinsky, 1964], Urtyn Obo, Shara Murun Region, Inner Mongolia); AMNH 26020, a partial skull lacking the occiput, basicranium and left zygomatic arch, with right C–M3. (Ulan Gochu [used in quotes] refers to a faunal zone rather than a formation; see Radinsky, 1964.)

The following mandibles could belong to Parabrontops gobiensis : (from the Urtyn Obo Formation, Urtyn Obo, Shara Murun Region, Inner Mongolia) AMNH 26019, a mandible with right and left incisors and canines (roots only), and p1–m3; AMNH 26021, a partial mandible with left p1–m3; (from the ‘‘Shara Murun’’ Formation [sensu Radinsky, 1964], East Mesa, Shara Murun Region, Inner Mongolia) AMNH 26131, a mandible with right and left i1–m3.

DIAGNOSIS: Parabrontops gobiensis is a large horned brontothere with short massive elliptical frontonasal horns. The horns are positioned low on the skull and directly above the orbits. The nasal incision extends to the anterior margin of the P4. The anterior rim of the orbit is above the anterolateral root of M1 and the posterolateral root of P4. The nasal process is very thick, unelevated, horizontal, strongly rounded anteriorly, with thickened lateral walls, and with a downturned distal tip. The premaxillomaxillary rostrum thickens posteriorly and it is not enclosed by bone dorsally. Other cranial characteristics include a saddle-shaped cranium, a dorsal cranial surface that is moderately constricted posteriorly by parasagittal ridges, a narrow emargination surrounding the posterior nares, nearly straight zygomatic arches, and a ventrally constricted and mediolaterally angled external auditory pseudomeatus. Ventral sphenoidal fossae and postzygomatic processes are absent. Large zygomatic swellings are absent.

Dentally, Parabrontops gobiensis is characterized by three small upper incisors that form a straight row, a postcanine diastema, a complex P1, a distinct P2 metacone, and distinct premolar hypocones on P3 and P4. The molars of Parabrontops gobiensis 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. Distinct central molar fossae and anterolingual cingular cusps are present. Paraconules and metalophs are absent.

Parabrontops gobiensis shares with Eubrontotherium clarnoensis the combination of very small upper incisors, paired horns that are widely spaced and unelevated, and relatively straight zygomatic arches that lack conspicuous swellings. Parabrontops gobiensis differs from Eubrontotherium most clearly in the retention of three upper incisors.

DESCRIPTION

SKULL: The holotype skull of Parabrontops gobiensis (AMNH 20354) is a nearly complete skull, although it is severely distorted. In addition to being crushed, it is laterally sheared and twisted so that the dorsal surface has been displaced far to the left of the ventral surface (figs. 137, 139). Many of the bone fragments, particularly in the facial area, are separated by plaster-filled gaps. AMNH 20260 is a partial skull missing the occiput, basicranium, and the left zygomatic arch (figs. 138, 140a). This skull, though less complete than the holotype, is not severely distorted.

There is conspicuous variation in the size and shape of the horns. The horns of the holotype (AMNH 20354) are large but short and slightly elliptical in cross section with the longest axis in an anteroposterior direction. The horns of AMNH 26020 are more massive and more strongly elliptical. On the side of the right horn of the holotype (AMNH 20354) the contact of the frontal and nasal bones can be seen revealing a configuration like most other horned brontotheres whose horn is composed of both the frontal and nasal bones. The frontonasal suture rises from the base of the horn and curves anteriorly along the side of the frontonasal horn. The peak and posterior surface of the horn is formed by the frontal. The proximal base and anterior side of the horn is formed by the nasal bone. The frontonasal contact is not readily discernable on AMNH 26020 where these elements appear to be more fully fused. The horns are essentially vertical in orientation, positioned low on the skull, and almost directly above the orbits. On AMNH 20354 the distal ends of the horns are roughened and each has a large pit at the apex. These unusual pits are unique to this specimen and appear to be an artifact of some form of damage. On AMNH 26020 the entire horn surface is roughened.

The holotype skull (AMNH 20354) is so distorted that the proportions of the face cannot be precisely described from that specimen although this area is more nearly intact in AMNH 26020. The nasal incision is very shallow and the posterior margin is not higher than the orbit. The nasal incision extends posteriorly to the anterior margin of P4. The orbit is situated directly above the anterior part of M2 and the posterolateral root of M1. The anterior rim of the orbit is above the anterolateral root of M1 and the posterolateral root of P4.

Despite the severe distortion, the nasal process of the holotype (AMNH 20354) is in good condition. From a dorsal view the nasal process is rather broad, of relatively constant width throughout its length, and the distal margin appears to be weakly rounded. From a lateral view the nasal process projects more or less horizontally from the skull. The sides of the nasal process form thickened and downturned lateral walls. The anterior margin is not as thickened as the lateral walls. The more complete nasal process of AMNH 26020 is similar to the holotype, although it is much thicker and the more nearly intact anterior edge is strongly rounded with a downturned distal tip. The lateral walls are very thick proximally and shallow distally. Additionally, from a dorsal view the nasal process tapers distally. The nasal bones are completely fused together, although at the distal tip they separate, creating a small median notch.

The nasal process and premaxillomaxillary rostrum are similar in length. The rostrum of the holotype (AMNH 20354) is severely twisted, but it is undistorted in AMNH 26020. From a lateral view the rostrum curves upward distally and deepens proximally. The dorsal margin of the rostrum rises posteriorly to the level of the midpoint of the orbit. The anterior margin of the premaxilla is flat and does not extend far beyond the canines. The premaxillomaxillary suture is not visible. Behind the premaxillary symphysis the dorsolateral margins of the rostrum diverge posterolaterally and the dorsal surface of the rostrum is not sealed by bone.

Though crushed, the holotype skull ( AMNH 20354 ) indicates a shallow saddle-shaped cranium. The dorsal surface of AMNH 26020 is incomplete. In the holotype the left parasagittal ridge is complete and appears to moderately constrict the posterodorsal surface. In AMNH 26020 the parasagittal ridges appear to be more distantly separated posteriorly ; however, there are significant plaster-filled gaps between the bone fragments in this part of the skull. Therefore, the width of the posterodorsal roof appears to be exaggerated in that specimen.

The left zygomatic arch of the holotype (AMNH 20354) is in relatively good condition and resembles the zygomatic arch of AMNH 26020. The zygomatic arch is thick and rectangular in cross section. From a dorsal view the zygomatic arch is straight and angled posterolaterally. Large central zygomatic swellings as seen in Protitanops , Duchesneodus , and Megacerops (sensu Mihlbachler et al., 2004b) are not present on any specimen of Parabrontops gobiensis . From a lateral view the jugal zygomatic process is horizontal. It is shallow anteriorly and deepens posteriorly. The squamosal zygomatic process is deep and straight, thus the zygomatic arch has essentially no curvature.

Granger and Gregory (1943) described the occiput (preserved only partially in AMNH 20354) as wide, although realistically the shape and proportions of the occiput cannot be readily described with any certainty. However, from the ventral view of AMNH 20354 the posterior end of the skull does not appear to have been greatly widened as seen in Metatitan or Rhinotitan andrewsi .

The anterior rim of the posterior nares is positioned between the protocones of M 3 in AMNH 20354, and slightly more posteriorly in AMNH 26020. A horseshoe-shaped rim emarginates the posterior nares. The elongate posterior narial canal does not appear to extend into the body of the sphenoid in AMNH 20354, although this area is poorly preserved. The entire basicranium of AMNH 20354 is poorly preserved, although one can discern that the foramen ovale and foramen lacerum are widely separated. The mastoid process is much shorter than the postglenoid process and it constricts the opening of the external auditory pseudomeatus, but it does not arch anteriorly to contact the postglenoid process. Thus, the external auditory pseudomeatus does not appear to be tube-shaped in AMNH 20354, although it is uncertain whether this reflects the actual condition of Parabrontops gobiensis or whether it is an artifact of distortion.

UPPER DENTITION: Close-ups of the upper dentition of Parabrontops gobiensis can be seen in figure 140. The holotype (AMNH 20354) lacks incisors and canines, although the remaining alveoli indicate an unreduced dental formula (3-1-4-3). Three small right incisor alveoli can be seen on the premaxilla of the holotype (fig. 140f). AMNH 26020 also lacks incisors, although portions of the roots of left I1, I2, and I3 are labeled in fig. 140e. The roots of the I2 and I3 are clearly preserved in AMNH 26020. Granger and Gregory (1943) state, ‘‘the alveoli of the medial pair of incisors (I1), if formerly present, are not evident’’ ( Granger and Gregory, 1943: 367). However, Granger and Gregory (1943) were mistaken; a remnant of the root of the left I1 is present. The right I1, on the other hand, appears to be absent. Probably, it was lost during life due to bone remodeling. There are two very small globular incisor crowns associated with AMNH 26020 although neither of these articulates with the fragments that are still rooted in the skull and they certainly do not belong to the same individual. The incisor row of AMNH 26020 is narrow, nearly straight, and positioned only slightly anterior to the space between the canines. The complete canines of AMNH 26020 are very small. There is no precanine diastema, but a distinct short postcanine diastema is present.

The cheek teeth of AMNH 20354 are smashed, although the morphology of the right premolars and left posterior molars is partially discernable (figs. 139 and 140d). Additionally, the right cheek-tooth row of AMNH 26020 is intact (fig. 140b, c). The P1 is heavily worn. It is smaller than those premolars that are posterior to it, but it is nearly as broad (transversely) as it is long (anteroposteriorly). The morphology of P1 is relatively advanced with two distinct labial cusps that are joined by an ectoloph, and a small lingual heel with a small protocone.

The P 2–P4 of AMNH 20354 are bisected by a large crack that separates the ectolophs from the lingual heels ; therefore the widths of these teeth are exaggerated. The dimensions of the P2–P4 are nearly intact in AMNH 26020 , although the P2 and P3 of that specimen are separated by an artificial gap. The premolars are nearly rectangular, although the anterior margins of P2 and P3 are slightly angled more posterolingually than the posterior margins. The labial paracone ribs are small but well defined and become progressively narrower in consecutively posterior premolars. The parastyle of P2 is straight or arched somewhat lingually. The P 3 parastyle is angled slightly labially, while the P4 parastyle is strongly arched labially. The metastyles of P2 and P3 are straight while that of P4 is angled slightly labially. The lingual side of the P2 of AMNH 20354 is damaged, but a large protocone and lingual crest extending posteriorly from it can be discerned. The hypocone is either absent or so small that the lingual crest has absorbed it. Two distinct lingual cusps can be found on P3–P4. The hypocones of P3 and P4 are smaller than the protocones and are not connected by a crest. The P 2–P4 of AMNH 26020 vary from the holotype in that the hypocones of the P3 and P4 are strongly connected to the protocone by a lingual crest. In both specimens a low but distinct preprotocrista is seen on P2. The preprotocrista is less distinct on P3 and is not discernable on P4. There is no evidence of anterolingual cingular cusps on any of the premolars. The premolar cingula are continuous around the lingual side of the crowns of AMNH 20354 , but in AMNH 26020 the lingual cingula are slightly discontinuous on P2 and P3 .

The upper molars of Parabrontops gobiensis show typical brontotheriine characteristics, including increased ectoloph height, convex and lingually angled labial walls, very weak labial ribs, thin lingual ectoloph enamel, and wedge-shaped lingual sides of the paracone and metacone in molars that are not heavily worn. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. Distinct central molar fossae are seen in M1–M3. All evidence of paraconules and metalophs is absent. The M3 exhibits a large anterolingual cingular cusp. This feature is less obvious on M1 and M2, but this appears to be due to the greater extent of wear of those teeth. The M3 has a very small pointed hypocone. The labial and lingual molar cingula are very weak and discontinuous.

cf. Parabrontops gobiensis mandibles

REFERRED SPECIMENS: (From the Urtyn Obo Formation, Urtyn Obo, Shara Murun Region, Inner Mongolia) AMNH 26019, a mandible with right and left incisors and canines (roots only), and p1–m3; AMNH 26021, a partial mandible with left p1–m3; (from the ‘‘Shara Murun’’ Formation [sensu Radinsky, 1964], East Mesa, Shara Murun Region, Inner Mongolia) AMNH 26131, a mandible with right and left i1–m3.

DESCRIPTION

MANDIBLE AND LOWER DENTITION: None of the AMNH skulls of Parabrontops gobiensis is directly associated with a mandible, although several mandibles in the collection might represent this species. Granger and Gregory (1943) referred two mandibles to P. gobiensis, AMNH 26019 and AMNH 26131. There are minor differences between these two specimens and the others that are described below.

Generally, these mandibles are consistent with what one would predict for the mandible of Parabrontops gobiensis with three small lower incisors that form a nearly straight row between the incisors and a short postcanine diastema. The combination of these characters rule out most other species of other large Asian brontotheres (e.g., Protitan , Rhinotitan , or Metatitan ). The most complete of these mandibles (AMNH 26131) is the larger of the two (fig. 141). The ventral margin of the symphysis is convex and steeper than 45 °. The symphysis extends to the talonid of the p4; however, this position of the posterior margin of the symphysis appears to have been effected by a paleopathology, possibly a dento-alveolar abscess near or on the lingual side of P4 and M1, that has resulted in a swollen right ramus.

Six small incisors form nearly a straight row between the anterior margins of the canines. The incisors appear to be wedge-shaped, although the apices of the crowns are worn. The incisors lack distinct lingual cingulids although this could be related to wear. The i2 is the largest incisor. The canines are of moderate size. There are no precanine diastema or gaps between the incisors. The postcanine diastema is about as long as p2.

The p1 is small with a small talonid heel. The p2 trigonid is much longer and slightly narrower than the talonid. The p3 and p4 trigonids and talonids are of similar width, although the trigonids are narrower. The paralophid of p2 is weakly angled lingually, creating a small lingual trigonid notch. The p2 protolophid is straight, but it is positioned lingually. The paralophids and metalophids of p3 and p4 are strongly arched lingually, creating nearly molariform trigonid basins. A large, lingually positioned metaconid is found on p3 and p4 but not on p2. The talonids of p2–p4 all have well-developed cristids obliqua and hypolophids with broad basins. Labial and lingual premolar cingulids are essentially absent. The molars are typical with thin lingual enamel, shallow talonid and trigonid basins, and m3 is elongate. There are very thin labial molar cingulids but no lingual molar cingulids.

The other mandible referred to Parabrontops gobiensis (AMNH 26019) by Granger and Gregory (1943) differs from AMNH 26131 in several ways (fig. 142). The inferior margin of the symphysis is angled slightly less than 45 °, although the extent to which distortion has affected this variable in either AMNH 26131 or AMNH 26019 is uncertain. The posterior margin of the symphysis is slightly more anterior, although comparison of this character is complicated by the seemingly pathological condition of the ramus in AMNH 26131. The most obvious difference between these two specimens is the premolars. The premolars of AMNH 26019 seem less molariform. The p2 talonid has a broad lingual notch, but lacks a basinlike depression. The p3 paralophid is only weakly lingually angled, the p3 protolophid is straight, and there is no sign of a p3 metaconid. Additionally, the m3 of AMNH 26019 is proportionately shorter.

REMARKS

Osborn (1925) named Parabrontops gobiensis from a nearly complete but extremely crushed and distorted skull. Osborn (1925) originally assigned this species to the North American genus Brontops (5 Megacerops sensu Mihlbachler et al., 2004b ). Osborn’s initial description of the holotype was very brief and was based on the ‘‘cranium reconstructed after comparison with that of Brontops brachycephalus ’’ ( Osborn, 1925: 5). Subsequently, Osborn (1929a) described and figured an interpretation of the skull had it been undistorted.

Because Osborn’s (1929a) reconstruction was based on North American Brontops , the reconstruction of the holotype skull figured by Osborn (1929a), and the cranial measurements that were provided are not necessarily reliable. Granger and Gregory (1943) were the first to actually figure and describe the specimen in its extremely distorted condition. They assigned this species to its own genus, Parabrontops .

The material from the AMNH collection that was originally described by Granger and Gregory (1943) is the only material assignable to P. gobiensis . Yanovskaya (1980) referred another skull of P. gobiensis from the Ergilin Dzo, but this specimen is more consistent with Eubrontotherium clarnoensis . Likewise, Mihlbachler et al. (2004a) wrongly suggested that other materials from the PIN collection (the material misidentified as Metatitan relictus by Yanovskaya [1980]) could belong to P. gobiensis ; all of these materials are now referred to Eubrontotherium clarnoensis . Hu (1961) referred a set of upper molars (IVPP V2490) from an unknown formation in the Hami Basin of China to Parabrontops (see Russell and Zhai, 1987), although this specimen lacks characters that are diagnostic of P. gobiensis .

Granger and Gregory (1943) considered Parabrontops to be very closely related to Metatitan ; they suggested that the only differences between these taxa are the larger horns and lesser width of the incisive border of the premaxillary. However, Parabrontops gobiensis differs from Metatitan in several other ways, such as the lack of paired ventral sphenoidal fossae, the more completely saddle-shaped skull, the presence of a postcanine diastema, and substantially more molarized premolars. Moreover, the horns and nasal process are not elevated, and the basicranium and occiput are not widened as in Metatitan . Parabrontops more closely resembles Dianotitan , Eubrontotherium , Protitanops , Duchesneodus , and Megacerops . Parabrontops shares with these species relatively large, widely separated frontonasal horns and reduced globular incisors. Recognizing these similarities, Wang (1982) considered Parabrontops to have closer phylogenetic affinities with North American ‘‘Brontopinae’’ than with other Asian brontotheres. Parabrontops gobiensis closely resembles Eubrontotherium , particularly in its small incisors and in lacking conspicuous zygomatic swellings. However, Parabrontops gobiensis differs from Eubrontotherium in its retention of three upper incisors.

Eubrontotherium clarnoensis Mihlbachler, 2007

HOLOTYPE: UCMP 126100, a skull missing the occiput with right and left I3–M3.

TYPE LOCALITY: UCMP locality I- V75203, Clarno Formation, Hancock Quarry, Wheeler, Oregon.

AGE: Late or Middle Eocene of North America (latest Uintan, Duchesnean, or Chadronian land mammal ‘‘age’’?), Late Eocene of Asia (Ulangochuian land mammal ‘‘age’’).

REFERRED SPECIMENS: (See Mihlbachler [2007] for a list of referred specimens from the Hancock Quarry locality); (from Ergilin Dzo (lower part), Dornogobi Province, Outer Mongolia) an associated partial skeleton all assigned to PIN 3109 with the following numbers: 90, a partial skull missing the left zygomatic and left side of cranium with right I1–M3, and left I1–M2, M3 (partial); 91, a mandible with right and left i1–m3; a partial postcranial skeleton with the following numbers 92–102, 103, 104, 106, 111–116, 142– 231, 232; PIN 3109-39, a flattened skull with right and left I2–M3.

DIAGNOSIS: Eubrontotherium clarnoensis is a large horned brontothere with small rounded frontonasal horns. The horns are positioned anterior to the orbits and high on the skull, but the horns and nasal process are not elevated to the degree seen in Diplacodon elatus . The nasal incision extends as far back as the anterior margin of the P4. The anterior rim of the orbit is between the anterior and posterior lateral roots of the M1. The nasal process is broad, horizontal, slightly downturned, of nearly constant width throughout its length, strongly rounded anteriorly, and with shallow and thickened lateral walls. The premaxillomaxillary rostrum thickens posteriorly and it is not enclosed by bone dorsally. Other cranial characteristics include a saddle-shaped cranium, a dorsal cranial surface that is moderately constricted posteriorly by parasagittal ridges, a narrow emargination surrounding the posterior nares, moderately curved zygomatic arches, and a ventrally constricted and mediolaterally angled external auditory pseudomeatus. Ventral sphenoidal fossae, postzygomatic processes, and lateral zygomatic swellings are absent.

Dentally, Eubrontotherium clarnoensis is characterized by two small, globular upper incisors that form a straight row, a long postcanine diastema, a complex P1, a distinct P2 metacone, and premolar hypocones on P2–P4. 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 and anterolingual cingular cusps are present. Paraconules and metalophs are absent. The lower dentition of Eubrontotherium clarnoensis includes three very small wedge-shaped lower incisors, a distinct postcanine diastema, a metaconid on p4 but not on p2 and p3, and a p2 trigonid that is less than twice the length of the talonid. The lower molars have shallow basins and the m3 is slender.

Eubrontotherium clarnoensis is the only brontothere species that simultanteously lacks large zygomatic swellings and has a a reduced number of upper incisors.

DESCRIPTION

The type specimen of Eubrontotherium clarnoensis and other material representing this species from the type Hancock Quarry locality of Oregon was described by Mihlbachler (2007) and is not repeated here. The material described below is additional material from Asia that is consistent with Eubrontotherium clarnoensis , and appears to document the occurrence of the same species in Asia.

SKULL: PIN 3109-90 is a partial skull that is missing the left zygomatic arch and much of the left side of the cranium (fig. 143). Although the surface of the skull is extensively fractured, it does not appear to be significantly crushed or distorted, although the right zygomatic arch seems to have been displaced slightly medially. From a lateral view this specimen provides a more faithful representation of the shape of the skull of Eubrontotherium clarnoensis in comparison to the type skull from Oregon (UCMP 126100) which is dorsoventrally flattened. A significantly more damaged skull, PIN 3109-39 (fig. 144), is also referable E. clarnoensis . This specimen is dorsoventrally crushed and is missing its frontonasal region and the dorsal portion of the occiput.

The horns of PIN 3109-90 are relatively smooth, short, and with slightly elliptical bases but with more rounded peaks. The horns project in an anterodorsolateral direction and are positioned between the orbits and the nasal incision. The contact of the frontal and nasal bones can clearly be seen on the lateral surface of the right horn in PIN 3109-90. The frontal forms the apex and posterodorsal surface of the horn, while the nasal forms the base and anterodorsal surface. The horns and nasal process are elevated well above the orbits in this specimen, although not to the degree seen in Diplacodon elatus . The right and left nasal bones are completely co-ossified. The nasal process is broad and slightly shorter than the premaxillomaxillary rostrum. From a lateral view the nasal process curves downward and its dorsal surface is somewhat convex. The lateral margins of the nasal process are very thick, curved downward, and become somewhat deeper proximally. The distal margin of the nasal is strongly rounded but it is not strongly downturned. The distal margin of the nasal process is thick but not as thick as the lateral margins. The nasal incision extends posteriorly to a point above the anterior margin of P4. The orbit is directly above the posterior part of M2 and the anterior part of M1, with the anterolateral root of M1 positioned directly below the anterior orbital rim.

From the lateral view, the premaxillomaxillary rostrum is slightly longer than the nasal process. The rostrum curves upward distally although the anteriormost portion is angled slightly downward. The rostrum deepens proximally while its lateral dorsal margin slopes upward and rises to about the midlevel of the orbit. The premaxilla is short and does not extend anterior to the canines.

The dorsal surface of the skull is fully concave. The parasagittal ridges are prominent and remain widely separated throughout their length, although they moderately constrict the transverse width of the posterodorsal cranial surface. From a lateral view the zygomatic process of the jugal is horizontal while the zygomatic process of the squamosal is sloped posterodorsally, thus giving the zygomatic arch a moderate curvature that is more pronounced than that of Parabrontops . From a dorsal view the zygomatic arch is straight, thin, and angled posterolaterally. Neither PIN 3109-90 nor PIN 3109-39 has strongly laterally bowed zygomatic arches or thick laterally projecting zygomatic swellings at the junction of the jugal and squamosals.

The occiput of PIN 3109-90 is poorly preserved, limiting its description. It is moderately tilted backward with a relatively flat nuchal crest. The dorsal portion of the occiput is similar in width to the ventral portion. The nuchal crest appears to have a small median notch. The nuchal crest of PIN 3109-90 is thinner than that of UCMP 126101, the only North American specimen of Eubrontotherium clarnoensis with a preserved nuchal crest. However, that particular skull (UCMP 126101) is rather robust in comparison to other North American specimens; nuchal crest thickness appears to covary with overall cranial robustness and could be related to sexual dimorphism.

The ventral surface of the skull of PIN 3109-90 is incomplete and is presently obscured by a steel mounting frame. However, a few details can be discerned from Yanovkaya’s (1980) original figure of it, reprinted in fig. 145a. The basicranium of PIN 3109-39 (fig. 144) is also complete although it is crushed. The anterior rim of the posterior nares is positioned anterior to M3. The elongate posterior narial canal does not appear to extend significantly into the sphenoid as it shallows off posteriorly, and large ventral sphenoidal fossae are not seen in either specimen. Like most other horned brontotheres, the external auditory pseudomeatus is tube-shaped and enters the skull in a mediolateral direction.

UPPER DENTITION: The dentition of PIN 3109-90 is less worn than that of PIN 3109-39 and thus provides the bulk of the following description (fig. 145). Both skulls indicate two pairs of upper incisors that form a nearly straight row directly anterior to the canines. In PIN 3109-90 both incisor pairs are present with a large median diastema between the central pair. The other skull, PIN 3109-39, retains the outer pair, although the alveoli for the central pair are still present. Both the medial and lateral incisors are very small (essentially vestigial) with small globular crowns. Neither specimen has any evidence of a third pair of incisors.

The canines of PIN 3109-90 are a little smaller than those of PIN 3109-39. There is a short diastema between the lateral incisor and canine in PIN 3109-39, but this diastema is absent in PIN 3109-90. However, both specimens retain a distinct postcanine diastema of variable length ranging from 1.2 cm (PIN 3109-39) to 3.5 cm (PIN 3109-90).

The P1 is small and ovoid in outline with the widest point near the posterior side of the crown. There are two distinct labial cusps, a paracone and a much smaller metacone. The lingual heel is small and posteriorly positioned. The lingual heel of the P1 has a small cusp with a small but distinct preprotocrista that attaches to the lingual side of the paracone; these features are more obvious on the right P1.

The ectolophs of the right P2–P4 are damaged. The left P2–P4 are nearly rectangular in shape with nearly parallel anterior and posterior sides; although P2 and, to a lesser extent, P3 are oblique. The P2 parastyle is angled anteriorly, while the parastyles of P3 and P4 have a more anterolabial orientation. The metastyles of P2–P4 are more or less straight and not distinctly angled labially. From an occlusal view the labial surfaces of the P2 paracone and metacone are broad convexities. The labial sides of the paracones and metacones of P3 and P4 are flatter with distinct labial ribs. Distinct labial ribs are not seen in the metacones of P3 and P4. The P4 of PIN 3109-90 has a small mesostyle. P4 mesostyles are occasionally found within numerous brontothere species, but usually only rarely. However, the P4 mesostyle appears to have been rather frequent in Eubrontotherium clarnoensis ( Mihlbachler, 2007) .

The P2–P4 of PIN 3109-90 tend to have dual lingual cusps. The P2 hypocone is poorly developed and connected to the protocone by a weak lingual crest. The hypocones of P3 and P4 are more developed, although they remain smaller than the protocone and are weakly connected to it by a small lingual crest. The P2 has a distinct preprotocrista. The P3 and P4 have discernable preprotocristae, but they are not as developed. The anterior premolar cingula thicken near their midpoints directly between the protocone and the lingual edge of the paracone. However, these cingular thickenings do not form distinct cingular cusps as occasionally seen in Duchesneodus uintensis and Megacerops coloradennsis . Labial premolar cingula tend to be weak while lingual premolar cingula are thicker and tend to be continuous or only slightly discontinuous around the lingual base of the protocone.

The upper molars of PIN 3109-90 are not well preserved, but they show a morphology that is essentially undifferentiated from closely related taxa. The ectoloph is rather tall and its labial wall is strongly angled lingually. The vertical labial ribs of the paracone and metacone are very weak. The lingual band of ectoloph enamel is much thinner than the labial band and the lingual margins of the paracone and metacone are wedge-shaped rather than rounded. The anterior cingulum of the molars is thin and passes proximally to the distal peak of the parastyle. Small anterolingual cingular cusps are present. Due to poor preservation central molar fossae are not visible on the holotype specimen; however, remnants of small central molar fossae can be seen on the extremely worn molars of the referred skull, PIN 3109- 39 (fig. 144). The M3 of PIN 3109-90 has a large hypocone, while PIN 3109-39 appears to have had only a small hypocone. Labial and lingual molar cingula are exceedingly weak or absent.

MANDIBLE AND LOWER DENTITION:

The mandible (PIN 3109-91) is missing only the left ascending ramus (fig. 146). In comparison to the North American mandible of Eubrontotherium clarnoensis (UCMP 126102), the horizontal ramus of PIN 3109-91 seems deeper and the symphysis seems more narrow; however, these differences seem to relate to minor lateral crushing in PIN 3109-91. From a lateral view of PIN 3109-91, the inferior margin of the mandibular symphysis is slightly convex and seems to be angled a little less than 45 °. From a dorsal view the symphysis seems rather narrow and extends posteriorly to the p4 talonid.

Although the upper incisors of Eubrontotherium clarnoensis are reduced to two pairs, three pairs of small lower incisors are retained. The lower incisors form a nearly straight row between the anterior margins of the canines. Each of the incisors is tiny and slightly wedge-shaped. A thin vertical rib can be seen on the lingual surface of i1 and i2. The i2 is distinctly larger than i1 or i3. Labial incisor cingulids are absent and lingual incisor cingulids are exceptionally weak. In contrast to the North American mandible of Eubrontotherium in which the incisors have small gaps between them, the incisors of PIN 3109-91 appear to be unusually crowded between the canines, although this difference may also relate to transverse crushing.

The lower canines of PIN 3109-91 are relatively large. A postcanine diastema of about 3 cm is present. The p1 is small and narrow, with a single cusp and a small, simple talonid heel. The p2 trigonid is slighly narrower and longer than the p2 talonid. The p3 and p4 trigonids are narrower than the talonids, but the trigonids are more similar in length to the talonids. The paralophid of p2 arches slightly lingually, creating a small lingual trigonid notch. The p2 protolophid is angled slightly lingually. The paralophid and protolophid of p3 are more strongly angled lingually, creating a broader lingual notch. The paralophid and protolophid of p4 are longer and arch fully lingually. Metaconids are absent on p2 and p3, but a large, molariform, lingually positioned metaconid can be found on p4. The talonid of the p2 has a short cristid obliqua and hypolophid with a shallow lingual-talonid notch. The talonids of p3 and p4 have longer cristids obliqua and hypolophids with more nearly molariform talonid basins. Labial and lingual premolar cingulids are essentially absent. The lower molars of PIN 3109-91 have shallow occlusal basins, thin lingual enamel, and an elongate m3, and they are not different from other advanced brontotheres. Lingual cingulids are absent while labial cingulids tend to be weak and discontinuous around each labial cusp.

REMARKS

The specimens from the Ergilin Dzo Formation of Mongolia described above were initially identified by Yanovskaya (1980) as belonging to other species. PIN 3109-90 and 3109-91 were referred to Metatitan relictus and PIN 3109-39 was referred to Parabrontops gobiensis . These identifications appear to be erroneous. The skull, PIN 3109- 90, is clearly different from M. relictus . The horns and nasal process are not as highly elevated, the orbits are more anteriorly positioned, they have a reduced number of upper incisors, there is a distinct postcanine diastema, basisphenoidal fossae are absent, the squamosal wing forms a much shallower angle with the zygomatic arch, the occiput is not extremely widened, and the basicranium is not anteroposteriorly shortened. Likewise, the jaw of PIN 3109-91 differs from Metatitan most notably by a distinct postcanine diastema. Additionally, the lower premolars are more slender, less molariform, and the p3 lacks a metaconid.

As noted above, the skull of PIN 3109-39 was assigned to Parabrontops gobiensis by Yanovskaya (1980). Indeed, all of the PIN material in question (3109-90, 3109-91, 3109- 39) is similar to Parabrontops . The tiny globular upper incisors and correspondingly small wedge-shaped lower incisors strongly suggest that both skulls (PIN 3109-90, PIN 3109-39) and mandible (PIN 3109-91) belong to a species similar to P. gobiensis . However, none of these specimens are directly referable to that species. Parabrontops retains three upper incisors, while the skulls PIN 3109-90 and PIN 3109-39 have only two. Further differences from P. gobiensis include the higher more anteriorly positioned horns, slighly more curved zygoamatic arch and slightly more distinctive preprotocrista.

The PIN specimens in question (3109-90, 91, 39) are consistent with Eubrontotherium clarnoensis ( Mihlbachler, 2007) , a recently described species that was previously known only from the Hancock Quarry, a prolific bone deposit in the Clarno Formation of Wheeler County, Oregon ( Hanson, 1989, 1996). E. clarnoensis is the only brontothere that simultaneously has a reduced number of upper incisors and lacks conspicuous zygomatic swellings. The PIN specimens share this combination of character states and are consistent with the North American E. clarnoensis in other respects. The few differences between the North American and Asian specimens that can be found are attributable to taphonomic deformation. For instance, the somewhat lower horns of the type skull from the Hancock Quarry (UCMP 126100) are related to the fact that that skull has been dorsoventrally crushed. Likwise, the apparently narrower symphysis of PIN 3109- 91 in comparison to the North American material, is related to the transverse crushing of that specimen.

‘‘Eubrontotheres’’ (sensu Schoch and Lucas, 1985) (infratribe Brontotheriita based on the classification presented in this paper) have been thought of primarily as a North American group notably because of their abundance in North America (i.e., Megacerops and Duchesneodus ) (Schoch and Lucas, 1992). However, there are multiple Asian eubrontotheres including Parabrontops gobiensis and Dianotitan lunanensis . Additionally, Eubrontotherium clarnoensis apparently occurs on both continents. Finally, it is notable that while the North American Hancock Quarry Eubrontotherium has been attributed to middle and late Eocene land mammal ages (latest Uintan [Lucas et al., 2004], Duchesnean or Chadronian [ Hanson, 1989; Lucas, 1992; Robinson et al., 2004]), the occurrence of E. clarnoensis in the Ergilin Dzo of Mongolia argues for a late Eocene occurrence of this species in Asia, although with so few occurrences, little can be said about the actual temporal duration of this species on either continent.