Mesatirhinus junius ( Leidy, 1872 )

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 : 22-33

Mesatirhinus junius ( Leidy, 1872)

HOLOTYPE: ANSP 10349 , a right p4; ANSP 10348 , a partial left m3, apparently from the same individual.

TYPE LOCALITY: Bridger Basin, Wyoming.

SYNONYMS: Mesatirhinus megarhinus ( Earle, 1891) ; Mesatirhinus petersoni Osborn, 1908a .

AGE: Middle Eocene (Bridgerian and possibly early Uintan land mammal ‘‘ages’’)

REFERRED SPECIMENS: (From the Bridger Basin, Wyoming) AMNH 1509 , a skull with right and left P1–M3 ; AMNH 1523 , a palate with right P1–M3, left P1, and P3–M3 ; AMNH 1520 , a left mandibular ramus with c–m3 ; AMNH 1551 , a partial left mandibular ramus with c and p2–m2 ; AMNH 1567 , a mandible with right c, p2–m3 and left c–m3 ; AMNH 1627 , a mandible fragment with right m3 ; AMNH 12184 (holotype of Mesatirhinus petersoni ), a skull missing the occiput and right zygomatic arch with right P2, P3, P4–M3 (partial), left P2–P4, M1–M2 (partial), and M3 ; AMNH 12191 , a right maxilla fragment with M2–M3 and a left mandibular fragment with p2–m3 ; AMNH 12199 , a mandible with right and left p3–m3 ; AMNH 12202 , a skull missing the premaxillae with right C–M3 and left P2–M3 ; AMNH 12206 , a skull fragment with right C–P4, M1–M2 (partial), left P1–P4, and M1–M3 (partial) ; AMNH 12211 , a mandible with right i3(?), p2, p3, p4 (partial) and left p2–m1 ; AMNH 12686 , maxillary fragments with right M2 and M3 ; BMNH. M10471 (formerly AMNH 1556 ), a skull with right P1–M3 and left C– M3 ; UCM 72430, a crushed skull fragment; USNM 26111, a mandible with right p2–p4, m3, left p3–p4, m1 (partial), and m2–m3; USNM 26116, a skull with right P2–M3 and left P4–M3; USNM 26123, a fragmented skull; USNM 26136, a skull with right C, P3– M3 and left C–M3; USNM 26148, a left mandibular ramus with p2–m3; USNM 26151, a mandible with right, left p3–m3, and fragmentary canines and incisors; USNM 363884, a left mandibular ramus with p2–m2, isolated incisors and p1; YPM PU 10242, a left maxilla fragment and associated upper teeth, including right P2– M1, left M2, and M3; YPM 11070, a partial skull with right P2–M3; YPM 11148, a skull with right P1–M3, left P3, M1–M3; YPM 11149, a skull with right and left M3; YPM 16420, a partial skull with left P2–P3, M1– M3; YPM 16423, a right maxilla with P2– M3; YPM 16421, a partial mandible with p4– m3, YPM 16722, a right maxilla fragment with P4–M2, M3 (partial), and left maxilla fragment with M1–M3; YPM 16725, a left mandibular ramus with p3–m3; YPM 16732, a left mandibular ramus with c, p2–m3; YPM 16743, a mandible fragment with left m2–m3; YPM PU10118, a mandible with right and left p3–m3; YPM PU10184, a partial mandible with right p3–m3 and left m1–m3; (from the Washakie Basin of Wyoming) AMNH 1512 , a mandible with right and left p2–m3 ; AMNH 1513 , an anterior half of skull with right P2, P3, P4, M3 (partial), and left C–M2, M3 (partial) ; AMNH 1514 , a premaxillamaxilla with I1–M3 ; AMNH 1571 , a palate with right I1–M3, left I2, C, P1–P3, and P4 (partial) ; AMNH 1575 , a partial mandible with right i2–c, left i2–c, and p2–m3 ; AMNH 1577 , a mandible with right and left p2–m3 ; AMNH 1651 a, a left maxilla fragment with M1–M3 ; AMNH 13178 , a partial mandible with right c, p2–p3 (partial), left i2 (?), c, p2, p4–m3 ; FMNH PM1676 , a crushed skull with right P2, P4–M1 and left P4–M1 ; FMNH PM27939 , a skull with C–M3 ; FMNH PM36045 , a skull with right P2– M3, left C–P4, and M1–M3 (all partial) ; FMNH PM39945 , a partially prepared palate with crushed cheek teeth ; FMNH PM54864 , a crushed skull fragment with left P2–M3 ; YPM PU10008 (holotype of Mesatirhinus megarhinus ), a partial skull with very poorly preserved molars; YPM PU10041, a posterior part of a cranium; (from the Sand Wash Basin of Moffat County, Colorado) DMNH 8103 , a right maxilla with C–M3 ; DMNH 9687 , a left mandibular ramus with p3–m3 ; DMNH 29950 , a partial skull (in two pieces) missing nasals and frontals with right P4–M1 (partial), and M2–M3; (no locality data) YPM PU25021, a crushed skull with right P2–M3, left C, P2–P4, M1–M3 (all partial), and a mandible with right and left p3–m3.

DIAGNOSIS: Mesatirhinus junius is a small hornless brontothere in which the frontal bone does not overlap or intrude into the nasal bone. The nasal incision extends posteriorly to between the anterior margin of P4 and the posterolateral root of M1. The nasal process is horizontal, unelevated, of relatively constant transverse width, narrow, with thin and deep lateral walls, and without a well-defined or strongly rounded distal margin. The orbits do not protrude laterally and are positioned over the M2 with the anterior lateral root of M2 and posterior lateral root of M1 below the anterior orbital rim. There is a prominent infraorbital process on the jugal. The premaxillomaxillary rostrum deepens posteriorly and it is not enclosed dorsally by bone. Other cranial characteristics include a flat or convex dorsal cranial surface, a sagittal crest, thin and weakly curved zygomatic arches, a ventrally open external auditory pseudomeatus, and wide occipital condyles.

Dentally, Mesatirhinus junius is characterized by large subcaniniform upper incisors, a postcanine diastema, a simple P1, a distinct P2 metacone, weak premolar preprotocristae and/or paraconules, and short crests extending posteriorly from the protocones of the premolars. Premolar hypocones are absent. The molars of M. junius have tall, lingually angled ectolophs with weak labial ribs, and somewhat thinned lingual ectoloph enamel with wedge-shaped paracones and metacones. A cingular parastyle shelf is absent. Mesatirhinus junius molars retain vestigial paraconules and metalophs, and they lack central fossae and anterolingual cingular cusps. The lower dentition of M. junius is characterized by large semispatulate incisors that are all of a similar size, a short p1–p2 diastema, an elongate p2 trigonid, a metaconid on p4 but not on p2 and p3, shallow molar basins, and a slender m3.

Mesatirhinus junius is very similar to Sphenocoelus uintensis but is clearly distinct from that species due to its smaller size, more brachycephalic proportions, and lack of paired ventral sphenoidal fossae. M. junius is similar in size to Metarhinus , but can be differentiated from it by its unspecialized rostrum.

DESCRIPTION

SKULL: Many skulls of Mesatirhinus junius are known, but because each one is damaged or distorted in some way it is necessary to refer to several specimens for a full description. The following description is primarily based on AMNH 1509 (fig. 6), AMNH 12202 (fig. 7a, fig. 8a), USNM 26116 (fig. 7b), and AMNH 12184 (fig. 7c). M. junius is a small (table 1) hornless brontothere similar in size to Metarhinus , but it is smaller than other late Bridgerian taxa, e.g., Telmatherium and Palaeosyops , that are contemporaneous with it. The general morphology of the skull most closely resembles Sphenocoelus uintensis , though it is smaller and more brachycephalic.

The frontonasal suture is not clear in most of the specimens. Osborn’s (1929a: fig. 328) illustration of AMNH 1556 (now in the British Museum with number BMNH. M10471) seems to portray a suture that looks more like a crack on the actual specimen. The frontonasal suture on the left side of the skull of AMNH 12184, also figured by Osborn (1929a: fig. 327), appears as a deeply convoluted line. This represents a true frontonasal suture and indicates the absence of a frontal process overlapping the nasal bone like that seen in Telmatherium validus .

The nasal bones tend to be poorly fused together. The nasal process is shorter than the premaxillomaxillary rostrum and is transversely narrower. The nasal process is slightly angled downward as it projects from the skull. The sides of the nasal process form relatively thin and deep vertical walls. From a dorsal view, the nasal process appears to be slightly constricted proximally in AMNH 1509, but in other specimens, such as AMNH

TABLE 1 Summary statistics for selected morphometric variables of Mesatirhinus junius See Methods for measurement definitions

12184, the nasal process is more nearly constant in width. The anterior margin of the nasal process is flat from a dorsal view, except for a median notch. The anterior edge of the nasal process is thin, roughened, and angled downward only slightly.

The nasal incision of Mesatirhinus junius is long and shallow. The position of the posterior margin of the nasal incision fluctuates slightly from the anterior margin of M1 (e.g., AMNH 1509, AMNH 12184) to the posterolateral root of M1 (YPM 11070). The orbits tend to be positioned more or less over the M2 although its position with respect to the molars varies slightly. For instance, in YPM 10008, the alveolus for the anterior lateral root of M1 is directly below the anterior edge of the orbital rim; this is somewhat more anteriorly positioned than the majority of M. junius specimens where the anterior lateral root of M2 and posterior lateral root of M1 are situated below the anterior orbital rim.

A shallow facial concavity exists between the nasal incision and orbit. The facial concavity is bordered superiorly by a prominent rim of bone that abruptly meets the nearly flat dorsal surface of the skull. The rim gives this part of the skull a somewhat thickened appearance. Osborn (1929a) described this species as having incipient horns, although Mader (1989) could not corroborate the presence of incipient horns. Later, McCarroll et al. (1996a) reported an incipient horn on FMNH PM27939, although it seemed to occur only on one side (right) of the specimen. Although, the prominent rim of bone above the facial concavity tends to gives this area of the face a thickened appearance, I agree with Mader’s (1989) judgment that there is no distinct hornlike structure on any specimen of M. junius .

From a lateral view, the dorsal margin of the premaxillomaxillary rostrum is steeply sloped, so that the posterior notch of the nasal incision is level with the top of the orbit. The premaxillae most often are not fused at the symphysis; however, they are occasionally fully co-ossified (e.g., AMNH 1571). The premaxillomaxillary suture is not always clear, but it is clearly visible on USNM 26136 (not figured). In this specimen, the ascending nasal process of the premaxilla does not reach the posterior notch of the nasal incision, and, therefore, does not contact the nasal bone. The premaxillomaxillary rostral cavity and the nasal cavity form a single continuous osteological cavity; this condition is normal, but differs substantially from Dolichorhinus and Metarhinus , in which the premaxillomaxillary rostral cavity is completely enclosed in bone.

Among the least distorted skulls of Mesatirhinus junius , the dorsal surface of the skull above the orbits is flat or slightly convex from a lateral view. The lateral profile of the dorsal surface of the posterior half of the skull varies from flat to slightly convex. The parasagittal ridges tend to be thin but distinct, and join medially to form a sagittal crest.

The zygomatic arches of Mesatirhinus junius are thin and weakly curved, like those of Sphenocoelus and Dolichorhinus . From a lateral view, the jugal zygomatic process is horizontal while the zygomatic process of the squamosal is slightly angled posterodorsally, giving the zygomatics a weak curvature. From a dorsal view, the zygomatic arches are slightly bowed laterally. M. junius possesses a large laterally projecting infraorbital process on the jugal. The size of this process varies (compare AMNH 1509 to AMNH 12184), although they tend to be large and conspicuous like those of Sphenocoelus , and they are more distinct than those of Metarhinus .

From dorsal views of the skulls, the nuchal crest is anteromedially angled. From lateral views, the occiput is slightly tilted backward. The occiput of Mesatirhinus junius is best preserved in USNM 26116. From the posterior view, the nuchal crest is arched dorsally. The dorsal portion of the occiput is nearly as wide as the posterior portion and the occiput is constricted in the middle. Occipital pillars are evident, but they are weak, and the central depression in the occiput is shallow. The occipital condyles are very wide, a condition shared with Sphenocoelus uintensis and Dolichorhinus hyognathus .

Unlike many other brontotheres, there is no horseshoe-shaped emargination around the opening of the posterior nares of Mesatirhinus junius . A small medial process typically projects posteriorly from the anterior rim of the posterior nares (although it is not preserved on AMNH 12202). The anterior margin of the posterior nares varies in position from between the M2 protocones (AMNH 12202) to between the M3 hypocones (AMNH 36045). Most frequently, the position of the posterior nares is anterior to the posterior margin of the M2. The elongate posterior narial canal formed by the posterior nares does not extend into the body of the sphenoid as in Sphenocoelus uintensis . The mastoid process does not contact the postglenoid process, thus the opening of the external auditory pseudomeatus is unconstricted ventrally. Other aspects of the basicranium of M. junius are typical of other brontotheres, such as a widely separated foramen ovale and foramen lacerum.

UPPER DENTITION: Few specimens of Mesatirhinus junius have preserved upper incisors; a partial skull, AMNH 1514, includes a complete row of three left incisors (fig. 8d, e). The incisors are large and arch anterior to the canines. The tips of I1 and I2 are worn off, but these incisors appear to have had conular crowns. The I3 crown is somewhat larger than those of I1 and I2 and is more nearly caniniform in shape. Each of the canines has a distinct lingual cingulum. The canines of Mesatirhinus junius are generally of moderate size, like those of Dolichorhinus hyognathus . The incisors of AMNH 1514 are separated by very short gaps, although this may have not been the case in other specimens. Finally, there are both short precanine and postcanine diastemata. The postcanine diastema is shorter than P2.

Complete cheektooth rows are preserved in many specimens of Mesatirhinus junius . The following description of the premolars and molars of M. junius is primarily of AMNH 12202 (fig. 8a–c) but information from other specimens is provided. P1 is slightly shorter than P2, although it is much narrower and with a simpler crown. The P1 crown has a single cusp and an elongate posterior heel. There is no P1–P2 diastema.

The anterior side of P2 is more steeply angled posterolingually than P3 and P4, thus giving the P2 crown a more oblique shape and a relatively narrower lingual side. P3 and P4 are progressively less oblique in outline; their anterior and posterior sides are nearly parallel. The parastyle and metastyle of P2 are angled slightly lingually. The P3 parastyle and metastyle are nearly straight, while those of P4 are slightly angled labially. The labial paracone rib of P2 is broad while those of P3 and P4 are progressively narrower and less distinct. The metacone of P2 is positioned somewhat more lingually in comparison to P3 and P4. Because of these differences the labial side of P2 is rounder than the labial sides of P3 and P4.

In AMNH 12202 a short lingual crest extends posteriorly from the protocone on P2–P4, but in some specimens this lingual crest is absent (e.g., AMNH 1514). Each premolar exhibits a small but distinct preprotocrista that extends from the anterior slope of the protocone and meets the ectoloph at a point anterior to the lingual base of the paracone. In AMNH 12202, the preprotocrista of P2 tends to be wider, while those of more posterior premolars (P3 and P4) are thinner and sometimes have a small but distinct paraconule. The preprotocrista of P2 is discontinuous with the small anterior cingulum. However, this morphology is highly variable. For instance, in AMNH 1523, the lingual side of the P2 crown is severely shifted posteriorly; the protocone is posterior to the metacone, and the preprotocrista attaches to the lingual base of the metacone. AMNH 1514 shows an entirely different morphology whereby the preprotocrista contacts the lingual base of the ectoloph at a point anterior to the protocone.

No hypocone is found on any of the premolars of Mesatirhinus junius . The anteri- or and posterior cingula of P2–P4 stretch around the anterolingual and posterolingual corners of the crown but they do not join on the lingual side. The labial premolar cingula of the P2 and P3 connect to the posterior slope of the paracone ribs, thus forming a ridge that runs from the posterolabial base of the crown to the occlusal peak of the paracone. The P4 labial cingulum stretches across the proximal base of the crown and does not join the paracone rib. However, in some specimens (e.g., AMNH 1513) the labial cingulum of P4 is incomplete and does not stretch across the base of the paracone.

The molars of Mesatirhinus junius exhibit numerous apomorphies shared with other brontotheriines. For instance, the anterior cingulum is very thin labially and does not form a thick shelf at the peak of the parastyle. The ectoloph is taller than the lingual cusps. The labial paracone and metacone ribs are thin and indistinct. Overall, the labial side of the ectoloph is strongly angled lingually. The lingual wall of the ectoloph enamel thickens slightly around the lingual sides of the paracone and metacone, but not to the degree seen in Palaeosyops or Eotitanops . The lingual ectoloph enamel that stretches between the cusps is about as thick as the labial sheet of enamel. In unworn teeth, such as the M3 of AMNH 12202, the lingual sides of the paracone and metacone are wedge-shaped, but in more worn molars such as the M2 and M1 of the same specimen the lingual sides of these cusps become progressively rounded near the proximal base of the cusp.

The molars of Mesatirhinus junius retain small and essentially vestigial paraconules. The presence of paraconules appears to be a fixed condition. There is a very small but distinct crest of enamel that runs along the anterolabial slope of the hypocone of M1 and M2 that connects to the inner base of the metacone. Sometimes this structure is long and crestlike as on the M2 of AMNH 12202, and other times it is shorter and cusp-like as on the M1 of the same specimen. This crest seems to be a vestigial remnant of a metaloph. It should be noted that evidence of the paraconules and the metaloph fades with wear, and in heavily worn molars the evidence for these structures is erased.

The molars of Mesatirhinus junius lack central molar fossae and anterolingual cingular cusps. A hypocone is not present on the M3, although a thick cingulum wraps around the posterolingual corner of the crown. Rarely (e.g., USNM 26136), a very small cusp is seen near the posterolingual corner of the M3 that could be interpreted as a rudimentary hypocone. Lingual cingula are absent on the molars, while the labial molar cingula are faint and discontinuous around the mesostyles.

MANDIBLE AND LOWER DENTITION: Four skulls of Mesatirhinus junius are associated with mandibles (AMNH 12191, FMNH PM36045, USNM 26116, and YPM PU25021). Unfortunately, these mandibles lack incisors and have incomplete, heavily worn sets of cheek teeth. However, several mandibles recovered from the Upper Bridger (Bridger C–D) and the lower parts of the Washakie Basin (Washakie A of Granger 1909 and TWKK–TWKA1 of McCarroll et al., 1996b) are consistent in size and morphology with the few mandibles that are directly associated with M. junius skulls. Among these jaws, the cheek teeth, particularly M3, are far too slender and thinenameled for any species of Palaeosyops . Telmatherium validus co-occurs with M. junius in the upper Bridger and lower Washakie formations and has similar lower dentition. Other than their smaller size, M. junius mandibles and lower dentition lack any obvious characters that clearly distinguish them from Telmatherium validus . However, those referred to M. junius are well below the lower size range of T. validus . There are no other small brontotheres from upper Bridger and/or lower Washakie sediments to whom these mandibles could belong.

Pictured is a complete mandible, AMNH 1567 (fig. 9a, b). The mandible is similar in proportion to most other Bridgerian and Uintan hornless brontotheres except Dolichorhinus , whose mandible is more slender. In Mesatirhinus junius the position of the posterior margin of the symphysis fluctuates slightly but it is generally positioned between the talonid of p2 and the trigonid of p3. The inferior angle of the mandible tends to be quite steep ($ 45 °).

Complete sets of relatively unworn lower incisors of Mesatirhinus junius are rare, although the alveolar surface suggests that the incisor row was arched and extended anterior to the canines. One partial jaw, USNM 363884, includes a complete set of nearly unworn but isolated left incisors (fig. 9d, e). The crowns are large, of similar size, and bilaterally asymmetrical. The i1 and i2 have rounded apices and are semispatulate. The i3 is more mesiodistally elongate than i1 or i2 is more lingually curved. Each incisor has a thin but distinct lingual cingulid. Another specimen, AMNH 1575 (fig. 9f), has a partial incisor row with more damaged incisors, but with essentially the same morphology. The i3 of this specimen is more subcaniniform in shape than the i2, with a short, lingually curved crown. There are no diastemata between the incisors or canines.

The following description of the lower premolars is based primarily on AMNH 1520 (fig. 9c), a specimen with nearly unworn premolars, but other specimens provide additional information on variation. The p1 is isolated by a postcanine diastema and a p1–p2 diastema. The postcanine diastema tends to be shorter than the p2. The p1–p2 diastema is even shorter. The p1 has a small simple crown with a single cusp and a very short talonid heel.

The trigonid of p2 is nearly twice as long as the talonid; the p3 trigonid is also distinctly longer than the talonid. However, the p4 trigonid is slightly shorter than the talonid. The talonid and trigonid are of nearly equal width in p2 and p3, although in most specimens (e.g., YPM 16725 and YPM PU10184) the p3 talonid is slightly wider than the trigonid. The p4 talonid is always wider than the trigonid. The paralophids of the p2 and p3 are angled in a slightly lingual direction, creating a small lingual trigonid notch, while the paralophid of p4 curves fully lingually, creating a much larger lingual trigonid notch. The protolophids of p2 and p3 are straight but are angled slightly lingually; in p4 the paralophid arches 90 ° lingually and is fully molariform. Metaconids are absent on p2 and p3; however, there is a large lingually positioned molariform metaconid on p4. The talonid of p2 tends to have a short and/or poorly developed cristid obliqua and hypolophid, and the lingual side of the p2 talonid is a slightly concave sloped surface. However, p3 and p4 have more or less basin-shaped talonids with longer cristids obliqua and hypolophids. Lingual premolar cingulids are absent and labial premolar cingulids are generally weak.

The molars of Mesatirhinus junius are typical with relatively thin enamel, shallow trigonid and talonid basins, and with an elongate m3. Labial molar cingulids are generally weak and lingual molar cingulids are absent. A thin beaded cingulid wraps around to the distal end of the hypoconulid of the m3 of some specimens.

REMARKS

Leidy (1872) originally assigned Mesatirhinus junius to the genus Palaeosyops Leidy (1870a) . This species was named from portions of a lower jaw from an unspecified stratigraphic level from Fort Bridger, Wyoming. The material was later described as ‘‘several small fragments of the right side of a lower jaw, together with a sketch of a larger fragment of the left side, containing the last premolar and the succeeding molars’’ ( Leidy, 1873: p. 57). Leidy (1872; 1873) did not figure any of these specimens or provide specimen numbers, but he did provide measurements for the specimen he had described as Palaeosyops junius . Later, Osborn noted, ‘‘Of this type material only p4 (right) and the posterior half of m3 (right) were located in the collection of the Academy of Natural Sciences of Philadelphia’’ ( Osborn, 1929a: 159); he considered these specimens (fig. 10) cotypes. However, if they are from a single individual, as Leidy’s (1873) description seems to imply, they are not cotypes, but a single holotype. The original measurements provided by Leidy (1873) for the p4 and m3 are consistent with ANSP 10349 (p4) and ANSP 10348 (m3 fragment): breadth (anteroposterior length) of last premolar 5 8 lines (16.9 mm), thickness (width) of last premolar 5 5.5 lines (11.6 mm), thickness of third molar at middle 5 7 lines (14.8 mm). Spamer et al. (1995) list these specimens as the syntypes of Mesatirhinus junius (fig. 10).

Earle (1891) erected another species, Mesatirhinus megarhinus , which he also assigned to the genus Palaeosyops . Palaeosyops megarhinus was based on a nearly complete skull, YPM PU10008, with poorly preserved teeth from an unspecified stratigraphic level of the Washakie Basin, Wyoming. It is worth noting that Earle’s (1891, 1892) figures of the holotype (YPM PU10008) show a complete left zygomatic arch, a complete nasal process, and a complete occiput. Currently, the distal end of the nasal process is not preserved, the zygomatic arches are incomplete, and the occiput is missing in the holotype.

Osborn (1908a) recognized that Earle’s Palaeosyops megarhinus differed from other Palaeosyops species and erected the genus name Mesatirhinus for it. Mesatirhinus megarhinus was diagnosed by its infraorbital process, sagittal crest, and upper molars with flattened outer cusps and reduced conules. In that same paper Osborn (1908a) erected a new species, M. petersoni , based on a partial skull (AMNH 12184) from Bridger D. Osborn (1908a) did not state how M. petersoni was distinct from M. megarhinus in clear morphological terms. He noted that the preorbital facial region was more elongate, and that the grinding teeth occupy more space. However, considering the fact that specimens are damaged and distorted to varying degrees, the subtle distinctions between these two species is clearly suspect.

Ultimately, Osborn (1929a) accepted three species of Mesatirhinus : M. junius (Leidy) , M. megarhinus (Earle) , and M. petersoni Osborn. Osborn (1929a) considered Mesatirhinus junius the earliest form of Mesatirhinus and suggested that the holotype was probably from Bridger B. Osborn states, ‘‘The type lower molar of M. junius , according to Leidy’s description, was found near Fort Bridger, Wyo., at a geological level that Granger places in Bridger B’’ (1929a: 388). However, Leidy’s description states that the fossils were ‘‘received from Dr. J. Van A. Carter, of Fort Bridger, Wyoming’’ (Leidy, 1892: 277). Leidy (1872, 1873) did not state that the fossil was ‘‘found near Fort Bridger’’ (contra Osborn, 1929a). Therefore, the stratigraphic provenience of the holotype of M. junius is unknown. There is no direct evidence of the occurrence of Mesatirhinus in Bridger B, although it is relatively common in Bridger C–D.

Mader (1998) recognized only a single species, Mesatirhinus megarhinus . He considered M. junius to be a nomen dubium but suggested that M. junius is a potential senior synonym of M. megarhinus . In contrast, Gunnell and Yarborough (2000) considered M. junius to be a synonym of Palaeosyops paludosus . Nonetheless, it can be shown that M. junius is a valid name, that it is distinct from Palaeosyops , and that it is the senior synonym of both M. megarhinus and M. petersoni . Among brontotheres that occur in the Bridgerian, the m3 of the holotype of M. junius (ANSP 10348) is smaller than Palaeosyops from Bridger B, or from Bridger C–D, and, more importantly, it is far too slender and thin enameled for a m3 of Palaeosyops . It is also much too large for Eotitanops . Strictly speaking, the morphology and proportions of the holotype of Mesatirhinus junius are not different from Telmatherium validus ; however, it is considerably smaller than all known specimens of T. validus . Moreover, the holotype of M. junius falls within the size range of other materials that have been referred to as Mesatirhinus (table 1). Therefore, M. junius is the earliest name that pertains to Mesatirhinus . M. megarhinus (Earle) and M. petersoni Osborn appear to represent the same species and are considered junior synonyms of M. junius (Leidy) .

The material referred to Mesatirhinus junius consists mostly of specimens from late Bridgerian deposits of the Bridger Basin (Bridger C–D) and Washakie Basin (Washakie A of Granger [1909] and TKWW– TWKA1 of McCarroll et al. [1996b]). There is evidence, however, that Mesatirhinus junius ranges into the early Uintan. A crushed anterior cranial fragment (FMNH PM54864) is from early Uintan deposits (Washakie Basin, TWKA2). Another specimen, FMNH PM1676, a crushed skull, is recorded from the ‘‘upper Washakie’’. Generally, ‘‘upper Washakie’’ refers to the Uintan portion of the Washakie deposits (see Granger, 1909), but the exact stratigraphic position of this specimen is uncertain. There are two probable specimens of M. junius from the Sand Wash Basin: DMNH 8103, a left maxilla; and DMNH 29950, a left partial skull. These specimens possess a prominent infraorbital process, a deep nasal incision, and they lack central fossae on the upper molars. This combination of characters is consistent with M. junius and rules out all other Uintan brontotheres except Sphenocoelus uintensis Osborn (1895) . These specimens are considerably smaller than S. uintensis . They are slightly larger than Bridgerian specimens of Mesatirhinus junius , but only by millimeters. It is therefore probable that (1) these specimens are a new species slightly larger than the M. junius from the Bridgerian land mammal ‘‘age’’, or (2) M. junius survived into the early Uintan but was slightly larger at that particular time and place. Because the material at hand does not clearly indicate a morphologically distinct species, the latter possibility is preferable; therefore, these specimens are referred to M. junius .