Metarhinus sp.

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 : 79-85

Metarhinus sp.

Includes nomina dubia: Metarhinus fluviatilis Osborn, 1908a ; Metarhinus earlei Osborn, 1908a ; Metarhinus cristatus Riggs, 1912 ; Heterotitanops parvus Peterson, 1914a ; Telmatherium accola Cook, 1926 ; Telmatherium advocata Cook, 1926 ; Metarhinus pater Stock, 1937

REFERRED SPECIMENS: (From the Wagonhound Member of the Uinta Formation, Uinta Basin, Utah) AMNH 1500 (holotype of Metarhinus fluviatilis ), a skull with right I1– M3 and left I2–M3 ; AMNH 1859 , a mandible with right c–m3 and left i3–m3 ; AMNH 1864 , a left maxilla fragment with P3–M1, M2 (partial) ; AMNH 1865 , a partial mandible with left p3–m3 ; AMNH 1877 , a crushed skull ; AMNH 1946 , right P3–M3, left P2– M3, and a partial mandible with right c–m3 ; AMNH 2059 , a partial mandible with right i1, c, p2, and left i1–m3 ; CMNH 2909 , a juvenile skull and skeleton (holotype of Heterotitanops parvus ) ; CMNH 3098 , a skull right P2–M3, left C–M3, and isolated incisors and canine fragment ; CMNH 3125 , a partial mandible with right p3–m3 ; CMNH 3133 , a mandible with right c–m1, left i2, and c–m3 ; CMNH 3142 , a maxilla fragment with partial molars and a partial mandible with right i1– c, p2–m3, and left i1–c ; CMNH 3371 , a partial mandible with left p1–m3 ; CMNH 3842 , a partial mandible with right i1–m2, m3 (partial), left i1–m3 ; CMNH 11924 , a mandible with right i1–i2, p3–m3, and left i1–m3 ; CMNH 16736 , a palate with right P4–M3 and left P1–M3 ; FMNH P12169 , a skull with right P1–M3 and left I3–M3 ; FMNH P12174 , a crushed skull with right and left M1–M3 ; FMNH P12178 , a mandible with right p3–p4 and left p2–m3 ; FMNH P12178 , a mandible with right p2–m3, left i1, and p1– m3 ; FMNH P12183 , a partially prepared skull and mandible with exposed left upper and lower cheektooth rows ; FMNH P12190 , a mandible with right and left m1–m3 ; FMNH P12194 (holotype of M. cristatus ), the posterior portion of a skull with right and left M2–M3 ; FMNH P12195 , a mandible with right c, p2–m3, left c, and p2–m3 ; YPM 11284, a partial mandible with right p4 and m1–m3 (partial); (from the Adobe Town Member of the Washakie Formation of Wyoming) AMNH 13166 (holotype of Metarhinus earlei ), a skull with right P4, M3, left P2–P4, and M2–M3 ; AMNH 13179 , a mandible with right p1–m3 and left c–m3 ; CMNH 9409 , a crushed skull with right P2– M3 and left P4–M3 ; FMNH PM1456 , a partial mandible with left p3–m3 (all partial) ; FMNH PM1511 , a partial mandible with right i2–c, p2–p4, and left i3–m3 ; FMNH PM1517 (in part), a right maxilla fragment with P2–P4 ; FMNH PM1519 , a palate with right P2–P4, and left P3–M3 ; FMNH PM1675 , a partial mandible with right and left dp2–dp4, m1, and m2 (unerupted) ; FMNH PM1680 , a rostrum with right P1– P2 (partial), and P3–M1 ; FMNH PM1715 , a mandible with right i1 (?), p2–p4, m3, left i1 (?) p1–p4, and m3 ; FMNH PM1730 , a rostrum with right P2–M1 ; FMNH PM1731 , the posterior portion of a skull with right and left M1–M2, and isolated right P3–P4 ; FMNH PM1732 , a left maxilla and jugal with P2–M3 ; FMNH PM1734 , a partial mandible with right p2–m3 ; FMNH PM39947 , a mandible with right p2–m3 ; UCMP 81273, a mandible with right c–m3 and left p1–m3; UCMP 81285, a right maxilla with P4–M2; YPM 16834, a partial mandible with right i1–i2, c, left i1–c, and p2– p4; (from the ‘‘ Metarhinus Quarry’ ’ of the Adobe Town Member of the Washakie Basin , Washakie Formation , Wyoming) FMNH PM28001 , a mandible with right i2, c, p2– m3, and left i1–m3 ; FMNH PM28002 , a partial mandible with left i2–m3 ; FMNH PM28003 , a partial mandible with right p1– m3 ; FMNH PM28004 , a partial mandible with right i2–m3 ; FMNH PM28006 , a mandible with right c, p2–m3, left c, and p2–m3 ; FMNH PM28014 , a partial mandible with right and left p1, dp2–dp4, and m1 (unerupted) ; FMNH PM28342 , a partial mandible with right p2–m3, left p1–p3, and m2–m3 ; FMNH PM28343 , a partial mandible with right p2–m3 and left p2–m2 ; FMNH PM28344 , a partial mandible with right p2– m2, left c, p2, p3–p4 (partial), m2–m3 (partial) ; FMNH P28345 , a skull fragment with right P2–M3 and left P2–M3 ; FMNH PM28348 , a palate with right I2 (?), C, P1– M3, left C, P1, and P3–M3 ; FMNH PM28359 , a right p2–p4 ; FMNH PM30388 , a right maxilla with P2–M3 ; FMNH PM30422 , a mandible with right i2 (?), p1– m3, left i3, and p1–m3 ; FMNH PM30432 , a crushed skull with right and left P2–P4 ; FMNH PM30434 , a partial mandible with right and left p1, dp2–dp4, m1 (unerupted) ; FMNH PM30435 , a mandible with right i2– i3, p1–m3, left i3, and p1–m3 ; FMNH PM35932 , a skull with right and left P1– M3 ; FMNH PM35933 , a mandible with right p3–m3 and left p2–m3 ; FMNH PM35970 , a mandible with right p2–m3, left canine, and p2–m3 ; FMNH PM35996 , a mandible with right i2–i3, p2–m3, left i2 (?), and p2–m3 ; FMNH PM36053 , a skull fragment with left P3–M1 (partial) ; FMNH PM36054 , a skull fragment with right P3–M3 (partial), left P1– M1, and M2 (partial); (from the Sand Wash Basin of Moffat County, Colorado) DMNH 543 , a partial mandible with left p2–m3 ; DMNH 544 (holotype of Telmatherium accola ), a mandible with right p2–m3 and left c–m3 ; DMNH 550 (holotype of T. advocata ), a mandible with right i3, left i1, c, p2–m2, and m3 (erupting) ; DMNH 2611 , a right maxilla with M1–M3; (from the Friars Formation of San Diego County, California) LACM / CIT 2037 (holotype of Metarhinus pater ), a right maxilla with C–M3 ; UCMP 95774, a right maxilla with M1–M3; UCMP 95808, a right maxilla with DP4 and M1; UCMP 95809, a left maxilla fragment with P3–P4; UCMP 95831, a left M1 or M2; UCMP 95780, a partial mandible with left m1–m3; UCMP 95813, a partial mandible with right m2 (partial), and m3; UCMP 95841, a mandible fragment with right p2–p3; UCMP 106011, right p3, p4, and m1; UCMP 113182, fragments of a skull, jaw and some isolated lower teeth; UCMP 113189, a right p2; UCMP 113194, a mandible fragment with right m2; UCMP 113201, a partial mandible with right dp3–dp4, and m1; UCMP 113203, a mandible fragment with left m3.

DESCRIPTION

Because the only clear distinction between Metarhinus fluviatilis and M. abbotti is the shape of the nasal bone, the vast majority of Metarhinus specimens cannot be assigned to either species. These specimens include jaws as well as skulls that lack preserved nasal bones. Among the fossil collections studied for this revision, specimens that could belong to either species of Metarhinus are known from the early Uintan Wagonhound Member of the Uinta Formation of Utah, the middle Adobe Town Member of the Washakie Formation of Wyoming, the Sand Wash Basin of Colorado, and from the Poway and Murray Canyon local faunas of the Friars Formation of San Diego County, California. These include 30 skulls and skull fragments. Four of these have associated mandibles and/or lower dental elements. Also included are an additional 50 complete and partial mandibles.

Among these specimens are the holotype skulls of Metarhinus fluviatilis Osborn (1908a) (AMNH 1500), as well as the holotypes of M. earlei Osborn (1908a) (AMNH 13166), M. cristatus Riggs (1912) (FMNH P12194), and M. pater Stock (1937) (CIT 2037). This large group of specimens provides further information on the morphology of Metarhinus . Therefore, they are described below, particularly as they pertain to intraspecific variation and missing phylogenetic data for M. fluviatilis and M. abbotti .

SKULL AND UPPER DENTITION: None of the Metarhinus sp. skulls have well-preserved upper incisors, however many specimens have preserved alveoli or partial sets of well-worn incisors. These all suggest large incisors, consistent in morphology with those of the holotype of Metarhinus abbotti . The incisor morphology of M. fluviatilis was probably not different from that of M. abbotti . A significant number of the skulls show variable premolar morphologies. Numerous specimens (e.g., AMNH 1864, FMNH PM1517, FMNH PM1730, FMNH PM1732, and FMNH P12169) have a small crest extending posteriorly from the protocone of P2 and P3. Occasionally a very small lingual crest can be seen on the P4 protocone. A premolar hypocone is never present. One atypical specimen (FMNH PM36054) has a P1 with an unusual lingual heel with a large protocone. The M3s of these specimens consistently have hypocones that are variable in size but they are always smaller than the hypocones of the M1 and M2.

MANDIBLE AND LOWER DENTITION:

The following description of the mandible of Metarhinus sp. is primarily based on AMNH 2059 (fig. 33), although additional information from other specimens is given. The inferior margin of the symphysis is angled about 45 ° or somewhat less than that. The posterior margin of the symphysis is between the talonids of the p 3 in AMNH 2059. However, its position fluctuates between the anterior margin of p3 and the posterior margin of p3. The dental formula is unreduced (3-1-4-3). The incisors are large, form a semicircular arch anterior to the canines, and are positioned closely together. The incisors are subcaniniform with short lingually curved crowns and blunt points. Each incisor has a strong lingual cingulid. The canines are somewhat variable in size, but generally, they are small and slender with lingual cingulids that are much weaker than those of the incisors. There is no precanine diastema in AMNH 2059 although a short precanine diastema is occasionally present in other specimens. The postcanine diastema of AMNH 2059 is shorter than the p2 although some specimens have a slightly longer postcanine diastema. AMNH 2059 lacks a p1–p2 diastema while other specimens (e.g., UCMP 81273) have a minor p1–p2 diastema.

The p1 is a simple tooth with a single cusp and a short talonid heel. The p2 and p3 trigonids are longer than the talonid, while the p4 trigonid is shorter than the talonid. The trigonids of p2–p4 are narrower than their respective talonids. The paralophid of p2 arches slightly lingually, creating a small lingual trigonid notch. The p2 protolophid is lingually positioned and posteriorly directed. The p2 lacks a metaconid. The p3 and p4 trigonids are more molariform with strongly

(C) dorsal view, (D) left incisors and canine, lingual view, (E) left incisors and canine, labial view.

lingually arched paralophids and protolophids, a broad lingual notch, and a large lingually positioned metaconid. The talonid of the p2 has a well-developed cristid obliqua, a small lingual notch, and a short hypolophid. The talonids of p3 and p4 are more developed with more basinlike depressions and longer hypolophids. The lower molars are typical with thin enamel, shallow trigonid and talonid basins, and an elongate m3.

Although the majority of the unidentified mandibles have a large metaconid on p3, it is occasionally absent (e.g., AMNH 1859, CMNH 3125, FMNH P12178) or very small (e.g., CMNH 3371). The presence or absence of a p3 metaconid is known to be variable in Lambdotherium ( Bonillas, 1936) and among some brontotheriids (e.g., Rhinotitan ). In this instance, it is possible that either this character is intraspecifically variable in Metarhinus fluviatilis and M. abbotti , or the m3 metaconid is present in M. fluviatilis but absent in M. abbotti . The former scenario is more likely based on the monospecific death assemblage of Metarhinus sp. from the Washakie Formation where the p3 metaconid is both absent and present. If this assemblage is truly monospecific, as it has been presumed to be ( Turnbull and Martill, 1988; see below), one must conclude that the p3 metaconid is intraspecifically variable. It is reasonably safe to infer that the jaws and lower dentition of M. fluviatilis and M. abbotti were not significantly differentiated. Therefore, these mandibles were used in coding the mandible and lower dental characters of M. fluviatilis and M. abbotti for the phylogenetic analysis.

REMARKS

In 1908, Osborn (1908a) named a new genus and species, Metarhinus fluviatilis , from a skull (AMNH 1500) from the Uinta Basin with worn dentition and missing the nasal process. The specimen shows a unique combination of characters that clearly sets it apart from other species that were known at the time. These characters include prominent orbits, a long nasal incision, and a small infraorbital process. In the same paper, Osborn (1908a) named another species, Metarhinus earlei , based on AMNH 13166, a skull also missing the nasal bones that is slightly larger than AMNH 1500 but similar to it in other respects. In 1912, Riggs (1912) erected two more species of Metarhinus . These include M. riparius , based on a complete skull (FMNH P12186) with nasal bones, and M. cristatus , based on the posterior portion of a skull (FMNH P12194). Riggs (1912) thought that he saw two lineages of Metarhinus evolving (presumably) from a small primitive M. fluviatilis . M. earlei and M. cristatus represented a broadheaded form, and M. riparius represented a narrow-headed form. Osborn (1929a) generally agreed with this interpretation. However, most of the differences in these ‘‘lineages’’ can be attributed to taphonomic distortion. For instance, the sagittal crest is exaggerated in skulls with crushed braincases. Other reported differences are simply unsubstantiated. For instance, the expansion of the nasal bones was one of the distinctions of these lineages, however, only one of the four holotypes has preserved nasals. Curiously, Osborn (1929a) recognized the influence of taphonomic distortion on the differences in the proportions of the various holotypes but he did not reject any of the supposed Metarhinus species. Finally, Stock (1937) named a fifth species, Metarhinus pater , based on CIT 2037, a right premaxilla and maxilla from the sandstones of the Poway Conglomerate of San Diego County, California. Although Stock (1937) questionably referred this species to Metarhinus , the holotype is consistent with other species that had been referred to Metarhinus .

Riggs (1912) based a new genus and species, Rhadinorhinus abbotti , on a complete skull (FMNH P12179). Riggs’ (1912) decision to erect a new genus for FMNH P12179 is curious because it closely resembles the other supposed species of Metarhinus . However, the nasal process of FMNH P12179 tapers distally, whereas some of the skulls that have been attributed to Metarhinus have a distally widening nasal process. However, Riggs’ (1912) and Osborn’s (1929a) referrals of species with the distally widening nasals to the genus Metarhinus and referrals of species with distally tapering nasals to the genus Rhadinorhinus is arbitrary because the morphology of the nasal processes of the type species of Metarhinus , M. fluviatilis , is unknown.

Despite the numerous species named by Osborn and Riggs, there appear to only be two diagnosable species, one with distally tapering nasals and one with distally broadening nasals. These species appear to be undifferentiated in every other respect. Unfortunately, the holotype of the type species of Metarhinus ( M. fluviatilis ) lacks a nasal bone and its specific identity is uncertain. Therefore the validity of the genus Metarhinus is questionable. On the other hand, the genus Rhadinorhinus is less problematic because the holotype of R. abbotti includes a complete (distally tapering) nasal bone. Nonetheless, in more recent revisions, Mader (1989) continued the dubious practice of using Metarhinus fluviatilis for the species with distally widening nasals, and Rhadinorhinus abbotti for the species with distally tapering nasals. In a later revision ( Mader, 1998), Rhadinorhinus abbotti was mistakenly considered a junior synonym of Metarhinus diploconus (now Fossendorhinus diploconus ). However, Fossendorhinus diploconus is clearly different from both M. fluviatilis and M. abbotti and can no longer be considered a potential synonym of either species.

Despite the fact that Metarhinus is problematic due to unknown nasal morphology of the type species, M. fluviatilis , Metarhinus is a well-known taxon with biostratigraphic importance (e.g., Prothero 1996; McCarroll et al. 1996b; Robinson et al. 2004). To reject Metarhinus outright would generate serious inconsistencies in the literature on North American mammal biostratigraphy. It has been long assumed that M. fluviatilis represents the particular species with the distally widening nasals ( Osborn 1929a; Mader 1989, 1998). Therefore, to continue the use of Metarhinus , I suggest that FMNH P12187, a complete skull with distally broadening nasals, be designated as the neotype for Metarhinus fluviatilis . Secondly, Rhadinorhinus abbotti is valid and represents the species with the distally tapering nasal bone. However, it is convenient to consider Rhadinorhinus a junior synonym of Metarhinus because the large number of museum specimens that pertain to either fluviatilis or abbotti but cannot be identified to either due to nonpreservation of the nasal bone are identified on specimen labels and museum catalogs as Metarhinus sp. Grouping fluviatilis and abbotti into one genus, Metarhinus , maintains accuracy in the identification of these specimens and published biostratigraphic data that are based on these specimens. The remaining species of Metarhinus are invalid. M. riparius is the only other Metarhinus species represented by a holotype with a preserved nasal bone. The nasal bone of that specimen indicates that M. riparius is a junior synonym of M. fluviatilis . Other species attributed to Metarhinus whose holotypes lack nasal bones, M. earlei , M. cristatus , M. pater , are nomina dubia.

In addition to the above nomina dubia, Peterson (1914a) described a skull and skeleton of a very young, possibly fetal brontothere from the early Uintan (Uinta B1) that he gave a new name, Heterotitanops parvus (CMNH 2909) . Given the very young ontogenetic age of the animal, this species is not valid and it probably represents a known species of similar age although this specimen cannot be clearly assigned to any particular species. Osborn (1929a) noted that it resembled Dolichorhinus in the absence of a sagittal crest but this similarity was dismissed as an artifact of ontogeny. Ultimately Osborn (1929a) considered it most likely to be Metarhinus fluviatilis , primarily based on comparisons of the deciduous dentition with the adult dentition of that species. However, the assignment to M. fluviatilis is uncertain since the neonate specimen lacks a nasal process. At any rate, the specimen is simply too young to clearly refer to any particular species of brontothere; juvenile brontothere materials are uncommon and too poorly documented to make an appropriate comparison of CMNH 2909 with other species. Heterotitanops parvus is a nomen dubium, but it could be a synonym of Metarhinus sp.

The summary statistics of those few specimens that are directly identifiable as Metarhinus abbotti and M. fluviatilis can be found in tables 4 and 5. M. abbotti appears to be larger than M. fluviatilis in premolar length and ventral skull length. These species slightly overlap in total cheektooth row length and completely overlap in molar row length. Although the few specimens identifiable as M. abbotti appear to be larger in some respects than those specimens identifiable as M. fluviatilis , plots of any of these variables (not shown) for the entire Metarhinus sp. group do no reveal any clear bimodal distributions that might suggest two size groups.

A catastrophic death assemblage found in overbank deposits associated with sandstone channels from the Adobe Town Member of the Washakie Formation provides a rare glimpse at the population variability of a brontothere species. This sample was initially identified as Mesatirhinus sp. ( Turnbull and Martill, 1988), but was demonstrated by McCarroll et al. (1996a) to be Metarhinus sp. No specimen with intact nasal bones is known from the quarry. Therefore, the quarry sample could represent either Metarhinus fluviatilis or M. abbotti . It is often

TABLE 6 Summary statistics for selected morphometric variables of Metarhinus sp. from the ‘‘ Metarhinus quarry’’ See Methods for measurement definitions presumed that large quarry samples, particularly catastrophic death assemblages, are monospecific. Turnbull and Martill (1988) assumed this sample to represent a monospecific herd. Modern ungulate catastrophic death assemblages are monospecific ( Berger et al., 2001), although they do not necessarily indicate herds ( Mihlbachler, 2003b). It is probable that this sample, though not necessarily representing a herd, is a monospecific death assemblage. (Indeed, I have assumed this as outlined in the explanation of the methodology used in this study). The summary statistics for the Metarhinus sp. quarry sample are given in table 6. The coefficients of variation are compatible with a monospecific population, although they do no falsify the possibility that two similarly sized taxa are present. The average values of the quarry sample seem slightly closer to the average values of M. fluviatilis . However, the size ranges of two of the variables (P3 length, P2– P4 length) virtually span the size range of both species. Additionally, M1–M3 length and P2–M3 length span the entire size range of M. abbotti . Clearly, size is not a realistic means for assigning specimens to either species of Metarhinus .