Metatelmatherium ultimum ( Osborn, 1908a )

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 : 122-133

Metatelmatherium ultimum ( Osborn, 1908a)

HOLOTYPE: AMNH 2060 , a complete skull with right I2, C (partial), P1–M3, left I2–I3, C (partial), P1–M3, and a partial mandible with symphysis and right ramus with c (partial), p3 (partial), p4–m3.

TYPE LOCALITY: Myton Member (lower part of Uinta C) of the Uinta Formation of Utah.

SYNONYMS: Manteoceras uintensis Douglass, 1909 ; Metatelmatherium cristatum Granger and Gregory, 1938 .

AGE: Middle Eocene (Uintan and Irdinmanhan land mammal ‘‘ages’’).

REFERRED SPECIMENS: (From the Myton Member of the Uinta Basin of Utah) AMNH 2004 , an anterior portion of a skull with right I2 (partial), I3–M3, left I2 (partial), I3, and P3–M3 ; AMNH 2029 , a crushed ventral surface of a skull with right P1, P2, P4, M1–M3 (partial), left P1–P4, and M1–M3 (all partial) ; AMNH 2033 , a mandible with partial symphysis and right ramus with p2– m3 ; CMNH 2339 , a partial skull with no teeth ; CMNH 2388 (holotype of Manteoceras uintensis ), a crushed anterior portion of a skull with right I1–M3, left I1, I3, C (partial), P1–P3, P4–M3 (all partial) ; CMNH 2354 , a skull fragment with right P2–M1 and left P2– P4 (partial); (from the Adobe Town Member of the Washakie Formation of Wyoming ) UCMP 81447, a skull with right I1–M3 (all broken) and left I3–M3; (from the Wind River Basin, Hot Springs County, Wyoming ) YPM 14158, a skull with no complete teeth; (from the ‘‘Irdin Manha’’ Formation [sensu Radinsky, 1964] Camp Margetts area , Inner Mongolia, China) AMNH 26411 (holotype of Metatelmatherium cristatum ), a complete skull with right and left I2–M3, and a mandible with right i1, i3–p1, p2 (partial), p3–m3, and left i1–m3 .

DIAGNOSIS: Metatelmatherium ultimum is a large hornless brontothere. The frontal bone intrudes into the nasal bone and splits off a small lateral splint of nasal bone from the main body of the nasal. The size and shape of the nasal splint is similar to that of Wickia brevirhinus and less pronounced than that of Telmatherium validus . The nasal incision is short and does not extend posteriorly beyond the P1. The nasal process is very short, horizontal, unelevated, of relatively constant transverse width, narrow, with thin and relatively deep lateral walls, and without a well-defined or strongly rounded distal margin. The orbits do not protrude laterally. The orbits are positioned directly above the M2 with the posterolateral root of M1 directly below the anterior rim of the orbit. The premaxillomaxillary rostrum deepens proximally and the rostral cavity is dorsally sealed by bone. Other cranial characteristics include a relatively flat dorsal cranial surface (from a lateral view), a short sagittal crest, an elevated occiput, deep and strongly curved zygomatic arches that are very strongly bowed laterally, a large ventral zygomatic flange on the jugal, and a ventrally open and mediolaterally angled external auditory pseudomeatus. Ventral sphenoidal fossae are absent.

Dentally, Metatelmatherium ultimum has large subcaniniform upper incisors, a simple P1, a distinct P2 metacone, a postcanine diastema, weak premolar preprotocristae, and short lingual crests extending posteriorly from the premolar protocones. Premolar hypocones are absent. The molars of M. ultimum have tall, lingually angled ectolophs with weak labial ribs, and thinned lingual ectoloph enamel with wedge-shaped paracones and metacones. Central molar fossae and cingular parastyle shelves are absent, but small anterolingual cingular cusps are present. The upper molars lack paraconules and metalophs. The lower dentition of M. ultimum includes large subcaniniform incisors that are all of similar size, a postcanine diastema, a p1–p2 diastema (variably present), an elongate p2 trigonid, a metaconid on p4 but not on p2 or p3, shallow molar basins, and a slender m3.

Metatelmatherium ultimum is most similar to Telmatherium validus and Wickia brevirhinus , but it is clearly distinct from these species primarily due to the unique combination of a very short nasal incision, a true sagittal crest, absence of strong occipital pillars, a deep notch in the dorsal rim of the foramen magnum, and a large ventral zygomatic flange.

DESCRIPTION

SKULL: The holotype skull of Metatelmatherium ultimum (AMNH 2060) is nearly whole and undistorted (fig. 56). Only the nasal process is fragmentary, but it is nearly complete and has been reassembled and reattached to the skull. Seven other skulls from North America are referable to M. ultimum , as well as an associated skull and mandible from Asia (AMNH 26411) (fig. 57). The following description of the skull of M. ultimum is based primarily on the holotype but is supplemented by other specimens where noted.

Metatelmatherium ultimum is a relatively large (table 8) hornless brontothere whose skull most closely resembles those of Telmatherium validus and Wickia brevirhinus . Upon close inspection the sutures in the facial area are discernable in AMNH 2060 although they are indistinct. The sutures of the facial bones are more distinct in a referred specimen (YPM 14158) (fig. 58). The frontonasal suture recedes posteriorly toward the midline, but near the midline it is acutely redirected anteriorly. The nasal bone is split by a small triangular process of the frontal just above and anterior to the orbits. The small splint of nasal bone that branches off the main body of the nasal arches posteroventrally between the frontal and the maxilla. Below this splint, the nasomaxillary suture is strongly dorsally arched. The face of M. ultimum is tall and the maxilla forms a shallow preorbital fossa. The shape and configuration of the facial bones of M. ultimum most closely resembles that of Wickia brevirhinus . The frontonasal suture in Telmatherium validus is more pronounced, the triangular frontal process is longer, and the nasal splint is broader, longer, and more arched. Previous figures of AMNH 2060 incorrectly depicted the configuration of the facial sutures of M. ultimum ( Osborn, 1908a: fig. 17; 1929a: figs. 126, 294–296, pl. 51a). In these earlier figures, the frontal bone is depicted as having a broad contact with the maxilla rather than being separated by the lateral nasal splint.

(C) anterior view, (D) posterior view.

Earlier descriptions of Metatelmatherium ultimum are misleading in other respects. For instance, Osborn (1929a: 346) described ‘‘horn rudiments’’ in male skulls. He considered the holotype (AMNH 2060) to be a female skull but regarded AMNH 2004 as male. In reexamining AMNH 2004, the triangular frontal process does not form a thickened protuberance; nor can I find a structure that could be interpreted as a ‘‘horn rudiment’’ in any other specimen of M. ultimum .

The nasal incision of Metatelmatherium ultimum is very short; in AMNH 2060 it extends only to the anterior margin of the P1. The position of the posterior margin of the nasal incision is similar to that of Wickia , but much shorter than Telmatherium . Because of the short nasal incision the preorbital region of the face appears to be quite long. The orbits are positioned directly above M2 while the posterior lateral root of M1 is positioned directly below the anterior rim of the orbit.

The nasal bones are poorly fused. The nasal process is thin, projects horizontally from the skull, and is shorter than the premaxillomaxillary rostrum. In the holotype the nasal process is straight with a flat dorsal margin. The sides of the nasal process form deep, thin, and vertical lateral walls. The depth of the lateral walls is relatively constant throughout the length of the nasal process, but they become shallower at the very distal end. From a dorsal view the nasal process is narrower than the rostrum. Other specimens with more intact nasal processes (e.g., AMNH 2004, CMNH 2339) are morphologically consistent with that of the holotype. These specimens also more clearly reveal that the anterior edge of the nasal process is thin, roughened, and very slightly downwardly deflected.

From the lateral view the dorsal border of the premaxillomaxillary process is steeply sloped posterodorsally so that the posterior margin of the nasal incision is elevated to the level of the upper rim of the orbit. The rostral cavity is open dorsally. The premaxillomaxillary suture is clearly visible on the right side of AMNH 2060 and the premaxilla termi-

TABLE 8 Summary statistics for selected morphometric variables of Metatelmatherium ultimum See Methods for measurement definitions nates near the posterior notch of the nasal incision. There is no contact of the nasal and premaxillary bones. The premaxillae are not co-ossified at the symphysis. The premaxillae are not nearly as massive as those of Wickia .

Like Wickia , the skull of Metatelmatherium ultimum is tall and the occiput is elevated. This condition is most extreme in CMNH 2339 (fig. 59). From a lateral view, the dorsal surface of the skull is flat or slightly convex

Fig. 58. Close-up view of the right face of a skull of Metatelmatherium ultimum showing the configuration of the maxilla, nasal, and frontal bones. (Courtesy of Division of Vertebrate Paleontology, YPM 14158. º 2005 Peabody Museum of Natural History, Yale University, New Haven, Connecticut, USA. All rights reserved.)

over the orbits. Postorbitally, the dorsal surface is flat, although it is angled posterodorsally. From the dorsal view of AMNH 2060 the skull is very narrow between the orbits. From the anterior view the dorsal cranial roof is vaulted. The appearance of a narrow arched skull roof seems to be slightly exaggerated by some lateral crushing in the holotype. In specimens that are less laterally crushed (e.g., CMNH 2399, YPM 14158, UCMP 81477), the dorsal roof of the cranium is wider and less vaulted than that of the holotype. Nonetheless, the superorbital dorsal skull roof is narrow in M. ultimum . The parasagittal ridges merge posteriorly to form a short but thin sagittal crest. Each parasagittal ridge forms a prominent ridge on the sagittal crest, although there is no distinct median pit between the parasagittal ridges as seen in Telmatherium .

From a dorsal view of AMNH 2060 the nuchal crest is very thin and concave. From a lateral view the occiput is slightly tilted backward. From a posterior view, the nuchal crest is strongly arched dorsally. The dorsal portion of the occiput is somewhat narrower than the ventral portion and the middle of the occiput is constricted. The occiput of AMNH 2060 is unusual in a number of respects. The entire surface of the occiput is roughened, particularly above the occipital condyles and below the nuchal crest. Two short crests originate near the top of the nuchal crest and converge ventromedially into a small process in the central depression of the occiput. The thick occipital pillars seen in most other brontothere skulls are not discernable in AMNH 2060. Finally, there is a tall and narrow median notch on the upper margin of the foramen magnum. On the lateral edges of this notch are two elongate, smooth, facetlike surfaces.

The zygomatics of Metatelmatherium ultimum are deep, relatively thin, and very strongly bowed laterally. The jugal portion of the zygomatic process is angled downward at a shallow angle, while the squamosal portion of the zygomatic is steeply angled posterodorsally, giving the zygomatic arch a strong curvature. The curvature of the zygomatic is exaggerated by a large flange on the ventral margin of the zygomatic process of the jugal just below the junction with the squamosal. A similar flange is occasionally seen on the skulls of other brontotheres (e.g., Epimanteoceras formosus ), but it is always less conspicuous in comparison to M. ultimum , where the ventral zygomatic flange is pronounced and greatly influences the overall appearance of the zygomatic arch. The ventral flange is always conspicuous but its size varies; in CMNH 2339 the prominence of the ventral zygomatic flange is extreme (fig. 59). An infraorbital process of the jugal, such as that of Sphenocoelus , is not seen in M. ultimum .

In AMNH 2060, the posterior nares are rimmed by a narrow horseshoe-shaped emargination (fig. 60a). The anterior rim of the posterior nares is situated between the anterior edges of the M3s. In other specimens, the anterior rim of the posterior nares fluctuates in its position from between the anterior edge of M3 to between the protocones of M2. A thin ridge of bone runs along the midline of the palate and extends past the anterior margin of the posterior nares. The posterior narial canal does not extend into the basisphenoid bone. The thin vomerine septum that bisected the posterior narial canal is not preserved in AMNH 2060, although remnants of it can be seen in the roof of the posterior narial canal. The basicranium of AMNH 2060 is quite short. The configuration of the basicranial foramina is typical, with the foramina of the alar canal, the foramen ovale, and the foramen lacerum being widely separated. The external auditory pseudomeatus is narrow; however, the mastoid process does not make contact with the postglenoid process and the external auditory pseudomeatus is unconstricted ventrally.

(B) left molars, and (C) left premolars of AMNH 2060, (D) upper incisors and canines of CMNH 2388.

UPPER DENTITION: The following description of the upper dentition is primarily based on the holotype (AMNH 2060) (fig. 60a–c), although this specimen has an incomplete incisor row. The incisors of AMNH 2060 are large and arch anterior to the canines. Alveoli indicate three pairs of incisors. The I1 crowns of AMNH 2060 are not preserved, although the root of the left I1 is preserved. Judging by the root of I1 it was the smallest incisor. The complete I1s of CMNH 2388 (fig. 60d) confirm that the I1 is the smallest incisor. The I1 is subcaniniform and similar in shape to the I2 and I3. I2 and I3 are large and subcaniniform with a single lingually curved cusp and a very thin lingual cingulum. No labial cingula are seen on the upper incisors. The I3 is larger in diameter and taller than the I2. The canine is bordered by a short precanine diastema and a longer postcanine diastema. The crowns of both canines are fragmentary in AMNH 2060, but in other specimens the canines vary in size from moderate to relatively large, except in the Asian skull (AMNH 26411), which has very large canines.

P1 is relatively large. It is about the same length as P2, but it is narrower and with a simpler morphology, including a single cusp and a posterior heel. A thin cingulum can be seen in the lingual side of the posterior heel. The posterior heel of the P1 of the Asian specimen (AMNH 26411) is somewhat wider than those of the North American specimens. There is no P1–P2 diastema in any specimen. The P2 of AMNH 2060 is oblique in shape due to the steeply posterolingually angled anterior margin. P2 is less oblique in AMNH 2004, although the P2s of other specimens are more similar to the holotype in this respect. In AMNH 2060 and other specimens P3 is much less oblique and P4 is not oblique.

The P2 parastyle and metastyle arch slightly lingually, while the P3 parastyle and metastyle are nearly straight. The P4 parastyle is strongly angled lingually, while the metastyle is straight. Because of these shape differences the outer wall of the ectoloph of P2 is more curved than those of P3 and P4. In AMNH 2060 each premolar (P2–P4) has a small distinct labial paracone rib though the ectolophs are too worn to compare their relative sizes. The premolars of AMNH 2004 are less worn and reveal that the labial paracone ribs are distinct and become narrower and shorter in more posterior premolars.

On the P2 of AMNH 2060 there is a small protocone with a small but distinct preprotocrista and a short lingual crest extending posteriorly from the protocone. The lingual morphology of P2 varies in other skulls. For instance, on the P2 of AMNH 2004 there is no distinct protocone; instead, a large crest arches around the lingual side of the crown. The lingual heels of P3 and P4, though larger and wider than that of P2, have less distinct preprotocristae and less distinct lingual crests. There are no hypocones on any premolars of Metatelmatherium ultimum . The labial cingula of the premolars tend to be discontinuous around the paracones except on the P4. The lingual cingula of the P2–P4 vary from being completely continuous to slightly discontinuous around the protocone.

The molars of AMNH 2060 are heavily worn and M1 and M2 have been shortened by interstitial wear. The M3 is minimally worn and preserves a number of brontotheriine traits including tall, lingually angled ectolophs, weak labial ribs, thin lingual ectoloph enamel, and wedge-shaped lingual sides of the paracone and metacone. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. There is a small anterolingual cingular peak on each molar. Central molar fossae are absent. The molars have no visible remnants of paraconules or metalophs. There is no hypocone on the M3, but a thick cingulum traces around the distolingual corner of the M3 crown. The labial molar cingula tend to be thick at the bases of the cusps, but they are discontinuous around the mesostyles. The lingual cingulum of the molars is thick, but it is only continuous around the base of the protocone of the M3.

MANDIBLE AND LOWER DENTITION:

The holotype of Metatelmatherium ultimum (AMNH 2060) includes an associated partial mandible with p4–m3. Another partial mandible, AMNH 2033, is consistent with the holotype mandible, and has a nearly complete symphysis with complete p2–m3 (fig. 61). The mandible associated with the Asian skull, AMNH 26411, is fully complete with a nearly complete set of lower dentition (fig. 62).

In comparison to AMNH 26411, the partial mandible of AMNH 2033 has a slightly more slender coronoid process and a somewhat narrower symphysis with a steeper inferior margin. However, these differences could be attributed to distortion and damage in AMNH 26411. The coronoid process of AMNH 26411 is crushed and partially reconstructed with plaster. In addition, the symphysis has gaps that are filled with plaster and the seemingly wider symphysis and shallower inferior margin could relate to the way in which this specimen has been distorted and subsequently reconstruct- ed. Additionally, the larger canines of the Asian specimen, with their rather bulbous roots, influence the size and shape of the symphysis. The posterior margin of the mandibular symphysis fluctuates in position from between the talonid of the p2 (AMNH 26411) to between the anterior margins of the p3 (AMNH 2033).

The lower incisors of AMNH 26411 are somewhat smaller than the upper incisors of the same specimen. The incisor row forms a short arch anterior to the canines. The apex of i1 is rounded although this seems due to wear. The i2 is larger and the apex is much more pointed. The i3 is about the same size as i2, although it is stouter and more subcaniniform. Lingual cingulids can still be discerned on these incisors, but labial cingulids are absent. The extremely procumbent condition of the incisors of AMNH 26411 is an artifact of distortion. In life the lower incisors would have been more nearly vertical. The canines of the Asian specimen are relatively large. There is no precanine diastema, but the postcanine diastema in these specimens is relatively longer.

The p1 of AMNH 26411 is small and simple with a single cusp and a short talonid heel. AMNH 2033 has a short p1–p2 diastema. However, AMNH 26411 lacks a p1–p2 diastema. The p2 trigonid is nearly twice as long at the talonid. The p3 trigonid is somewhat longer than the talonid. The talonid and trigonid of the p4 are of similar length. The trigonids of p2, p3, and p4 are narrower than the talonids. The p2 paralophid curves slightly lingually as it projects from the protoconid, creating a slight lingual notch in the trigonid. The p2 protolophid is straight but slightly angled lingually. The paralophid of p3 is angled about 45 ° lingually, creating a distinct lingual notch in the trigonid. The p3 protolophid is straight but weakly angled in the lingual direction. Finally, the p4 trigonid is essentially molariform with a lingually arching paralophid and protolophid. A distinct metaconid is seen only on the p4. The talonids of the p2–p4 are well developed with distinct cristids obliqua and hypolophids. The lingual surface of the p2 talonid forms a steeply sloped surface, although the p3 and p4 have nearly molariform talonid basins.

The molars of Metatelmatherium ultimum are typical with thinner lingual enamel, shallow talonid and trigonid basins, and an elongated m3. The labial cingulid is discontinuous around the bases of the cusps. The m3 cingulid does not wrap around the distal end of the m3.

REMARKS

Osborn (1908a) originally referred Metatelmatherium ultimum to the genus Telmatherium . Although this species was based on a very well-preserved skull and a partial right mandible (AMNH 2060), Osborn’s (1908a) initial description amounted to no more than four brief sentences. Douglass (1909) named another species, Manteoceras uintensis , on an anterior portion of a cranium with a complete set of upper dentition (CMNH 2388). Douglass’ only stated reason for differentiating this species from Telmatherium ultimum is, at best, a dubious distinction, ‘‘The zygomatic arch is not very heavy…. It is not nearly so heavy as in Telmatherium ultimum ’’ ( Douglass, 1909: 307). The figure of CMNH 2388 in Douglass (1909: figs. 4, 5) shows this specimen to be more complete and better preserved than it actually is. Although the specimen is figured as having zygomatic arches, only the anteriormost portion of the right arch is actually preserved.

Osborn (1929a) continued to accept Telmatherium ultimum and Manteoceras uintensis as separate species, and he believed them to belong to different lineages. He distinguished M. uintensis from T. ultimum based on ‘‘the obliquely flattened form of the infraorbital portion of the malars’’ ( Osborn, 1929a: 374). However, when one considers that the specimens are subtly distorted in different dimensions, one cannot realistically consider this difference as significant. Mader (1989) considered these two species to be synonymous and that revision is upheld here.

Granger and Gregory (1938) named a new genus and species, Metatelmatherium cristatum from a complete skull and mandible (AMNH 26411) from the Camp Margetts area of Inner Mongolia. Granger and Gregory (1943) noted that North American specimens referred to Telmatherium ultimum are nearly identical to M. cristatum . Therefore, they referred the North American species, T. ultimum , to their new genus, Metatelmatherium . They noted that the type skull of the North American species ‘‘so closely resembles the type of our Metatelmatherium cristatum that one can barely discover specific differences between them, while their congeneric relationship becomes more evident the longer they are studied’’ ( Granger and Gregory, 1943: 356).

Granger and Gregory (1943) distinguished Metatelmatherium cristatum from M. ultimum by its apparently larger size, the shorter and broader coronoid process, and the relatively longer and more sloping mandibular symphysis. However, all of these distinctions are dubious. The Asian skull is larger than the holotype but not significantly. In fact, many of its dimensions fall well within the range of North American M. ultimum . Though the skull is uncrushed, it has experienced severe expanding matrix distortion (sensu White, 2003). The skull looks larger than it probably was when it was intact, and some aspects of its shape could be subtly distorted. In the jaw of the Asian specimen the coronoid process is fragmented, its distal tip is missing, and there are copious amounts of plaster filling gaps between the bone fragments. The symphysis seems wider that that of the North American specimen, AMNH 2033. However, there are large amounts of plaster in the mandibular symphysis of the Asian jaw, and the degree to which the shape of the symphysis has been influenced by distortion and reconstruction is uncertain. Given the variability in canine size of other brontotheres species, the influence that the unusually large canine of the Asian specimen had on the overall shape of the anterior portion of the mandible is probably not taxonomically significant. Another difference, the wider posterior heel of the P1, is not much more compelling particularly when one considers the high level of intraspecific variation in brontothere premolars. For instance, wide and narrow P1 posterior heels can be found in a single individual of Protitanotherium emarginatum (YPM PU11242).

In more recent revisions Mader (1989; 1998) continued to recognize Metatelmatherium cristatum and M. ultimum as distinct species. However, M. cristatum Granger and Gregory (1938) is here considered to be a junior synonym of Metatelmatherium ultimum ( Osborn, 1908a) . Thus, M. ultimum becomes the type species of Metatelmatherium .

Metatelmatherium ultimum as defined here primarily occurs in the late Uintan deposits of the Uinta Basin, but it is also known from the Adobe town member of the Washakie Formation (UCMP 81447), the Wind River Basin (YPM 14158), and the ‘‘Irdin Manha’’ (put in quotes to denote uncertain relationship with the type Irdin Manha beds [ Radinsky, 1964]) of the Camp Margetts area of Inner Mongolia (AMNH 26411). These specimens are morphologically consistent with the M. ultimum material from the Uinta Formation, although the Washakie Basin specimen is notably smaller and could represent an earlier (early Uintan) species of Metatelmatherium , although, based on the present material, the only differentiating factor would be size. Presently, I include this specimen with M. ultimum .

Some authors have not accepted the generic distinction of Metatelmatherium from Telmatherium ( Qi, 1987) , while most others have accepted it ( Russell and Zhai, 1987; Prothero, 1996; Mader, 1989, 1998). Metatelmatherium ultimum is similar to Telmatherium validus , but can be differentiated from it on the following features: the very large ventral flange on the zygomatic arch, the extremely shallow nasal incision, short nasal processes, thinner sagittal crest, the narrow vaulted cranial roof, and small anterolingual cingular peaks on the molars. M. ultimum is more similar to Wickia brevirhinus , but it can be distinguished from that species primarily by the large autapomorphic ventral zygomatic flange, thin sagittal crest and, possibly, the more developed p2 talonid. Several other species have been assigned to Metatelmatherium , although they are all now considered to be nomina dubia and are further discussed in the section dealing with miscellaneous dubious taxa.