Metarhinus abbotti ( Riggs, 1912 )

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 : 74-79

Metarhinus abbotti ( Riggs, 1912)

HOLOTYPE: FMNH P12179 , a complete skull, somewhat crushed dorsoventrally, with complete dentition.

TYPE LOCALITY: Wagonhound Member (Uinta B) of the Uinta Formation, Uinta Basin, Northeast Utah.

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

REFERRED SPECIMENS: (From the Wagonhound Member of the Uinta Basin, Utah) CMNH 2866, a complete skull with heavily worn teeth including right P2–M3 and left P1–M3; CMNH 3510, an anterior portion of a skull (partially prepared) with heavily worn teeth including right I2?–I3?, P1 (partial), P2–M1, left I3?, P1–M3.

DIAGNOSIS: Metarhinus abbotti is a small hornless brontothere. The nasal incision extends to the anterior margin of M2. The nasal process tapers distally. It is thin, horizontal, unelevated, and with very shallow lateral walls. The orbits are positioned above the M2 and strongly protrude laterally as in Metarhinus fluviatilis . The premaxillomaxillary rostral cavity is enclosed by bone dorsally and its dorsal surface is nearly horizontal. Other cranial characteristics include a small infraorbital process, a sagittal crest, a dorsal cranial surface that is flat or slightly convex postorbitally, strongly curved and unbowed or weakly bowed zygomatic arches, and a ventrally open and mediolaterally angled external auditory pseudomeatus. Ventral sphenoidal fossae are absent among the known specimens.

Dentally, Metarhinus abbotti has large subcaniniform upper incisors, a simple P1, a distinct P2 metacone, occasional weak premolar preprotocristae, and occasional short lingual crests. Premolar hypocones are absent. The molars of M. abbotti have tall, lingually angled ectolophs with weak labial ribs, and thinned lingual ectoloph enamel with wedge-shaped paracones and metacones. Central molar fossae and anterolingual cingular cusps are present. Cingular parastyle shelves, paraconules, and metalophs are absent.

Metarhinus abbotti shares with Dolichorhinus hyognathus and M. fluviatilis a rostrum that is sealed dorsally by bone. The skull of M. abbotti most clearly differs from D. hyognathus in its shorter proportions, prominent sagittal crest, and laterally protruding orbits. The skull of M. abbotti is undifferentiated from M. fluviatilis except for the distally tapered nasal process.

DESCRIPTION

SKULL: The holotype of Metarhinus abbotti ( FMNH P12179 ) is a complete skull although it is somewhat crushed dorsoventrally (fig. 30). Small portions of the right premaxilla and right zygomatic arch are reconstructed with plaster. The dentition of FMNH P12179 is complete ; however, substantial portions of the canine crowns are plaster so that measurements of the canine crowns are probably not reliable. There are no visible sutures on FMNH P12179 . Two additional skulls, CMNH 2866 and CMNH 3510 , are identified as Metarhinus abbotti . Although it is more poorly preserved than the holotype, CMNH 2866 (fig. 31) is undistorted and more faithfully reveals the general shape of the cranium from a lateral view .

Metarhinus abbotti is a small (table 5) hornless brontothere most similar in size to M. fluviatilis and Fossendorhinus diploconus . The nasal incision extends as far back as the posterolateral root of M1. The orbit is positioned directly above M2, while the posterolateral root of M1 is positioned below the anterior rim of the orbit. The face is greatly constricted in FMNH P12179 but somewhat less than that seen in some specimens of M. fluviatilis . However, CMNH 2866 shows greater facial constriction. The orbits of M. abbotti protrude laterally to a degree similar to that of M. fluviatilis (see description of M. fluviatilis for further explanation).

The nasal process of the holotype is very thin, narrow, shorter than the premaxillomaxillary rostrum, and it projects in a slightly upward direction. The upward orientation of the nasal process of the holotype appears to be a taphonomic artifact related to the dorsoventral crushing above and behind the orbits. The nasal process of CMNH 2866 is horizontal. The side of the nasal bone forms a very shallow lateral wall. The lateral wall is truncated at the midpoint of the nasal process and the distal half of the nasal process is nearly flat. In FMNH P12179 the nasal process tapers continuously from the proximal end to the distal end and the distal margin is rounded. However, the shape of the nasal bone appears to be variable. In CMNH 2866 and CMNH 3510 the proximal two

(C) anterior view.

thirds of the nasal process has a constant width while the distal third tapers.

The premaxillomaxillary rostrum is undifferentiated from that of Metarhinus fluviatilis . From a lateral view, the premaxillomaxillary rostrum is long, of relatively constant dorsoventral depth, and slightly curved upward. The dorsal surface of the rostrum is horizontal and the rostrum does not deepen proximally. The premaxillary symphysis is very long and it extends the entire length of the rostrum. Consequently, the dorsal surface of the rostrum is completely covered by a solid layer of bone. From the anterior view the premaxillae form a dome with a tall ridge of bone running mesially along the full length of the rostrum.

From a lateral view the dorsal surface of FMNH P12179 is flat above and behind the orbits. However, in the uncrushed specimen, CMNH 2866, the postorbital dorsal surface is more convex. The sagittal crest is very thin. Likewise, the zygomatic arches are very thin, dorsoventrally shallow, and are not laterally bowed. From a lateral view, the zygomatic arches are strongly curved. The infraorbital jugal process of Metarhinus abbotti can be seen most clearly from the anterior view (fig. 30c). It is small like that of M. fluviatilis .

From a lateral view the occiput is moderately tiled backward. From a dorsal view the nuchal crest is deeply notched medially. From a posterior view (not shown) the dorsal margin of the occiput is arched and the center of the occiput is deeply recessed between two prominent occipital pillars.

In comparison to specimens of Metarhinus fluviatilis (particularly FMNH P12187, seen in fig. 28), the holotype of M. abbotti is gracile. However, other specimens such as CMNH 2866 are more robust with somewhat deeper zygomatic arches and less slender proportions. In particular, CMNH 2866 resembles some of the more gracile specimens of Metarhinus fluviatilis . Therefore, it is difficult to separate these two species based on the depth of the zygomatic arches or general robusticity of the skull.

The ventral view of FMNH P12179 (fig. 32a) does not notably differ from Metarhinus fluviatilis . The anterior rim of the posterior nares is positioned between the M2s, the emargination of the posterior nares is narrow, and there are no ventral sphenoidal fossae, although the posterior narial canal extends onto the anterior part of the sphenoid. The foramen oval is widely separated from the foramen lacerum and the external

TABLE 5 Summary statistics for selected morphometric variables of Metarhinus abbotti See Methods for measurement definitions

(C) left premolars, (D) lingual view of left incisors and canine.

auditory pseudomeatus is open ventrally. A pair of thin bony choanal pouches can clearly be seen in the anterior portion of the posterior narial canal of FMNH P12179.

UPPER DENTITION: The holotype of Metarhinus abbotti (FMNH P12179) is the only specimen with well-preserved teeth (fig. 32), although some information on variation in the premolars can be gleaned from other specimens. M. abbotti has an unreduced upper dental formula (3-1-4-3). The incisors are large and form an arched row that extends anterior to the canines. The incisors are subcaniniform with short, lingually curved crowns, and distinct lingual cingula. The incisors increase in size laterally, with i3 being the largest and most caniniform incisor. There is both a short precanine diastema and a postcanine diastema. The crowns of the canines are fragmented and reconstructed with plaster. The remnants of real enamel embedded in the plaster reconstruction suggest that the canines were rather small.

The P1 is a small and simple tooth with a single cusp and an elongate posterior heel. There is a distinct lingual cingulum on the P1. The P2 is more oblique in outline than the P3 and P4 because of a slightly more posterolingually angled anterior margin and a slightly more lingually shifted metacone. The P2 parastyle is straight, while those of P3 and P4 are angled slightly labially. The metastyles of P2–P4 are nearly straight. The labial paracone ribs of P2–P4 are distinct and become smaller in more posterior premolars. The protocones of P2–P4 are relatively tall. In P2, the protocone is slightly ovoid, but those of P3 and P4 are progressively more cone-shaped. There are no additional lingual cusps or crests on the premolars of FMNH P12179; however, the premolars of CMNH 3510 exhibit vestigial yet distinct preprotocrista on P2 and P3. The anterior and posterior premolar cingula typically join lingually, forming continuous lingual cingula, but this is not always the case, as in the P2 of FMNH P12179, where the lingual cingula are discontinuous.

The molars of Metarhinus abbotti show typical brontotheriine apomorphies including tall, lingually angled ectolophs, weak labial ribs, thin lingual ectoloph enamel, and wedge-shaped lingual sides of the paracone and metacone in molars that are not heavily worn (M2 and M3). The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. Each molar has a shallow central molar fossa. There are no traces of paraconules or metalophs. There are strong anterolingual cingular peaks on the M2 and M3. It is weaker on M1, but this is because it is more heavily worn. A M3 hypocone is present, but it is not bilaterally symmetrical; on the right side it is a small but distinct cusp and the distal cingulum ascends to the apex of the right M3 hypocone; on the left side the hypocone is smaller and the cingulum appears to wrap around the mesiodistal corner of the M3. The labial molar cingula are thin and discontinuous around the labial bases of the mesostyles. The lingual molar cingula are discontinuous around the protocone and hypocone.