Metarhinus fluviatilis 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 : 69-74

Metarhinus fluviatilis Osborn, 1908a

NEOTYPE: FMNH P12187 , a complete skull missing only the incisors.

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

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

SYNONYMS: Metarhinus riparius Riggs, 1912 .

REFERRED SPECIMENS: (From the Wagonhound Member of the Uinta Formation of Utah) FMNH P12186 (holotype of Metarhinus riparius ), a skull with heavily worn dentition including right C–P1, P2–P3 (partial), M3, left C–P4, and M2–M3 ; FMNH P12201 , a subadult skull with right I3–C (erupting), P1, P2–P3 (erupting), DP4, M1– M2, M3 (erupting), left P1, P2–P3 (erupting), DP4, M1–M2, and M3 (erupting) ; YPM 13125, a skull with heavily worn dentition including right C, P2, P3–M3, left C–M3, and a partial mandible with right i2 (?), and left p2–m3; FMNH P12173 , a skull with right C, P3–P4, M2–M3 (all partial), left P2–P4, M2–M3; (from the Adobe Town Member of the Washakie Formation of Wyoming) FMNH PM1733 , a palate with a partial nasal bone, right C–M3, and left P1–M3 ; FMNH PM3935 , an anterior half of a skull with right P2–M3 and left P1–M3 ; FMNH PM44655 , a fragmented skull with right M2– M3, left P2–P4, M1–M2 (partial), and M3 ; UCM 44939, a complete skull with right P2– M3 and left M3 (partial); UCMP 81278, a skull missing the posterior end with right and left C, and P2–M3.

DIAGNOSIS: Metarhinus fluviatilis is a small hornless brontothere in which the frontal bone intrudes slightly into the surface of the nasal bone splitting off a small lateral nasal splint from the main body of the nasal. The posterior margin of the nasal incision is above the anterior margin of the M2. The nasal process broadens distally, is thin, horizontal, unelevated, with very shallow lateral walls, and without a strongly rounded distal margin. The orbits are positioned above the M2 and strongly protrude laterally. 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.

Dentally, Metarhinus fluviatilis can be characterized as having large subcaniniform upper incisors, a simple P1, a distinct P2 metacone, weak premolar preprotocristae on P2 and P3, and with short lingual crests occasionally extending posteriorly from the premolar protocones. Premolar hypocones are absent. The molars of M. fluviatilis 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. The mandibular symphysis extends to the p3. The p3 has a distinct metaconid (although this character may be variable). The lower molars of M. fluviatilis can be characterized as having shallow molar basins and a slender m3.

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

DESCRIPTION

SKULL: The proposed neotype of Metarhinus fluviatilis (FMNH P12187) is an exceptionally complete and undistorted skull with lightly worn teeth (figs. 27, 28a). It is missing only the incisors. Riggs’s original figure of FMNH P12187 ( Riggs, 1912: pl. VIII) depicts a left I2 (?) although it is presently lost, or not with the specimen. In addition, several views of another well-preserved specimen, UCM 44939, are shown (fig. 29).

Metarhinus fluviatilis is a rather small (table 4), hornless brontothere that is most similar in size to the contemporaneous early Uintan brontotheres M. abbotti and Fossendorhinus diploconus . In most specimens the sutures of the facial bones are not discernable, although they can be seen in UCM 44939. The posterolateral portion of the nasal bone is split by a short triangular process of frontal bone. The lateral portion of the frontal bone forms a short but distinct lateral nasal splint that extends in a posteroventral direction between the frontal and maxilla and makes a narrow contact with the lacrimal bone. This configuration is also seen in several other hornless brontotheres (e.g., Telmatherium ), but in M. fluviatilis , the triangular process and lateral nasal splint are relatively short. Riggs (1912) and Osborn (1929a) described rudimentary horns in specimens of M. fluviatilis (FMNH P12186, FMNH P12187, FMNH P12194), but no such structures can be discerned on the actual specimens. The lateral views of some skulls, such as FMNH P12187, are deceptive because the nasal process is angled slightly downward; this gives the appearance of a raised area between the orbits and the nasal incision. Often, the nasal bone is forced further downward taphonomically, exaggerating the effect. However, there are no distinct hornlike protuberances on any specimen of M. fluviatilis .

The face of Metarhinus fluviatilis is highly constricted by the long nasal incision. The nasal incision extends to the anterior rim of the orbit and to the anterior margin of the M2. The orbit is situated over the M2, with the posterolateral root of M1 and anterolateral root of M2 situated below the anterior orbital rim. The orbits of this species have been described as prominent, or laterally protruding ( Osborn, 1908a, 1929a). The appearance of prominent orbits is manifested by a broad and relatively flattened frontal between the orbits, a transverse constriction of the dorsal surface of the skull anterior to the orbits, and a recessed surface of bone between the deep nasal incision and the anterior rim of the orbit that forms a narrow wall of bone facing anteriorly just in front of the orbit.

The nasal process of Metarhinus fluviatilis is long. In FMNH P12187 it is as long as the premaxillomaxillary rostrum. The nasal process is angled slightly downward and it is somewhat bowed downward in the center so that the dorsal surface is concave. The lateral walls of the nasal process are thin, shallow, and nearly constant in dorsoventral depth from the proximal end to about the midpoint of the nasal process. On the distal half of the nasal process the lateral walls become shallower distally. The anterior edge of the nasal process is thin, roughened, and deflected downward. From the dorsal view the width of the nasal process is constricted proximally, broadens distally, and has an imperfectly rounded anterior edge. Other specimens with complete (or nearly complete) nasal processes are consistent with this description except that the nasal process is slightly shorter than the premaxillomaxillary rostrum most of the time (e.g., FMNH P12173, FMNH PM3935, UCM 44939, UCMP 81278, YPM 13145).

From the lateral view, the premaxillomaxillary rostrum is long, slightly upturned, and of relatively constant thickness throughout its length. The dorsal margin of the rostrum

TABLE 4 Summary statistics for selected morphometric variables of Metarhinus fluviatilis See Methods for measurement definitions is not raised above the lower half of the orbit. However, the posterior margin of the nasal incision extends to the level of the upper rim of the orbit. The rostrum of Metarhinus fluviatilis is highly specialized in the same manner as Dolichorhinus and M. abbotti where the rostral cavity is entirely enclosed by bone. In the available specimens of M. fluviatilis the dorsal covering is solid and no distinct sutures can be seen. The premaxillary symphysis is elongate and extends the entire length of the rostrum. This can be best seen in UCM 44939, where the symphysis is unossified. From an anterior view of FMNH P12187 the premaxillae form a solid dome of bone with a thick median ridge of bone (the osteological marker for the nasal septum) running anteroposteriorly along the superior margin of the premaxillomaxillary process. This ridge continues along the dorsal surface of the rostrum into the skull.

From a lateral profile of FMNH P12187, the dorsal surface of the skull is essentially flat over the orbits, and convex over the posterior half of the skull. From the dorsal view of the skull, the parasagittal ridges converge into a short sagittal crest. The zygomatic arches are thin and not strongly bowed. The jugal portion of the zygomatic arch is dorsoventrally shallow and horizontal, while the squamosal portion is deeper and rises posteriorly, giving the zygomatic arch a strong curvature. Some of the specimens are more gracile than the holotype with longer and thinner sagittal crests and thinner zygomatic arches (e.g., FMNH P12186, UCM 44939). Despite this variation, there are no discrete broad-skulled and narrow-skulled groups as hypothesized by Riggs (1912). There is a small infraorbital process on the jugal. The infraorbital process is much smaller than those of Dolichorhinus , Sphenocoelus , or Mesatirhinus , but is similar to that of Metarhinus abbotti . In the center of the zygomatic arch the ventral margin of the jugal is extended ventrally forming a small flange, although this structure is less pronounced in other specimens of Metarhinus fluviatilis and is always much less conspicuous than the large ventral zygomatic flange seen in Metatelmatherium ultimum .

From a dorsal view the nuchal crest is strongly notched medially. From a lateral view the occiput is moderately tilted backward. From a posterior view the dorsal border of the occiput is arched. The dorsal half of the occiput is narrower than the ventral portion and it is somewhat constrict- ed in the middle. The center of the occiput is deeply recessed between two prominent occipital pillars.

The ventral surface of the skull of Metarhinus fluviatilis is well preserved in both FMNH P12187 and UCM 44939. The anterior rim of the posterior nares varies in its position from between the M2 hypocones (FMNH P12187) to between the anterior edges of the M3s (UCM 44939). A narrow horseshoe-shaped rim emarginates the anterior and lateral margins of the posterior nares. The anterior rim of bone and the emargination each have a small posteriorly projecting median process. Remnants of posteriorly shifted turbinates can be seen in the posterior narial canal of UCM 44939, where the matrix inside the skull has been removed. Though the matrix has not been removed from inside the skulls of other specimens, thin slivers of bone suspended in the matrix filling the posterior narial choanae can be seen in some skulls (e.g., FMNH P12201) that also appear to represent posteriorly extended turbinates. The posterior narial canal is elongate and tends to extend onto the anterior part of the sphenoid, but large ventral sphenoidal fossae are absent. In other ways the basicrania of these specimens are otherwise typical with a widely separated foramen ovale and foramen lacerum. The mastoid process does not contact the postglenoid process ventrally; therefore, the external auditory pseudomeatus is open ventrally.

UPPER DENTITION: No known skull of Metarhinus fluviatilis contains upper incisors. Earlier figures of the neotype ( Riggs, 1912: pl. VIII) depict a large, subcaniniform I2(?), but that incisor is no longer attached to the skull and it is apparently lost. The incisor alveoli and/or partial roots indicate an unreduced number of incisors (three pairs) that form an arched row anterior to the canines. The canines are generally small, but tend to vary in size more than the other teeth. Short precanine and postcanine diastemata are consistently present.

The description of the cheek teeth of Metarhinus fluviatilis is primarily based on FMNH P12187 (fig. 28b, c), but is supplemented by information from other specimens. The P1 crown is a small and simple tooth with a single cusp and an elongate posterior heel. In FMNH P12201 (not shown), the P1 is unworn; the cusp of that tooth curves lingually and a thin lingual cingulum is present. The P2 of FMNH P12187 is slightly more oblique in outline than P3 and P4 due to a more posterolingually angled anterior margin. In other specimens the P2 can be less oblique (e.g., FMNH PM3935). The parastyle of P2 is straight, while those of P3 and P4 are somewhat labially directed. The metastyles of P2 and P3 are essentially straight, while the P4 metastyle is angled slightly labially. The labial walls of P2–P4 have distinct labial paracone ribs that become shorter and narrower in more posterior premolars. The lingual heel of the P2 is as developed as those of P3 and P4. There is only a single large lingual cusp (protocone) on the P2–P4; premolar hypocones are absent. On P2 and P3 there is both a small but distinct preprotocrista and a short lingual crest. In P4, the preprotocrista is extremely faint, and there is no lingual crest. The labial cingula are discontinuous around the paracone of P2 and P3, but there tends to be continuous cingulum around the labial base of the crown of P4. The anterior and posterior premolar cingula extend around the lingual sides of the crowns but do not join in FMNH P12187. Occasionally (e.g., FMNH P12173), there are continuous lingual premolar cingula.

Molar apomorphies seen in Metarhinus fluviatilis include 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. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. The molars invariably have shallow central molar fossae. Very small anterolingual cingular peaks can be seen on FMNH P12187, although in other specimens (e.g., UCM 44939 and UCMP 81287), small anterolingual cingular cusps are usually more distinct. There are no traces of paraconules or metalophs. The M3 of FMNH P12187 lacks a hypocone, but the posterolingual cingulum is thick and rough. However, an M3 hypocone is occasionally present (e.g., FMNH P12186), though it is never as large as that of the M1 or M2 hypocone. The labial molar cingula are thin but distinct and discontinuous around the labial bases of the mesostyles; lingual molar cingula are essentially absent.

MANDIBLE AND LOWER DENTITION: Only one identifiable skull of Metarhinus fluviatilis (YPM 13125) is associated with a partial mandible. The p3 of this mandible has a large metaconid, a character that is variable among the Metarhinus sp. mandibles that are further described below.