Rhinotitan andrewsi ( Osborn, 1925 )

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 : 174-182

Rhinotitan andrewsi ( Osborn, 1925)

HOLOTYPE: AMNH 20271 , a skull missing the nasal, with right I2–M3, left I2–C, and P2–M3.

TYPE LOCALITY: Shara Murun Formation, Ula Usu, Baron Sog Formation, Shara Murun Region, Inner Mongolia, China.

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

REFERRED SPECIMENS: (From the Shara Murun Formation, Ula Usu, Baron Sog Mesa, Shara Murun Region, Inner Mongolia) AMNH 20254, a skull with right I1–I2 (damaged), I3, C (damaged), P1–M3, left I2 (damaged), I3, C (damaged), and P1–M3; AMNH 20261, a laterally crushed skull with right C–M3 and left I3–M3; AMNH 20263, a palate with right and left I2–M3; IVPP V3254-1, a partial skull with right I1–C, P2–M3, left I1–I3, P2–M3, a mandible with right i1–c, p2–m3, left i1–c and p2–m3, and a skeleton; IVPP V3254-2, a complete skull with complete dentition except left I1, and a mandible with right p1–m3, left p2, and p4– m3; PIN 2198-3, a partial skull with right P2– M2, M3 (partial and unerupted) and left P1– M2; PIN 2198-5, a skull with right P2–M3 and left I2–C, P1–M3: PIN 7130-3 with right and left P2–M3.

DIAGNOSIS: Rhinotitan andrewsi is a large horned brontothere with small, elliptical, and widely separated frontonasal horns. The horns are positioned slightly in front of the orbits and high above the orbits. The nasal incision is dorsoventrally deep and extends to the P4. The orbit is above the M2 with the posterolateral and anterolateral roots of M1 below the anterior orbital rim. The nasal process is either horizontal or slightly angled upward, relatively broad, not strongly rounded anteriorly, and with thick lateral walls that do not arch around the anterior margin. Proximally, the lateral walls of the nasal process deepen and angle ventromedially, greatly constricting the dorsal portion of the nasal cavity. The premaxillomaxillary rostrum thickens posteriorly and it is not enclosed by bone dorsally. Other cranial characteristics include a marginally saddle-shaped or incompletely saddle-shaped cranium, a dorsal cranial surface that is not constricted posteriorly by parasagittal ridges, a narrow emargination surrounding the posterior nares, nearly straight zygomatic arches, and a ventrally constricted and posteromedially angled external auditory pseudomeatus. Ventral sphenoidal fossae and postzygomatic processes are absent.

Dentally, Rhinotitan andrewsi has three large incisors. The I1 and I2 are subglobular, while I3 is more subcaniniform. P1 is complex, there is a distinct P2 metacone, and premolar hypocones are occasionally present, although in P2 and P3 the protocone and hypocone sometimes take the form of a single lingual crest. The molars have tall, lingually angled ectolophs with weak labial ribs, and thinned lingual ectoloph enamel with wedge-shaped paracones and metacones. A cingular parastyle shelf is absent. Distinct central molar fossae are present, but anterolingual cingular cusps are absent. Paraconules and metalophs are absent. The lower dentition of R. andrewsi includes three large incisors; the i1 and i2 are semispatulate, while the i3 is more subcaniniform. There is a distinct postcanine diastema and the p2 trigonid is nearly twice the length of the talonid. The lower molars have shallow basins and the m3 is slender.

Rhinotitan andrewsi can be distinguished from most other horned brontotheres by the combination of large incisors and an incompletely saddle-shaped cranium. R. andrewsi is most easily distinguished from R. kaiseni by the wider basicranium, the more posterior position of the frontonasal horns, the less downturned anterior margin of the nasal process, and the more subglobular I1 and I2.

DESCRIPTION

SKULL: The holotype of Rhinotitan andrewsi (AMNH 20271) is a large skull missing the nasal bones (fig. 83). The ventral surface of the skull and the zygomatic arches are well preserved, although large portions of the cranium are incomplete and reconstruct- ed with copious amounts of plaster. There are several additional skulls of R. andrewsi , all from the Shara Murun Formation, including material in the AMNH collection, the IVPP collection previously described by Wang (1982), and undescribed material in the PIN collection. Each skull is crushed, heavily reconstructed with plaster, and/or damaged in some way, rendering it difficult to precisely interpret the shape of the skull without considering several specimens. In addition to the holotype, I have figured lateral views of AMNH 20254, IVPP V3254-1, IVPPV 3254-2 (fig. 84), and a more extensive set angles of PIN 7130-3 (fig. 85).

Rhinotitan andrewsi is larger than R. kaiseni , but the size difference is probably not so extreme that there was no overlap in body size among members of these species. The horns of all R. andrewsi specimens are highly elliptical with the longer axis in the anteroposterior direction, and they tend to project strongly laterally. There is a moderate amount of size variation in the horns and some are rather smooth while others have more rugose surfaces. The horns are widely separated laterally and they are elevated above the orbits to a degree similar to Diplacodon elatus . From a lateral view the horns rest upon a superorbital pillar that rises from the orbit at about a 45 ° angle. The horns are more posteriorly positioned than those of R. kaiseni . They are positioned in front of the orbits and rest directly above the posterior margin of the nasal incision (IVPP V3254-2) or in front of it (PIN 7130-3). Such minor differences in the position and height of the horns may relate to taphonomic distortion.

The nasal incision is so dorsoventrally deep that its posterior margin is significantly higher than the orbit. The nasal incision of Rhinotitan andrewsi is longer than that of R. kaiseni . In the holotype (AMNH 20271), the incomplete posterior margin of the nasal incision appears to extend to the anterior edge of P4. The posterior margin of the nasal incision of PIN 7130-3 is also positioned about at the anterior margin of P4. There is some fluctuation in the exact position of the posterior margin of the posterior nares; in AMNH 20254 it is slightly more anterior than the P4, while those of IVPP V3254-1 and IVPP V3254-2 are slightly posterior to the anterior margin of P4.

The position of the orbit of Rhinotitan andrewsi is not significantly different from that of R. kaiseni . For instance in the holotype (AMNH 20271), the M2 is directly below the orbit and the lateral roots of M1 are below the anterior rim of the orbit. Among other specimens, the exact position of the orbit fluctuates slightly. For instance, in AMNH 20254, the posterolateral root of the M1 is clearly behind the anterior orbital rim.

In some of the skulls (e.g., IVPP V3245-1, AMNH 20254) the nasal process slants upward; this character has been used as evidence for drawing phylogenetic lines between Rhinotitan and embolotheres ( Protembolotherium and Embolotherium ) ( Yanovskaya, 1980; Wang, 1982). However, for the most part, the upward slant of the nasal processes of these specimens seems related to taphonomic distortion. In IVPP V3254-1, the horns and the entire nasal process are strongly angled upward at about a 45 ° angle. However, there is an abrupt kink in the dorsal surface of the skull just behind the orbits, suggesting that the entire frontonasal region has been artificially rotated upward. Other specimens of R. andrewsi with upwardly angled nasals (e.g., AMNH 20245) also clearly suffer from heavy distortion in the anterior portion of the skull. IVPP 3254-2 is minimally distorted and suggests that the nasal process may have been slightly angled upward, similar to that seen in R. kaiseni . However, another skull, PIN 7130-3 has a more normal horizontal nasal process with a slightly downward bend to it. In conclusion, the nasal process was either horizontal or it was only slightly angled upward.

The nasal process is similar in length to that of the premaxillomaxillary rostrum in some specimens (e.g., IVPP V3254-1, and IVPP V3254-2), but in others (PIN 7130-3 and AMNH 20254) the nasal process clearly extends anterior to the rostrum. The most complete nasal processes (IVPP V3254-2 and PIN 7130-3) have distally tapering widths. The distal margin of the nasal process is flat or slightly rounded with a small median notch. From a strictly lateral view, the lateral wall of the nasal process appears to be relatively shallow. However, the view of the ventral side of the nasal process of PIN 7130- 3 reveals that the lateral walls are strongly directed medially (fig. 85C). Proximally, the lateral walls become thicker, deeper, and they are strongly angled ventromedially, severely constricting the upper portion of the nasal cavity. The lateral walls of the nasal process are very shallow toward the distal end of the nasal and they do not continue around the anterior margin of the nasal as they do in Rhinotitan kaiseni .

The premaxillomaxillary rostrum is not clearly differentiated from Rhinotitan kaiseni . From the lateral view of the holotype (AMNH 20271), the rostrum is relatively shallow. At a point above the P3 the dorsal margin of the rostrum curves sharply upward, giving the dorsal surface of the rostrum a concave appearance. PIN 7131-3 has a similar appearance. On the other hand, the dorsal margins of the rostra of IVPP V3254-1 and IVPP V3254-2 are flatter and angled sharply posterodorsally. The premaxillary symphysis is completely ossified. Behind the symphysis, the dorsolateral margins of the rostrum diverge laterally and the rostral cavity is not sealed over by bone. Like in R. kaiseni , the nasal opening of the skull is very tall and narrow.

Interpretation of the shape of the dorsal surface of the skull of R. andrewsi is difficult. The dorsal surfaces of the holotype (AMNH 20271) and AMNH 20254 are nearly flat, but these specimens are unreliable due to the degree of plaster reconstruction. The dorsal surface of IVPP V3254-2 is barely concave, while that of PIN 7130-3 is somewhat more deeply concave. However, IVPP V3254-1 suggests a more bulbous cranium, with a somewhat convex postorbital dorsal surface similar to that of Metatitan . Like R. kaiseni , the dorsal surface of the skull broadens posteriorly and the parasagittal ridges do not greatly constrict the posterodorsal surface of the skull. (Note that the dorsal surface of PIN 7130-3 is artificially narrowed due to transverse buckling.) The zygomatic arches of R. andrewsi tend to be thicker and somewhat deeper than those of R. kaiseni , although in at least one specimen (AMNH 20261), the zygomatics are thin. From lateral views the entire zygomatic arch is straight and nearly horizontal. From a dorsal view the zygomatic arches are nearly straight are not strongly bowed laterally.

The occiput is nearly vertical like that of Rhinotitan kaiseni . The nuchal crest is slightly concave from a dorsal view. From the posterior view the nuchal crest is barely arched dorsally. Overall, the occiput is large, tall, and broad. The dorsal and ventral halves of the occiput are the about the same width, and the width of the occiput does not appear to have been constricted in the middle. The surface of the occiput has weak occipital pillars with a shallow pit in the center of the occiput.

The ventral surface of the holotype skull (AMNH 20271) is seen in fig. 86a. The anterior rim of the posterior nares is positioned slightly anterior to the M3 protocones. The posterior nares are rimmed by a very narrow emargination. In AMNH 20271 this emargination is partially covered by plaster, but it is completely exposed on AMNH 20254 (not shown) and slightly narrower that that of Rhinotitan kaiseni and more squarish (rather than horseshoe-shaped). The posteri- or narial canal does not extend into the sphenoid as it does, for instance, in Diplacodon elatus . The arrangement of the basicranial foramina is typical, with a widely separated foramen lacerum and foramen ovale. The mastoid process curves anteroventrally and contacts the postglenoid process forming a tube-shaped external auditory pseudomeatus. Like R. kaiseni , the external auditory pseudomeatus enters the skull in a strongly posteromedial direction.

One peculiar aspect of the basicranium of Rhinotitan andrewsi is its width. In specimens where the basicranium is complete and undistorted (AMNH 20271, AMNH 20254, IVPP V3254-2) the width of the basicranium (measured as the distance between the lateral sides of the mastoid processes) is consistently wider than the width measured between the lateral sides of the M3s. For instance, in the holotype (AMNH 20271), the distance across the mastoid processes (315 mm) exceeds the outside width of the M3s (260 mm). The condition seen in R. andrewsi is intermediate between most brontotheres, where the basicranium is narrower than the distance across the M3s, and Metatitan , where the basicranium is even more extremely widened. Nonetheless, R. andrewsi consistently differs in this regard from skulls of R. kaiseni where the outside width of the M3s is greater than the width of the basicranium.

UPPER DENTITION: Most of the specimens of Rhinotitan andrewsi have well-preserved and lightly worn teeth. In addition to the ventral surface of the holotype (AMNH 20271) (fig. 86a), pictured in close-up are the upper premolars and molars of AMNH 20254 (fig. 86b–c), the upper premolars, canines, and incisors of AMNH 20271 (fig. 86d–e), and the unworn incisors and canine of AMNH 20263 (fig. 86f–g).

The incisors are large and form a slightly arched row anterior to canines. Complete I1s are only seen with IVPP V2354 -1 and IVPP V3254-2. The I1s of these specimens are worn, but they appear to be similar to the I2 although somewhat smaller. The I2 and I3 are intact in a number of specimens, including AMNH 20271. Another specimen, AMNH 20263, has incisors that are very similar to those of AMNH 20271, but they are essentially unworn. In these specimens, I2 is nearly a featureless crown, although in the least worn specimen the crown is subglobular in appearance with a very short cusplike apex. The I3 is slightly larger than the I2 and it is taller and with a more conical crown and a more distinct lingual cingulum. However, the incisors of AMNH 20271 and AMNH 20263 are more globular than those of Rhinotitan kaiseni . The size of the canines among specimens of R. andrewsi varies; they are relatively large in all specimens except AMNH 20261 (not shown). There are distinct precanine and postcanine diastemata in R. andrewsi . The postcanine diastema tends to be shorter than the P2.

The crown of P1 is nearly round in outline. There are two lingual cusps, a large paracone, and a smaller metacone that is positioned at the very posterior end of the P1 ectoloph. The lingual heel of P1 is well developed with a small protocone that is partially absorbed by a crest that runs along the lingual side of crown. P2–P4 are more rectangular; the anterior and posterior sides of these teeth are nearly parallel and the lingual sides are only slightly rounded. The parastyle and metastyle of P2 are straight and the metacone is not strongly shifted lingually, thus the ectoloph is nearly straight. On P3 and P4 the parastyles are strongly angled labially, while the metastyles are weakly angled labially. The outer swelling of the P2 paracone is minor, but there are small but distinct labial paracone ribs on P3 and P4. A mesostyle is not seen on any of the premolars of Rhinotitan andrewsi .

The lingual features of the premolar crowns have low relief. In the holotype, AMNH 20271 , the lingual sides of the P2– P4 have two cusps, a protocone and hypocone that are connected by a lingual crest. A vestigial preprotocrista can also be seen on P2 and P3, but it is not detectable on P4. In P 2, the protocone and hypocone are spaced apart, but these cusps are almost fully absorbed by the connecting crest. In P 3, the cusps are also indistinct, but they are positioned more closely together. Finally, on the P4, the protocone and much smaller hypocone are widely separate and less strongly connected. Other specimens of Rhinotitan andrewsi show variable lingual premolar morphology. For instance, in AMNH 20261 (not show) there are distinct hypocones on P2–P4. On this specimen, the hypocones are only weakly connected to the protocone and the P3 hypocone is positioned more distantly behind the protocone. Yet in AMNH 20254 there are no distinct lingual cusps on P2 and P3 ; instead, there is a single lingual crest that arches along the lingual side of the crown. The P 4 of AMNH 20254 has a protocone and a smaller hypocone that are distinctly separated. In AMNH 20263 , the P2 and P3 are similar to those of AMNH 20254 , but the P4 of that specimen has only a centrally positioned protocone with no hypocone or connecting crest. Labial premolar cingula are very weak ; the lingual premolar cingula vary from continuous (e.g., AMNH 20271 ) to slightly discontinuous (e.g., AMNH 20254 ) .

The upper molars of Rhinotitan andrewsi show numerous brontotheriine features, including tall, lingually angled ectolophs, very 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. Distinct central molar fossae are present. However, anterolingual cingular cusps are not seen. No traces of paraconules or metalophs remain. One specimen, AMNH 20254, has a small hypocone on the M3, although other specimens show no trace of an M3 hypocone. Labial molar cingula are generally weak, and lingual molar cingula are very weak between the protocone and hypocone.

MANDIBLE AND LOWER DENTITION:

The skulls, IVPP V3254-1 and IVPP V3254- 2, are associated with mandibles. The mandible of IVPP V3245-1 is presently articulated with the skull of IVPP V3254-2 and is seen in fig. 84c. I was not able to find the mandible belonging to the skull of IVPP V3254-2, but a photo of it can be seen in Wang (1982: pl. 4). The horizontal ramus of IVPP V3254-1 is slender and deepens posteriorly, while that of IVPP V3254-2 is deeper overall. Additionally, the angle of the inferior margin of IVPP V3254-1 is very shallow (much less than 45 °), while that of IVPP V3254-2 is angled about 45 °. The symphyses of these mandibles extend to the trigonid of p4. The lower incisor row of IVPP V3254-1 is nearly intact and is not differentiated from Rhinotitan kaiseni . The incisors are large, tall, semispatulate, and form a slightly arched row anterior to the canines. The i2 is distinctly the largest incisor. Each of the incisors has a small lingual cingulid. The canines of that particular specimen are relatively large. The lower cheek teeth of R. andrewsi are not clearly differentiated from R. kaiseni . Both specimens lack evidence of a p3 metaconid. They fall within the morphological range of the specimens described below as Rhinotitan sp.