Rhinotitan kaiseni ( 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 : 167-174

Rhinotitan kaiseni ( Osborn, 1925)

HOLOTYPE: AMNH 20252 , a skull and mandible with extremely worn teeth.

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

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

REFERRED SPECIMENS: (From the Shara Murun Formation, Ula Usu, Baron Sog Mesa, Inner Mongolia) AMNH 20257, a ventral surface of a skull with a complete set of upper dentition; FMNH P14048 (formerly AMNH 20260), a skull and mandible with complete sets of upper and lower dentition.

DIAGNOSIS: Rhinotitan kaiseni is a large brontothere with small, elliptical frontonasal horns. The horns are positioned far in front of and high above the orbits. The nasal incision is dorsoventrally deep and extends posteriorly to P3. The orbit is positioned above M2 with posterolateral and anterolateral roots of M1 below the anterior orbital rim. The nasal process is unelevated, slightly angled upward, relatively narrow, not strongly rounded anteriorly, and with deep lateral walls that arch around the anterior end of the nasal process. Proximally, the lateral walls of the nasal process deepen and angle ventromedially. The premaxillomaxillary rostrum thickens posteriorly and it is not enclosed by bone dorsally. Other cranial characteristics include an 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 kaiseni has three large but short subcaniniform upper incisors. The P1 crown is complex and there is a distinct P2 metacone. Premolar hypocones are present although the protocones and hypocones of P2 and P3 are sometimes fused into a single lingual crest. The molars of R. kaiseni 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. kaiseni includes three large incisors. The i1 and i2 are semispatulate, while the i3 is more subcaniniform. There is a distinct postcanine diastema and an occasional small p3 metaconid. The p2 trigonid is nearly twice the length of the talonid. The lower molars have shallow basins and the m3 is slender.

Rhinotitan kaiseni can be distinguished from most other horned brontotheres by the combination of large incisors and an incompletely saddle-shaped cranium. R. andrewsi shares these traits, but R. kaiseni can be further distinguished from R. andrewsi by the narrower basicranium, more anterior position of the frontonasal horns, downfolded anterior margin of the nasal process, and subcaniniform upper incisors.

DESCRIPTION

SKULL: The holotype of Rhinotitan kaiseni

(AMNH 20252) includes a complete skull

(fig. 79) and an associated mandible. The skull is complete, but it has experienced some shearing distortion and the dorsal surface of the skull is warped. Additionally, there is another complete skull (fig. 80) and associated mandible (fig. 82) (FMNH P14048: originally AMNH 20260). The skull consists of hundreds of small bone fragments held together with plaster, yet the specimen retains its general shape although its overall size appears to be swollen due to expanding matrix distortion (sensu White, 2003). A third specimen (AMNH 20257) consists of an undistorted ventral surface of a skull (fig. 81a).

Rhinotitan kaiseni is a large horned brontothere similar in size to Protitan grangeri and Protitanotherium emarginatum . There are no discernable sutures on the holotype skull (AMNH 20252), but judging by the extreme dental wear this animal died at a very old age. FMNH P14048 is an ontogenetically younger individual with minimally worn adult dentition. On that specimen the posterodorsal surfaces of the right and left horns are less damaged than the surrounding bone. Portions of the frontonasal suture are visible on both the lateral and dorsal views, indicating that the horn is composed of both frontal and nasal elements.

The horns of AMNH 20252 are small but prominent and project almost horizontally from the skull. They are proximally elliptical, but the distal peaks of the horns form round roughened knobs. In FMNH P14048 the horns do not project laterally. In both skulls, the horns are positioned far anterior to the orbits. This differs from Rhinotitan andrewsi where the horns are more posteriorly positioned. The horns of R. kaiseni are positioned well out onto the nasal process and are high above the orbits.

The nasal incision extends posteriorly to a point above the anterior margin of the P3. The nasal incision is dorsoventrally deep. The posterior notch of the nasal incision is positioned slightly higher than the orbit. The orbit is above M2 while the posterolateral and anterolateral roots of M1 rest below the anterior orbital rim.

From the lateral view the nasal process is slightly longer than the premaxillomaxillary rostrum. The nasal process of AMNH 20252 appears to be angled slightly upward. The sides of the nasal process form deep lateral walls; these are deeper and thinner than those of R. andrewsi . The lateral walls are shallower toward the distal end of the nasal process. From the dorsal view, the width of the nasal process is relatively constant throughout its length and the anterior margin is nearly flat. From the anterior view the distal end of the nasal process is downfolded. Toward the proximal end of the nasal process the lateral walls are weakly angled ventromedially and they constrict the space between them, but this condition is not as severe in R. kaiseni as it is in R. andrewsi .

From a lateral view the premaxillomaxillary rostrum is thin distally, but it deepens proximally. The dorsal margin of the rostrum is steeply sloped posterodorsally. From the anterior view the premaxillary symphysis is completely fused. The dorsolateral margins of the rostrum diverge laterally behind the symphysis and the rostral cavity is not sealed over by bone. From the anterior view the nasal opening of the skull is very tall and narrow.

The dorsal surface of the skull is artificially warped. However, unlike many horned brontotheres, the skull does not appear to have been saddle-shaped. Rather, the shape of the skull in lateral profile more closely resembles Rhinotitan andrewsi and more primitive hornless brontotheres (e.g., Telmatherium ) in which the dorsal surface is concave over the middle of the skull, but the dorsal surface of the skull is more convex over the posterior end of the skull.

The parasagittal ridges are not prominent on AMNH 20252 and the dorsal surface of the skull is not laterally constricted posteriorly. From a lateral view, the zygomatic arches are relatively shallow. The jugal portion of the zygomatic is horizontal, while the squamosal portion rises posteriorly at a very shallow angle and the zygomatic arch is nearly straight. From the dorsal view, the zygomatic arches are thin, straight, and not strongly angled laterally. A postzygomatic process, as seen in Protitan grangeri and Metatitan relictus , is absent in Rhinotitan kaiseni .

From a lateral view the occiput is nearly vertical. The nuchal crest is damaged, but from a dorsal view, it appears to have been concave. From the posterior view, the nuchal crest is dorsally arched. The dorsal and ventral halves of the occiput are the same width, although the occiput seems to have been moderately constricted in the middle. The surface of the occiput has weak occipital pillars with a shallow pit in the center.

In the holotype specimen ( AMNH 20252 ) the anterior rim of the posterior nares is positioned somewhere behind M2, although its exact position is obscured by plaster. In AMNH 20257 the anterior edge of the posterior nares is slightly anterior to the M3 protocones. The posterior nares are rimmed by a narrow emargination. The vomerine septum seen in AMNH 20257 bisecting the posterior narial canal is mostly made of plaster except at its posteriormost end. There are no sphenoidal fossae in any of the specimens of R. kaiseni . The external auditory pseudomeatus is tube-shaped and enters the skull at a strongly posteromedial angle. The configuration of the basicranial foramina is typical, with widely separated foramen ovale and foramen lacerum. The width of the basicranium, measured at the mastoid processes, is slightly narrower than the width across the M3s ; this distinctly differs from skulls of Rhinotitan andrewsi whose basicranium is wider that the total width across the M3s.

UPPER DENTITION: The teeth of the holotype skull are too worn to describe, but the upper dentitions of AMNH 20257 and FMNH P14048 are complete and not heavily worn (fig. 81). The incisor rows of AMNH 20257 and FMNH P14048 are complete, although those of the latter are the least worn. The incisors number three per side, are large, and form an arched row that extends anterior to the canines. The incisors increase in size laterally. The incisors of Rhinotitan kaiseni are less globular than those of R. andrewsi . When the I1 and I2 are nearly unworn, as in FMNH P14048, the crowns retain a relatively subcaniniform shape, although they are very short and have smooth lingual cingula. The I3 has a similar shape, but it is taller and more subcaniniform. The canines of all R. kaiseni specimens tend to be relatively small. There is a short precanine diastema and a longer postcanine diastema.

The P1 crown of AMNH 20257 is not preserved. The P1 of FMNH P14048 is much smaller than more posterior molars, but its morphology is relatively advanced and the crown has a nearly round outline. A metacone is distinctly present and is similar in size to the paracone. There is a small lingual heel with a small protocone. In both specimens the crown of P2 seems more obliquely shaped than the crowns of P3 or P4 due to its more posterolingually angled anterior margin. The P3 and P4 are less oblique in shape and the anterior and posterior margins of these teeth are nearly parallel. The P2 metacone is not shifted lingually. The parastyle and metastyle of P2 are straight. The parastyles of P3 and P4 are directed somewhat labially, although the metastyles of these teeth are essentially straight. The labial margin of the P2 paracone is rounded, while the paracones of P3 and P4 have small but distinct labial ribs.

The lingual features of the crowns of P2– P4 have low relief and invariably include a protocone and a lingual crest that extends posteriorly from the protocone. Vestigial preprotocrista are seen on P2 and P3, but this trait is not evident on P4. In other respects the lingual premolar morphology is variable. For instance, there is notable bilateral asymmetry in the presence and distinctness of premolar hypocones in AMNH 20257. The left P2 has a hypocone that is incompletely separated from the protocone, while the P3 and P4 each have small, well-separated hypocones. The right P2 is similar to the left side, but unlike their left counterparts, the right P3 and P4 lack distinct hypocones. The premolars of FMNH P14048 are more bilaterally symmetrical; P2 has a distinct hypocone that is connected to the protocone, but P3 and P4 lack hypocones. The lingual premolar cingula are also variable. The lingual premolar cingula of AMNH 20257 are discontinuous, while the lingual premolar cingula of FMNH P14048 are continuous on the P2 and P3 but discontinuous on the P4. Likewise, the labial premolar cingula of FMNH P14048 tend to be weaker than those of AMNH 20257.

The upper molars of Rhinotitan kaiseni show typical brontotheriine apomorphies, 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 shallow central molar fossae are present, but anterolingual cingular cusps are absent. All evidence of paraconules and metalophs is lost. There is no trace of a hypocone on any M3 of R. kaiseni , although the distolingual cingulum of the M3 is thickened. Labial molar cingula are weak and lingual molar cingula are absent.

MANDIBLE AND LOWER DENTITION: Although the holotype (AMNH 20252) includes an associated mandible, the lower teeth are extremely worn. However, a second skull, FMNH P14048, is associated with a complete and essentially undistorted mandible with a complete set of lightly worn dentition (fig. 82). The inferior margin of the symphysis has a shallow angle (, 45 °). The symphysis extends to the talonid of the p3. The incisors number three per side and form an arch anterior to the canines. The lower incisors are large, while i2 is clearly the largest incisor. The crowns of i1 and i2 are tall and semispatulate with rounded apices. The i3 crown is shorter and mesiodistally more elongate. There are slight lingual incisor cingulids and no labial incisor cingulids. There are no diastemata between the incisors or the canines. The canines are rather small. The postcanine diastema of FMNH P14148 is similar in length to p2, although that of the holotype is slightly longer.

A left p1 was originally associated with FMNH P14048, but it has been lost. Fortunately, the left p1 is still present on a cast of the jaw (AMNH 20260). The p1 cast is shown in proportion to the original specimen in fig. 82a and 82c. The p1 is a small narrow tooth with a single cusp and a short talonid heel. The p2 trigonid is less than twice the length of the talonid, while the trigonid and talonid have similar widths. The talonid and trigonid of p3 and p4 are about equal in length, but the talonid is distinctly wider than the trigonid. The paralophid of p2 arches slightly lingually, creating a small lingual trigonid notch. The p2 protolophid is straight but lingually positioned. The p2 lacks a metaconid. The trigonid of the p3 is more molariform with a strongly lingually paralophid and metalophid that create a much broader lingual-trigonid notch. In addition, p3 has a small but distinct metaconid. The trigonid of p4 is essentially molariform with a fully lingually arched paralophid and protolophid, and a large lingually positioned metaconid. The talonid of p2 has a relatively short cristid obliqua and hypolophid; these features create a small lingual-talonid notch. The cristids obliqua and hypolophids of p3 and p4 are longer and the talonid basins are more molariform. There are no lingual cingulids on the premolars and the labial cingulids are weak.

The lower molars of Rhinotitan kaiseni are typical. They have relatively thin lingual enamel, shallow trigonid and talonid basins, and the m3 is elongate. There are no lingual cingulids. Labial molar cingulids are weak and they tend to be discontinuous around the labial cusps. A cingulid is not seen around the hypoconulid of the m3.