Microtitan mongoliensis ( 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 : 59-68

Microtitan mongoliensis ( Osborn, 1925)

NEOTYPE: AMNH 22099 , a partial mandible with right p1 alveolus and p2–m3.

TYPE LOCALITY: Ulan Shireh Formation, eight miles north of Tukhum Lamasery, Inner Mongolia, China.

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

REFERRED SPECIMEN: (From the same locality as the neotype) AMNH 21611, a left maxilla with C–M3.

DIAGNOSIS: Microtitan mongoliensis is one of several relatively small Asian brontotheres. The dorsal surface of the maxilla rises steeply posteriorly indicating that the rostrum lacks the specializations seen in Metarhinus or Dolichorhinus . Dentally, Microtitan mongoliensis is characterized by a simple P1, a distinct P2 metacone, exceedingly weak labial paracone ribs, and weak premolar preprotocristae. M. mongoliensis premolars lack lingual crests (although this trait could be intraspecifically variable). Premolar hypocones are absent. The molars of M. mongoliensis 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. M. mongoliensis molars lack paraconules or metalophs. Central molar fossae and anterolingual cingular cusps are absent. Lower dental characters of M. mongoliensis include the absence of a p1– p2 diastema, an elongate p2 trigonid, a large metaconid on p4 but not on p2 and p3, shallow molar basins, rounded molar crescents, and a very elongate m3.

Microtitan mongoliensis can be readily differentiated from the vast majority of brontotheres on the basis of size, although it is within the size range of Metarhinus . Microtitan can be readily differentiated from Metarhinus by the ascending dorsal surface of the rostrum and lack of central molar fossae.

DESCRIPTION

SKULL: Microtitan mongoliensis is much smaller than the majority of brontotheres, yet it is distinctly larger than the three smallest species, Pygmaetitan panxianensis , Acrotitan ulanshirehensis , and Nanotitanops shanghuangensis . However, Microtitan mongoliensis is within the size range of Metarhinus . The best cranial fragment referred to M. mongoliensis is AMNH 21611, a left maxilla with a canine and a complete cheektooth series (fig. 21). Although the premaxilla of that specimen is not preserved, the remaining contact surface for the premaxilla can be seen on the inner side of the maxilla above the root of the canine. This surface ends above the P2. A small intact portion of the ventral edge of the nasal incision is visible on the maxilla above the P2. Although the nasal incision extended at least to this point, the specimen does not reveal the position of the posterior margin of the nasal incision. The anterodorsal surface of the preorbital portion of the maxilla rises posteriorly at a steep angle. This indicates that the premaxillomaxillary rostrum lacked the specializations seen in Metarhinus , in which the rostrum does not deepen posteriorly. The exact position of the orbit is indeterminate, although the surface of the orbital floor (formed by the maxilla) is intact and positioned above the M2 and M3. The orbit would have been positioned directly above the M2 or slightly behind it.

UPPER DENTITION: The canine of AMNH 21611 is of moderate size, is slightly elliptical in cross section, and has no distinct cingulum. The length of the postcanine diastema is similar to the length of the P2. The P1 is a simple tooth with a single cusp and an elongate posterior heel (fig. 21c). Most of the enamel of P1 on the labial side of the cusp and on the posterior and lingual sides of the posterior heel is missing. Despite this damage, it is clear that P1 was a very narrow tooth. The P2 and P3 are obliquely shaped due to their posterolingually angled anterior margins, while P4 is more rectangular. The parastyle of P2 is arched lingually, while the parastyle of P3 is nearly straight, and the P4 parastyle is slightly angled labially. The P2 metastyle is angled slightly lingually while those of P3 and P4 are straight. Labial paracone ribs can be seen on the P3 and P4 but they are weak. The metacone of P2 is strongly shifted lingually while those of P3 and P4 are positioned directly behind the protocone. Because of these differences the ectoloph of P2 is more rounded than the ectolophs of P3 and P4. The lingual heels of P2–P3 have large protocones. Premolar hypocones are absent. A small preprotocrista can be seen on the P2. In P3, there is a faint preprotocrista, while P4 lacks this feature completely. None of the premolars exhibits a lingual crest. The labial premolar cingula are extremely weak. The anterior and posterior premolar cingula arch around the lingual sides of the crowns, but they do not join lingually to form continuous lingual cingula.

The upper molars of Microtitan mongoliensis exhibit 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. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. Paraconules and metalophs are completely absent. M. mongoliensis molars lack both anterolingual cingular cusps and central molar fossae. The M3 of AMNH 21611 has a well-developed hypocone that is somewhat small- er than the hypocones of M1 and M2. Lingual and labial molar cingula are essentially absent. One peculiar aspect of the molars of this specimen is the deep notch formed at the labial side of the paracone of each molar.

MANDIBLE AND LOWER DENTITION: The neotype, AMNH 22099, is a partial right ramus with a p1 alveolus (p1 missing) and complete p2–m3 (fig. 22). The partial ramus is reconstructed from fragments with plaster filling a significant gap on the ventral border of the ramus. The dorsal view of the partial ramus suggests that the symphysis extended to the talonid of p2. There is no diastema between the p1 alveolus and the p2. The premolars are slender, particularly p2. The trigonid of p2 is elongate and at least twice as long as the talonid. The p3 trigonid is much longer than the talonid as well. Finally, the p4 trigonid and talonid are of similar length. The trigonids of all the premolars are somewhat narrower than their talonids. The paralophid of p2 curves slightly lingually, although there is almost no lingual trigonid notch. The p2 protolophid is lingually positioned but directed posteriorly. The p3 paralophid is strongly angled lingually, creating a more distinct lingual trigonid notch. The p3 protolophid is straight, but it is angled slightly lingually. The paralophid and protolophid of the p4 are more nearly molariform and arch fully lingually. Among the premolars, only the p4 exhibits a large, lingually positioned metaconid. However, a small metaconid-like feature can be seen at the junction of the protolophid and cristid obliqua on the p3. Give the rudimentary nature of this structure, it is possible that a p3 metaconid was variably present. The talonid of p2 has a short but well-developed cristid obliqua and a very short hypolophid. The lingual surface of the talonid is a sloped, slightly convex surface. On the other hand, the talonids of p3 and p4 are more nearly molariform with longer cristids obliqua, longer hypolophids, and much broader basins. Labial and lingual premolar cingulids are absent.

The lower molars of AMNH 22099 have relatively thin enamel, shallow talonid and trigonid basins, and very weak lingual ribs. The m3 is one of the most elongate among brontotheres. The molars of AMNH 22099 have unusually rounded crescents, as noted by Granger and Gregory (1943). The hypoconulids of m1 and m2, though small in all brontotheres, are quite pronounced in AMNH 22099. Labial molar cingulids are exceedingly faint, and lingual molar cingulids are absent. The m3 of AMNH 22099 has a groove near the labial base of its crown that is probably a hypoplasia.

REMARKS

Osborn (1925, 1929) described a new species, ‘‘ Metarhinus ?’’ mongoliensis , from a mandible fragment with a p4 and m1 (AMNH 20167) from the Irdin Manha Formation of Inner Mongolia (fig. 23). Despite the fragmentary nature of this specimen, it is clearly much smaller than any species of Asian brontothere that was known at the time. However, the size of AMNH 20167 is within the reach of the

(B) dorsal view, (C) right p2, p3, and p4.

North American species Metarhinus fluviatilis . Therefore, Osborn (1925) questionably referred this new species to Metarhinus . Better material was available to Granger and Gregory (1943) and they erected a new genus, Microtitan , for this species.

Because the original holotype of Microtitan mongoliensis lacked any diagnostic feature other than size, Granger and Gregory (1943) designated a neotype, AMNH 22099, a partial right jaw with p2–m3. Due to the fragmentary nature of the holotype of Microtitan mongoliensis , a reconsideration of these specimens is warranted to determine which, if any, can be realistically referred to M. mongoliensis , or whether that species should even be considered valid. The holotype of M. mongoliensis (AMNH 20167) is a small fragment of mandible with right p4 and m1 (fig. 23a, b). Overall, it is a rather unremarkable fossil, but several peculiarities about AMNH 20167 suggest that it is actually made up of multiple individuals. For instance, AMNH 20167 includes a barely worn DP4 that is obviously from a very young individual (fig. 23c). On the other hand, the mandible fragment of AMNH 20167 contains an erupted p4 and m1. The m1 shows a significant amount of wear. The lower dentition obviously represents an older individual than the nearly unworn deciduous premolar. More evidence suggests that the mandible fragment itself is a composite specimen. The lower teeth are disproportionately small in comparison to the actual mandible fragment. Closer inspection reveals that the dorsal surface of the mandible fragment is actually a weathered surface. The true alveolar surface has been weathered or broken away. Because the teeth are resting directly upon the weathered surface of bone, the conclusion that the teeth are not in situ is inescapable. Various adhesive materials and plaster can be traced around the borders of the p4 and m1. Therefore, the mandible fragment is probably from a larger individual than those represented by the actual teeth. The two lower teeth are more consistent with a single individual in size and in their degree of wear. Therefore, the m1 and p4 are possibly from the same individual, but this is not certain. In terms of size, the teeth of AMNH 20167 are similar to the North American brontothere Metarhinus fluviatilis , but because the specimen is too fragmentary to readily distinguish it from this taxon, Microtitan mongoliensis could be considered a nomen dubium.

Granger’s and Gregory’s (1943) neotype (AMNH 22099) is a more sufficiently diagnostic specimen, particularly due to the relatively slender p2 and p3 with elongate trigonids, the rather rounded molar crescents, and the hyperelongate m3. I recommend that AMNH 22099 continue to be recognized as the neotype specimen for Microtitan . The alternative to designating a neotype, rejecting Microtitan outright, is not recommended. At present, it is well understood that Microtitan represents a small Irdinmanhan aged brontothere that, until this paper, was most adequately diagnosed and described by Granger and Gregory (1943).

The maxilla and upper cheektooth series (AMNH 21611) is not directly referable to Microtitan mongoliensis (due to a lack of associated skulls and jaws) but there is little doubt that it represents that same species because of its occurrence in the same locality, its nearly identical size, and relatively elongate M3. Therefore, I continue to include AMNH 21611 in M. mongoliensis .

No other specimens reported since Grang- er and Gregory (1943) can be assigned to Microtitan . The mandible (PIN 3107-25) assigned to Microtitan by Yanovskaya (1980) from the Khaichin Formation does not actually belong to a brontothere. ‘‘ Microtitan ?’’ elongatus Qi (1987) is presently a nomen dubium, although it is possibly a synonym of M. mongoliensis . Dental fragments from the southern Jiangsu Province of China referred to Microtitan sp. by Qi and Beard (1996) (herein referred to cf. Metarhinus sp. ) belong to an unnamed new species of Microtitan -sized brontothere.

Fossendorhinus diploconus (Osborn, 1895) new genus

HOLOTYPE: AMNH 1863 , a partial skull with right C–P1 (roots only), P2 (partial), P3–M3, left P1–P2 (roots only), and P3–M3.

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

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

ETYMOLOGY: Fossendorhinus combines the Latin term fossa (‘‘ditch’’) with Greek terms, endo (‘‘inside’’) and rhinus (‘‘nose’’). This combination refers to the internal fossae seen within the nasal cavity of this species.

DIAGNOSIS: Fossendorhinus diploconus is an intermediate-sized hornless brontothere. The nasal incision extends posteriorly as far back as the anterior margin of the M1. The orbits are positioned above the M2 and protrude laterally, though not to the degree seen in Metarhinus . The premaxilla is robust and does not contact the nasal bone. The premaxillomaxillary rostral cavity is open dorsally and there are two large fossae inside the nasal chamber. The premaxillomaxillary rostrum is strongly upturned and is relatively constant in thickness throughout its length. Other cranial features include a well-developed sagittal crest, a strongly concave midcranial dorsal surface, a strongly convex posterior dorsal surface, thin and strongly curved zygomatic arches, and a ventrally open and mediolaterally directed external auditory pseudomeatus.

Dentally, Fossendorhinus diploconus has three large upper incisors, a distinct P2 metacone, weak premolar preprotocristae, short lingual crests extending posteriorly from the premolar protocones, and small hypocones on P2 and P3. 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 and an anterolingual cingular cusp are absent. Central molar fossae are present. All traces of molar paraconules and metalophs are lost.

Fossendorhinus diploconus is most similar to Metarhinus , but can be clearly differentiated from Metarhinus by the autapomorphic fossae inside the nasal chamber, the less laterally protruding orbits, and the more strongly upturned rostrum.

DESCRIPTION

SKULL: The holotype of Fossendorhinus diploconus (AMNH 1863) is a skull lacking the nasal process (fig. 24). The skull has been subjected to a minor amount of sheering distortion. Prior figures of the specimen in Osborn (1895: fig. 6) and in Osborn (1929a: figs. 362 and 363) misleadingly portray the skull as undistorted, and less damaged than it actually is. The shape of the right side of the skull is well preserved, except for a significant portion of the squamosal, which has been reconstructed with plaster. The left side of the skull is mostly complete, but it has been more severely crushed and the zygomatic arch has been forced inward.

There is no hornlike protuberance on this specimen. Osborn’s (1929a: fig. 363) figure of this specimen includes an unlabeled line that seems to portray the frontonasal suture. However, the line drawn in Osborn’s figure actually corresponds to a large crack in the specimen, not a suture. There is no discernable frontonasal suture in this specimen.

The nasal incision of Fossendorhinus diploconus strongly constricts the face, but not to the degree seen in Metarhinus . The nasal incision extends as far back as the anterior margin of the M1. The orbit is positioned directly over the M2. The posterolateral root of M1 is positioned below the anterior rim of the orbit. The right orbit protrudes somewhat laterally from the skull, but not to the degree seen in Metarhinus . The proximal base of the nasal process is preserved on the right side. This remnant suggests a rather thin nasal process that was nearly flat or had very shallow lateral walls, similar to those of Metarhinus .

The most distinctive characteristics of Fossendorhinus diploconus are in the premaxillomaxillary rostrum and the nasal chamber. From the lateral view of the skull it can be seen that the premaxillomaxillary rostrum is a consistent depth from the proximal end to the distal end. The dorsal margin of the rostrum does not rise above the midlevel of the orbit. In comparison to Metarhinus , the premaxillomaxillary rostrum is more strongly curved upward. The pronounced upward curvature of the rostrum does not appear to be a result of taphonomic distortion. On the right side there are a few large cracks in the face, but the proportions appear to be essentially intact.

The premaxillae have become detached at the symphysis and the left premaxilla has been displaced laterally, ventrally, and anteriorly. A roughened groove of bone on the right side, probably representing the premaxillomaxillary suture suggests that the premaxilla did not extend to the posterior base of the nasal incision. The median symphyseal contact surface of the right premaxilla is flat and long (,7.1 centimeters), although Osborn (1895) described it as short. The dorsal surface of the rostrum is not covered by bone as seen in Metarhinus or Dolichorhinus . However, the rostrum of F. diploconus shows

(C) dorsal view.

a number of autapomorphic specializations (fig. 25a, b). The premaxillary symphysis arches dorsally. Behind the symphysis are two large internal ovoid fossae. The internal fossae are recessed below the dorsal surface of the rostrum and extend well behind the nasal incision. An additional smaller fossa appears in the upper corner, just beneath what was the proximal base of the nasal process. This small fossa is intact on the right side, but only the very bottom of the fossa is preserved on the left side. The bone that forms these internal fossae fills a large portion of the volume of the nasal chamber. Consequently, the internal nasal cavity is very narrow.

From a lateral view the midsection of the dorsal surface of the skull is strongly concave. Although two large cracks that run through the right side of the frontal area may exaggerate the concave superorbital profile, the individual bone fragments on this surface are concave as well. The dorsal surface of the posterior portion of the cranium is convex in lateral profile. The parasagittal ridges converge medially into a long sagittal crest.

The zygomatic arch is thin and bladelike in cross section. The right zygomatic is less damaged. It is only slightly bowed laterally. The jugal section of the zygomatic is dorsoventrally shallow and is more or less horizontal while the squamosal portion is deeper and angled posterodorsally, giving the zygomatic arch a moderately curved shape. Although Osborn (1908a) described this specimen as lacking an infraorbital jugal process, as seen in Metarhinus , the surface of the jugal is flaked off on the ventral surface of the inferior rim of the orbit. This damage seems superficial and it is doubtful that this specimen possessed a large infraorbital process, though it is possible that a small infraorbital process was present.

From the dorsal view, the nuchal crest is very narrow and swept backward, although this appears to be exaggerated by lateral crushing. From the posterior view (fig. 25c), the nuchal crest is strongly arched dorsally. The overall proportions of the occiput are distorted, although the occiput is narrower dorsally than it is ventrally, and it is slightly waisted. There are small but distinct occipital pillars, and a median depression in the occiput between the occipital pillars.

The ventral surface of the skull is poorly preserved, but several features are discernable (fig. 26a). Although the palate has been crushed laterally, the right side of the posterior nares is preserved and is positioned slightly anterior to the M3. The posterior narial canal appears to have been elongate, but it is severely damaged. Finally, the

(C) right premolars.

basicranium, though badly damaged, is typical with a foramen ovale well separated from the foramen lacerum. The opening for the external auditory pseudomeatus, preserved on the left side, is wide and unconstricted ventrally.

UPPER DENTITION: No incisors or canines are preserved with AMNH 1863, however there are clearly three pairs of incisor alveoli. The size of the alveoli and shape of the premaxillae suggest relatively large incisors and an arched incisor row. The canine alveoli suggest relatively small canines that are comparable in size to those of Dolichorhinus and Metarhinus . There is both a short precanine diastema and postcanine diastemata.

The P1 is not preserved but the remnants of the roots indicate a small, double-rooted premolar (fig. 26c). The ectoloph of P2 is not preserved although the shape of the P2 appears to have been similar to those of P3 and P4. The parastyle and metastyle of P3 are relatively straight. The parastyle of P4 is labially angled, although the P4 metastyle is relatively straight. Prominent labial paracone ribs can be seen on P3 and P4; the P4 labial paracone rib is smaller. There is a large bulge at the proximal base of the P4 ectoloph near the metacone. This bulge is similar in position to the mesostyles to one that is occasionally seen on the P4s of some brontothere species. Each premolar (P2–P4) has a large centrally positioned protocone. Additional lingual features in the P2 include a rudimentary preprotocrista with a distinct paraconule and a distinct crest extending posteriorly from the protocone that is connected to a very small hypocone-like swelling. The lingual side of P3 is morphologically similar to that of P2. However, the preprotocrista, the lingual crest extending posteriorly from the protocone, and the hypocone are less distinct. Finally, the lingual side of the P4 crown is devoid of any features except for the large protocone. The labial premolar cingula of P3–P4 are very thin. The anterior and posterior premolar cingula wrap around the lingual side of the crown but do not join lingually.

The molars of Fossendorhinus diploconus exhibit numerous 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) (fig. 26b). The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. F. diploconus molars lack anterolingual cingular cusps but possess distinct central molar fossae. All traces of paraconules and metalophs are absent. The trivial name ( diploconus ) alludes to the two lingual cusps of the M3, a protocone, and a large hypocone that is similar in size to the hypocones of the more anterior molars. However, the size and/or presence of a M3 hypocone could vary in this species as it does in many other brontothere species. The labial molar cingula are distinct but weak and they are discontinuous around the mesostyles. The lingual molar cingula are distinct between the protocone and hypocone, but they are discontinuous around the protocone and hypocone.

REMARKS

Fossendorhinus diploconus (Osborn, 1895) is based upon a single skull (AMNH 1863) from the Uinta Basin (Uinta B). Osborn (1895) originally referred this species to the genus ‘‘ Telmatotherium ’’ (a variant on the spelling of Telmatherium ), but he subsequently reassigned it to the genus Metarhinus ( Osborn, 1908a) . Osborn (1908a) differentiated this species from other species of Metarhinus based on the lack of an infraorbital process. However, AMNH 1863 is damaged in the area where the infraorbital process would occur if it were present. It is unclear whether a small infraorbital process was present on the holotype skull.

Riggs (1912) and Osborn (1929a) reassigned ‘‘Metarhinus’ ’ diploconus to another genus, Rhadinorhinus (now considered a synonym of Metarhinus ). More recently, Mader (1998) reassigned this species to the genus Metarhinus and synonomized it with Metarhinus abbotti ( Riggs, 1912) . The repeat- ed revisions and associations of this species with Metarhinus are, nonetheless, all contradicted by the distinct characteristics of AMNH 1863, which seem to clearly differentiate it from other species of Metarhinus ( M. fluviatilis and M. abbotti ). These distinctions include the less prominently protruding orbits, the more strongly upturned rostrum, the more deeply concave dorsal midcranial surface, and the small premolar hypocones. More compellingly, the autapomorphic features found in the rostrum of AMNH 1863 are distinctly different from Metarhinus . Recent removal of the hard sandstone matrix from the nasal chamber of the holotype (AMNH 1863) revealed a pair of large fossae in the floor of the rostrum and inside the nasal chamber. These peculiar nasal fossae, not known to prior authors, warrant the designation to the new genus.

Currently, AMNH 1863 is the only specimen that clearly represents Fossendorhinus diploconus . However, AMNH 2055, a poorly preserved and incompletely prepared skull probably represents this species, but the diagnostic morphology of the rostrum is obscured by sandstone matrix.