Rhinotitan sp.

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 : 182-185

Rhinotitan sp. Includes the nomen dubium, Rhinotitan mongoliensis ( Osborn, 1923)

REFERRED SPECIMENS: (From the Shara Murun Formation Ula Usu, Baron Sog Mesa, Shara Murun Region , Inner Mongolia) AMNH 18653 (holotype of Rhinotitan mongoliensis ), a mandible fragment with right p2–m3 ; AMNH 20251 , a left mandibular ramus with p2–m3 ; AMNH 20256 , a palate with right C–M2, left P1–M2, M3 (unerupt- ed), a mandible with right i1–i2, c (erupting), p1, p3–m2, m3 (erupting), left i1–i2, p1–m2, and m3 (erupting) ; AMNH 20262 , a left mandibular ramus with p1–m2, m3 (erupting), four isolated incisors and a canine ; AMNH 20269 , a partial mandibular symphysis with right and left i1–i2, isolated canine and right p1 ; AMNH 20270 , a right maxilla with P1–M2 ; AMNH 20272 , mandible fragment with right i2?, c, p3–m3, and a left ramus with i2?, c, and p2–m3 ; AMNH 20273 , right and left mandibular rami with p2–m3 ; AMNH 20280 , a fragmented skull with right P2–P3, dp4, P4 (unerupted), M1, M2 unerupted, left P1–P3, dp4, P4 (unerupt- ed), and M1 ; AMNH 21598 , a palate with right and left P1–P3, dp4, P4 (unerupted), and M1–M2; (from the Shara Murun Formation, four miles north of Baron Sog Lamasery, Baron Sog Mesa , Shara Murun Region , Inner Mongolia) AMNH 21603 , a partial mandible with right i2–p4 and left i1–m3 ; AMNH 21605 , a partial mandible with poorly preserved incisors and right c, p2 (partial), and p3–m3 ; PIN 2198-2 , a mandible with right and left p2–m3 .

DESCRIPTION

UPPER DENTITION: Many specimens from the Shara Murun Formation with well-preserved upper and/or lower sets of cheek teeth clearly belong to a species of Rhinotitan , but because the two known species, R. kaiseni and R. andrewsi , do not have clearly differentiated cheek teeth, the majority of these specimens can be assigned only to the genus level (i.e., Rhinotitan sp. ) Four palates with essentially unworn upper cheek teeth (AMNH 20256, AMNH 20270, AMNH 20280, AMNH 21598) demonstrate a degree of variability in the lingual features of the premolars (P2–P4) similar to both R. kaiseni and R. andrewsi . The lingual side of the P2 varies from having a single crest with no distinct cusps to having a slightly discernable protocone within the lingual crest. P3 varies in similar way, but occasionally there is a very small hypocone present. Finally, P4 has a large protocone, a very low crest is occasionally present, and a tiny hypocone is sometimes present.

MANDIBLE AND LOWER DENTITION:

The mandibles referable to Rhinotitan sp. demonstrate variability in the presence of the p3 metaconid. In the majority of these specimens the p3 metaconid is absent. However, AMNH 18653 (holotype of Rhinotitan mongoliensis ), AMNH 20251, and PIN 2198-2 exhibit small metaconids on the p3. The metaconid is smaller and not as lingually positioned as that of p4 or those of the molars. The canines of these mandibles are highly variable in size as well, with some specimens having large curved crowns and bulbous roots (AMNH 20269, 20272) and others with smaller less curved crowns and less bulbous roots (AMNH 21605).

REMARKS

The first brontothere described from the Central Asiatic Expeditions of the American Museum of Natural History is Protitanotherium mongoliensis Osborn, 1923 . Osborn (1923, 1925, 1929a) based this species on a partial lower cheektooth row (AMNH 18653). In 1925 Osborn named two more species. One of these, P. andrewsi , was based on a large skull (AMNH 20271) lacking the horns and nasal region. No mandible or lower teeth were associated with this skull but Osborn conjecturally assigned another mandible, AMNH 20251, as the paratype of P. andrewsi . Osborn then distinguished P. mongoliensis from P. andrewsi based on ‘‘progressive mutations and rectigradations warranting a specific separation’’ ( Osborn, 1925: 6), but he did not indicate specifically what the distinguishing characters were.

The third species, Dolichorhinus kaiseni Osborn (1925) , was based on an associated skull and mandible (AMNH 20252). Osborn assigned this species to Dolichorhinus because its elongate cranium and long nasal bones seemed similar to that of the North American genus Dolichorhinus . However, Osborn (1925) noted that it differed from North American Dolichorhinus primarily in its larger size and more distinct horns. Osborn (1925) did not compare D. kaiseni with Protitanotherium mongoliensis or P. andrewsi . The teeth of the holotype of D. kaiseni are so worn that any comparison to the dentition of other brontotheres is not possible. However, another specimen (FMNH P14048; formerly AMNH 20260) assigned to Dolichorhinus kaiseni by Osborn (1925) includes a nearly complete and minimally worn set of upper and lower teeth that Osborn could have compared to P. mongoliensis and P. andrewsi .

Granger and Gregory (1943) were the first to recognize the similarities among the numerous specimens that were referred Protitanotherium mongoliensis , R. andrewsi , and Dolichorhinus kaiseni . These were assigned to a new genus, Rhinotitan , to which R. kaiseni was considered the type species. Rhinotitan was characterized by Granger and Gregory (1943) as having long upturned nasals, small oval horns, a wide occiput, premolars with ‘‘tetartocone ridges’’, and a metaconid variably present on p3. Granger and Gregory (1943) continued to accept all three species, but unfortunately they provided dubious species diagnoses. Rhinotitan kaiseni was diagnosed by its smaller dimensions and relatively wider P4. Even more dubious is their distinction of R. mongoliensis and R. andrewsi . The specific character of R. mongoliensis , ‘‘P2–P 4 in neotype with tetartocones [hypocones] less distinct from the deuterocone [protocone] crest than in the type of R. andrewsi ’’ ( Granger and Gregory, 1943: 365) is based on a set of upper dentition (AMNH 20263) that they conjecturally referred to R. mongoliensis and considered to be the neotype.

Ironically, Granger and Gregory (1943) recognized that their specific distinctions of the three supposed species of Rhinotitan did not hold up when all the specimens in the AMNH collection were considered, and that the dentition of other specimens bridged the gaps between these species. Granger and Gregory (1943) considered the various holotypes, paratypes, and neotypes of the three species of Rhinotitan to represent arbitrary divisions between evolutionary stages for a continuous series represented by R. kaiseni – R. mongoliensis – R. andrewsi . Granger and Gregory (1943) did not consider the alternative possibility that the seemingly continuous variation, particularly that of the premolars, might represent intraspecific variation rather than progressive evolutionary stages. This alternative hypothesis is supported by the fact that nearly all of the Rhinotitan material is from the same locality and formation. Moreover, most other brontothere species exhibit similar levels of seemingly random intraspecific variation in the lingual morphology of the premolars.

Other attempts to revise the species-level taxonomy of Rhinotitan are problematic as well. Prior to Granger and Gregory’s (1943) work, Takai (1939) recognized problems with Osborn’s taxonomy and considered R. mongoliensis and R. andrewsi to represent opposite sexes of the same species. More recently Wang (1982) described two associated skulls and jaws (IVPP V3254-1 and IVPP V3254-2) and drew similar conclusions to those of Takai (1939). Wang (1982) considered R. andrewsi to be a junior synonym of R. mongoliensis , and considered the minor variations in the cheek teeth to represent individual variation. However, neither Takai (1939) nor Wang (1982) considered Rhinotitan kaiseni , although the cheek teeth of that species are indistinguishable from the other supposed Rhinotitan species.

My observations on the Rhinotitan material suggest two distinct species, R. kaiseni and R. andrewsi , that can be differentiated by the following characteristics: relative width of the basicranium, position of the frontonasal horns, shape of the nasal process, and the morphology of the upper incisors. Because the cheektooth morphologies of these species are not differentiated, much of the Rhinotitan material, consisting of partial jaws and palates, must be referred to Rhinotitan sp. For this reason, Rhinotitan mongoliensis ( Osborn, 1923) , whose holotype consists of a lower cheektooth row, must be considered a nomen dubium.

Nanotitanops shanghuangensis ( Qi and Beard, 1996)

HOLOTYPE: IVPP V11032 , a right premolar, most probably a P2 or P3.

TYPE LOCALITY: IVPP locality 93006D, a fissure-filling located in the Shanghuang Limestone Quarry, near the village of Shanghuang, Liyang County, southern Jiangsu Province, China.

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

SYNONYMS: Nanotitan Qi and Beard, 1996 , not Nanotitan Sharov, 1968

REFERRED SPECIMENS: (From the same locality as the holotype) IVPP V11002 , a right P2 or P3 ; IVPP V11003 , a left P2 or P3 ; IVPP V11004 , a left P2 or P3 ; IVPP V11005 , a left

P1 or P2; IVPP V11006, a right P1; IVPP V11007, a left M1 or M2; IVPP V11008, a left M1 or M2; IVPP V11010, a left m1 or m2; IVPP V11011, a right m1 or m2; IVPP V11013, a right m1 or deciduous premolar; IVPP V11014: a right m1 or deciduous premolar.

DIAGNOSIS: Nanotitanops shanghuangensis is the smallest known brontotheriid. It can be characterized as having a P1 with a weak metacone and small lingual heel, a distinct P2 metacone, weak premolar preprotocristae, and with short lingual crests extending posteriorly from the premolar protocones with an occasional premolar hypocone. The molars of N. shanghuangensis 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. Central molar fossae are present but anterolingual cingular cusps are absent. N. shanghuangensis molars retain vestigial paraconules, but all traces of metalophs are lost. The lower molars of N. shanghuangensis have shallow molar basins and weak lingual ribs.

Nanotitanops shanghuangensis can most easily be distinguished by its very small size. In addition to minuscule size, this species is the only brontothere to have a relatively complex P1 (with two labial cusps and a small lingual heel) but to retain vestigial paraconules in the upper molars.

DESCRIPTION

UPPER AND LOWER DENTITION: Nanotitanops shanghuangensis is known only from isolated teeth (fig. 87). In the original description, Qi and Beard (1996) identified all the specimens to specific dental homologues, but they did not explicitly state how or why these exact identifications were made. Upon reexamining the material, I am unconvinced that all of these isolated dental elements can be identified to their specific dental homologues. There are problems, in particular, with several of the original premolar identifications.

The original assignment of IVPP V11006 (fig. 87f) as a P1 seems accurate. It is significantly smaller than the premolars, IVPP V11004, IVPP V11032, IVPP V11003, and IVPP V11002, and it is semirounded in outline. There are two poorly differentiated labial cusps, there is no discernable labial rib on the paracone, the cingulum is thin, and the lingual shelf is very small with small protoconelike cusp and a short loph connecting the lingual cusp to the lingual base of the metacone.

IVPP V11005 (fig. 87g) was originally identified as a P2, although it might be a P1. The specimen is nearly the same size as IVPP V11006. Among brontotheres, P2s are usually much larger than P1s. IVPP V11005 differs from IVPP V 11006 in some ways. The paracone and metacone are more distinct, and the small lingual shelf has a loph that arches around the anterolingual corner of the crown that connects to the lingual base of the paracone. This loph is absent in IVPP V11006. There is a short loph at the lingual base of the metacone. In IVPP V11006 this loph is continuous with the protocone, but in IVPP V11005 it is discontinuous from the protocone. The exact morphology of the lingual features of the premolars tend to vary in most brontothere species and it is therefore possible that morphological differences between IVPP V11005 and IVPP V11006 represent intraspecific variation of the P1.

Based on their larger size, labial paracone ribs, large lingual shelves with large protocone lingual crests, occasional small hypocones (IVPP V11032), rhomboidal or rectangular outlines, and thick cingula, IVPP V11032, V V11002, V11003, and V11004 most certainly represent premolar elements other than P1.

IVPP V11004 (fig. 87d) was originally identified as a P3 although this is questionable. A P4 can probably be ruled out because the parastyle arches lingually; in brontotheres, the P4 parastyle usually projects labially while the parastyles of P3 can be straight or slightly lingually angled. Therefore, this tooth could be either a P2 or P3. The size of the labial rib of the paracone suggests that a P2 is more likely, although the rectangular shape of the tooth is more