Species Taxonomy, Phylogeny, and Biogeography of the Brontotheriidae (Mammalia: Perissodactyla) Author Mihlbachler, Matthew C. text Bulletin of the American Museum of Natural History 2008 2008-06-03 311 1 1 475 http://dx.doi.org/10.1206/0003-0090(2008)501[1:stpabo]2.0.co;2 journal article 10.1206/0003-0090(2008)501[1:stpabo]2.0.co;2 0003-0090 13223833 Rhinotitan quadridens Xu and Chiu, 1962 HOLOTYPE : IVPP V2651 , a left P3–P4, and isolated left p2, and right p3, p4, and m1. TYPE LOCALITY : Lumeiyi Formation, Lunan Basin, Yunnan Province , China . AGE : Middle Eocene (Sharamurunian land mammal ‘‘age’’). DETERMINATION : Nomen dubium: holotype lacks diagnostic features. REMARKS Rhinotitan quadridens is a nomen dubium based on a series of upper and lower cheek teeth (fig. 181). P3 and P4 are rectangular with parallel anterior and posterior sides, and flat lingual and labial margins. P3 has a well-developed lingual crest, while P4 does not. P4 has a poorly developed mesostyle and cusp on the anterior cingulum. The p2 trigonid is only slightly longer than the talonid and it lacks a metaconid. There is an anomalous small cusp at the lingual edge of the p2 talonid basin. The teeth identified as p3 and p4 each have a well-developed metaconid. The lower premolars lack cingulids. Fig. 181. Part of the holotype of Rhinotitan quadridens (IVPP V2651) . (A) Left P3–P4 (from Xu and Chiu, 1962 ), (B) left p2, (C) right p3, and (D) right p4. Xu and Chiu (1962) noted that the degree of premolar molarization resembles Rhinotitan mongoliensis (a nomen dubium) and R. andrewsi , but that it differs from other Rhinotitan species in its larger size, higher crowns, flatter lingual edges of P3 and P4, and well-developed secondary wrinkles (?) on P3 and P4. However, none of these specimens retains diagnostic characters that would indicate Rhinotitan , nor do they indicate a unique taxon. Given the considerable amount of intraspecific variation seen in the upper premolars of brontotheres, the rectangular shape of P3 and P4, prominent lingual crest on P3, lack of well-developed hypocones, and (occasional) P4 mesostyle and anterolingual cingular cusps do not sufficiently differentiate this species from several large Asian brontothere species, such as Qufutitan zhoui , Epimanteoceras formosus , Rhinotitan kaiseni , Rhinotitan andrewsi , Gnathotitan berkeyi , Aktautitan hippopotamopus , and possibly others. Moreover, size variation among these species is poorly understood because of limited sample sizes. The lower premolars are also consistent with multiple taxa, particularly Rhinotitan , Gnathotitan , and Aktautitan . To summarize, the specimens upon which R. quadridens was based do not indicate a unique taxon; nor is there enough evidence for a species-level identification. However, it is noteworthy that the material is distinct from Dianotitan lunanensis , a species from the same formation with well-developed premolar hypocones. Other material from the Lumeiyi Formation, assigned to another nomen dubium, Protitan major , consisting of an upper molar series, may also be pertinent, although comparisons are not possible due to nonoverlapping parts.