Protitan major Huang and Zheng, 2004

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 : 364-366

Protitan major Huang and Zheng, 2004

HOLOTYPE: IVPP V13802 , a maxillary fragment with right P5–M3.

TYPE LOCALITY: Nearby Lumeiyi Village , Lunan County, Yunnan Province ; lower part of middle Eocene Lumeiyi Formation.

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

DETERMINATION: Nomen dubium: holotype lacks diagnostic features, but it may be synonymous with Metarhinus .

REMARKS

Protitan major Huang and Zheng (2004) is a nomen dubium that was based on a maxilla fragment with P4–M3 (IVPP V13802). The fossil lacks sufficient character data to justify a new species. Moreover, the assignment of the specimen to the genus Protitan is erroneous. As Huang and Zheng (2004) note, the specimen is too large for any other known species of Protitan . More importantly, the molars of IVPP V13802 clearly display derived features not seen in species originally referred to Protitan by Granger and Gregory (1943), including a prominent anterolingual cingular cusp and a distinct central molar fossa. These same traits are seen in many advanced brontothere species. The holotype specimen has a molar-row length (220.5 mm) that is within the acceptable size range of many brontotheres that possess a set of molar characters identical to those of IVPP V13802, including anterolingual cingular cusps and central molar fossae. The P4 of the holotype, and the referred P2 (IVPP V13803) limit the taxonomic identity of IVPP V13802 considerably. These premolars lack hypocones, thus eliminating Dianotitan (from the same geologic formation), Parabrontops , Pachytitan , Eubrontotherium , and other similarly sized taxa that have well-developed hypocones on their premolars. Other taxa, including Metatitan , Aktautitan , and Gnathotitan , on the other hand, show more unstable lingual premolar morphologies, with more poorly developed premolar hypocones that are variably present and absent. Considering the intraspecific variability in premolar morphology, IVPP V13802 is consistent with any of these taxa. The proportions of the molars and the high relief on the lingual features of the P4 are particularly reminiscent of Metatitan . Therefore, Protitan major is a nomen dubium, and its holotype IVPP V13802 possibly belongs to a species of Metatitan .

cf. Palaeosyops sp. ( Palaeosyops sp. sensu Gabounia, 1977)

AGE: Middle Eocene ( Russell and Zhai, 1987).

REFERRED SPECIMEN: (From Aksyr River, Lower Obayla Subsvita in the Zaysan Basin, central Kazakstan) a right M1 or M2 (no number provided by Gabounia, 1977).

DETERMINATION: this tooth represents a small, unnamed Palaeosyops -like species, but there is not a sufficient amount of character data to sufficiently describe or diagnose it.

REMARKS

The upper molar identified as Palaeosyops sp. by Gabounia (1977) from the Zaysan Basin of Kazakstan indicates a relatively primitive Palaeosyops - or Eotitanops -like brontothere (fig. 188). The tooth lacks derived brontotheriine characters and is more similar to Palaeosyops and Eotitanops ; it has a short ectoloph, prominent labial ribs, rounded lingual margins of the paracone and metacone, a thickened parastyle cingular shelf, and a distinct paraconule. The ectoloph is wide, more than half as wide as the entire crown. In this respect, the tooth more closely resembles Palaeosyops ; Eotitanops has slightly narrower ectoloph. Howev- er, this tooth is much smaller than any North American Palaeosyops . It also has very crenulated enamel, similar to Bunobrontops savagei . This specimen seems to represent an Asian species of Palaeosyops -like brontothere. Other Palaeosyops -like materials known from southeast Asia are described further below.

cf. Metarhinus sp. ( Microtitan sp. sensu Qi and Beard, 1996)

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

REFERRED SPECIMENS: (From IVPP locality 93006D, a fissure-filling located in the Shanghuang Limestone Quarry, near the village of Shanghuang, Liyang County, southern Jiangsu Province, China.) IVPP V11017, a partial upper right M1 or M2, IVPP V11016, a left m1 or m2.

DETERMINATION: This material represents an additional species of Asian brontothere, possibly a species of Metarhinus . However, any species named from this material would be dubious; it cannot be differentiated from North American species of Metarhinus .

REMARKS

Qi and Beard (1996) referred a couple of small brontothere dental elements to Microtitan sp. The only complete element, the lower molar (m1 or m2) has a length of 26.1 mm and a width of 14.7 mm and is the right size for Microtitan . However, in other details, this material does not compare well with M. mongoliensis , the only valid species of Microtitan . The lower molar (IVPP V11016) has a less prominent hypoconulid in comparison to the neotype of M. mongoliensis (fig. 189a). The partial upper molar (IVPP V11017) exhibits a well-developed central molar fossa and a small anterolingual cingular cusp (fig. 189b); these are derived dental features not seen in M. mongoliensis . Pygmaetitan panxianensis , another small Asian brontotheriid, approaches these specimens in size; however, the lower molar of Pygmaetitan is higher crowned with much deeper trigonid and talonid basins. Among the remaining brontotheres with similarly advanced upper molars, only Metarhinus approaches the size range of these specimens. Although the material is consistent with Metarhinus it lacks specific characteristics that are diagnostic of Metarhinus . Therefore, the taxonomic identity of the material is dubious, although it can be differentiated from all other known Asian species and therefore represents an additional species on that continent.

cf. Palaeosyops sp. ( Bunobrontops sp. sensu Holroyd and Ciochon, 2000)

AGE: Middle Eocene.

REFERRED SPECIMEN: (From ‘‘ 1 mi. N. Koniwa’’, Pondaung Formation, Myanmar) two right m3 fragments, AMNH 32523a and AMNH 32523b.

DETERMINATION: Problematic taxon; this represents a Palaeosyops -like species, but the material lacks a sufficient amount of character data to justify a new species, nor can it be assigned to any particular genus or species.

REMARKS

Holroyd and Ciochon (2000) referred two right m3 fragments, AMNH 32523a and AMNH 32523b, from ‘‘ 1 mi. N. Koniwa’’, to Bunobrontops sp. (fig. 190). The specimens are roughly 20 percent smaller than the material referred to Bunobrontops savagei by Holroyd and Ciochon (2000), but such minor size differences are of dubious taxonomic significance. The more complete of the two specimens, AMNH 32423b, a right m3, is less elongate than the m3 of B. savagei (UCMP 128414) and is more similar to Palaeosyops in its length/width ratio (,1.71). More importantly, it has developed a wear pattern that seems inconsistent with that of the upper molars of Bunobrontops savagei , but is consistent with more plesiomorphic brontotheres such as Palaeosyops . The peaks of the labial cusps have formed rounded pondlike exposures of dentin, bordered labially and lingually by very thick enamel. This is a wear pattern seen in the upper and lower dentition of basal brontotheres such as Palaeosyops and Eotitanops . Bunobrontops savagei upper molars seem to exhibit a more derived wear pattern seen in more advanced brontotheres where the upper and lower molars develop a strongly W-shaped wear facet along the ectoloph. A lower molar fragment referred to Bunobrontops savagei (UCMP 147048) exhibits the more derived lower molar wear pattern. However, AMNH 32523b exhibits the plesiomorphic wear pattern with rounded pondlike exposures of dentin. Therefore, these molars (AMNH 32523a and AMNH 32523b) seem to represent a species other than Bunobrontops savagei . The molar is similar in size and proportion to those of Palaeosyops , although the enamel is much thicker than that of Palaeosyops and the molar displays deep crenulations in the talonid and trigonid basins, similar to that of Bunobrontops savagei . Along with an upper molar of a small Palaeosyops -like species from Kazakstan reported by Gabounia (1977) (referred to above as cf. Palaeosyops sp. ), this material represents one of the most primitive brontothere species in Asia.