Eotitanops gregoryi Osborn 1913

Mader, Bryn J., 2010, A species-level revision of the North American brontotheres Eotitanops and Palaeosyops (Mammalia, Perissodactyla), Zootaxa 2339, pp. 1-43 : 15

Species Eotitanops gregoryi Osborn 1913

= E. minimus Osborn 1919

Holotype: AMNH 14889, an incomplete lower jaw with some of the right cheek dentition intact and fragments of the left maxilla containing M2 and M3.

Referred specimens: AMNH 17418, AMNH 17439 (type of E. minimus ), AMNH 56539, AMNH 96281, AMNH 104773, YPM 16439, YPM 16462, UCM 24690, UCM 32171, UCM 42725, UCM 42740.

Diagnosis: Small species of Eotitanops distinguished from E. borealis by its much smaller size and by the presence of an entoconid on p4.

Discussion: This species is still poorly known and it is not absolutely certain that it belongs to the genus Eotitanops . The list of referred specimens for Eotitanops gregoryi cited above is taken from Wallace's unpublished Master's Thesis (1980) and consists of the specimens that he referred to E. minimus . According to Wallace, these specimens are from Huerfano Basin localities I, II, and V (see Robinson 1966), which are all in the upper Huerfano faunal zone (Gardnerbutte Local Fauna).

According to Wallace, and confirmed here, a distinct entoconid is present on the p4 of all three specimens in the hypodigm (AMNH 17418, AMNH 17439, and AMNH 96281) that have this tooth preserved, although it is worn down on AMNH 96281.

The entoconid on the p4 of the type of Eotitanops gregoryi (AMNH 14889) is smaller than the p4 entoconids of the two Huerfano specimens in which this tooth is well preserved. According to Wallace (1980, p. 28) the p4 entoconid in the type of E. gregoryi is not actually a cusp, but is simply a "crenellation" or " minor inflation" of the hypolophid, giving "the impression of incipient entoconid ... development". Although the entoconid is very small in the type of E. gregoryi , however, it is certainly a distinct cusp and not simply a " minor inflation". The Huerfano E. gregoryi sample is too small to determine the size variation of the p4 entoconid and it is possible that specimens will be discovered in which the p4 entoconid more closely approaches the size of this cusp in the type of E. gregoryi .

Regardless of the character polarity of the p4 entoconid, the presence of this cusp in both the type of Eotitanops gregoryi and specimens once referred to E. minimus , in contrast with its absence in E. borealis , suggests a close relationship. These factors coupled with the close similarity in size between the type of E. gregoryi and specimens once referred to E. minimus , and the fact that the size variation among all of these individuals is well within the range of a single extant mammalian species, justifies synonymizing E. gregoryi and E. minimus .

As noted, Gunnell and Yarborough (2000) regarded E. gregoryi as a junior synonym of E. borealis and regarded E. minimus as the valid name for the species described here.