Epihippus sp.

Murphey, Paul C. & Kelly, Thomas S., 2017, Mammals from the earliest Uintan (middle Eocene) Turtle Bluff Member, Bridger Formation, southwestern Wyoming, USA, Part 2: Apatotheria, Lipotyphla, Carnivoramorpha, Condylartha, Dinocerata, Perissodactyla and Artiodactyla, Palaeontologia Electronica (Cambridge, England: 2003) 20 (2), pp. 1-51 : 35-36

Epihippus sp. , cf. E. gracilis ( Marsh, 1871)

Figure 13.5-6 , Table 3

Referred specimen. From SDSNH Locality 5844: Rp3, SDSNH 110408.

Description. SDSNH 110408 has a forward projecting paralophid, a character that is typical of the p3 of Epihippus . It can be differentiated from the dp3 of Epihippus ( MacFadden, 1980; Hanson, 1996; McCarroll et al., 1996) by being slightly wider relative to its length and by having a better developed paralophid (larger and more expanded anteriorly). In the p4 of Epihippus , the paralophid is slightly taller and relatively shorter than that of the p3, wherein it extends slightly less anterolingually from the protoconid to about the midline of the tooth and then turns sharply posteriorly, like those of m1-3.

SDSNH 110408 is fully molariform with the metaconid and entoconid well developed and about equal in size to the protoconid and the entoconid, respectively. The paralophid is a robust, relatively tall cristid extending lingually in a wide arc from the protoconid apex to the anterolingual base of the metaconid. The metaconid is expanded anteroposteriorly with a distinct metastylid present just posterior to its apex. A complete protolophid is present, connecting the protoconid to the metaconid. The posterior cingulid (postcristid) extends posterolingually from about the center of the hypolophid to the posterior base of the entoconid.

Remarks. The p3 of Epihippus is more molariform than that of Orohippus and further differs by the following ( Granger, 1908; Stirton, 1940; Forstén, 1971; MacFadden, 1975, 1980, 1998; Hanson, 1996; McCarroll et al., 1996; Kelly and Murphey, 2016b): 1) a better developed paralophid (in Orohippus , paralophid lower and less transversely expanded); 2) the metaconid nearly equal in size to the protoconid (in Orohippus , metaconid significantly smaller than the protoconid); 3) the entoconid nearly equal in size to the hypoconid (in Orohippus , entoconid significantly smaller than the hypoconid); and 4) a tendency to bifurcate metaconid at its apex due to the presence of a small metastylid just posterior to the metaconid apex (in Orohippus , a distinct metastylid usually lacking). SDSNH 110408 agrees well with all of the above differentiating characters of the p3 of Epihippus and is assigned to the genus.

Two species of Epihippus are currently recognized from the Uintan; Epihippus gracilis (including E. parvus Granger, 1908 ), and Epihippus uintensis Marsh, 1875 . A third species that is sometimes included in Epihippus is Duchesnehippus intermedius Peterson, 1931 , from the Duchesnean (e.g., MacFadden, 1980, 1998; Hanson, 1996). However, this species exhibits even further molarization of the lower premolars (especially p2), greater hypsodonty and increased size, which justifies its referral to a separate genus from Epihippus ( Rasmussen et al., 1999; Kelly et. al., 2012; Kelly and Murphey, 2016b). Species of Epihippus are differentiated primarily by size, with E. uintensis being larger than E. gracilis . The Epihippus p3 from the TBM agrees well in size to those E. gracilis ( Granger, 1908; MacFadden, 1980, table 1) and probably represents this species. However, until additional material of the TBM Epihippus is available to confirm its specific assignment, SDSNH 110408 is left in open nomenclature as Epihippus sp. , cf. E. gracilis .

Figures

FIGURE 13. Perissodactyla from TBM. 1-4, Triplopus sp., cf. T. obliquidens, partial left dentary with alveolus for p1, p2, partial p3, p4 and partial m1 (UCM 69053 with 1, 3-4 original specimen, 2 cast showing detail better) in occlusal (1- 2), labial (3) and lingual (4) views. 5-6, Epihippus sp., cf. E. gracilis, Rp3 (SDSNH 110408) in occlusal (5) and lingual (6) views. Scale bars for 1-2 and 3-4 equals 5 mm. Scale bar for 5-6 equals 1 mm.