Tillomys parvidens, (Marsh, 1872)

Kelly, Thomas S. & Murphey, Paul C., 2016, Mammals from the earliest Uintan (middle Eocene) Turtle Bluff Member, Bridger Formation, southwestern Wyoming, USA, Part 1: Primates and Rodentia, Palaeontologia Electronica 7 (8), pp. 1-55 : 24

T.? parvidens ( Marsh, 1872)

Figure 7.8-9 , Table 4

Referred specimens. From SDSNH Locality 5841: m1 or 2, SDSNH 110379. From DMNH Locality 4672: m1 or 2, DMNH 75329; m3, DMNH 75288. From UCM Locality 92189: partial m1 or 2, UCM 68533.

Description. The three m1 or 2s are very similar in occlusal morphology to those referred above to Tillomys senex including the following: 1) a robust, tall trigonid; 2) an incomplete metalophulid II; 3) an isolated mesoconid; 4) an incomplete hypolophid that is orientated obliquely posteriorly; and 5) a robust posterior cingulid. They differ from those referred to T. senex in being slightly smaller and by having a double-cusped mesoconid that is slightly less elongate transversely and a slightly shorter hypolophid.

The m3 is unworn and has a rectangular, elongate occlusal outline. The primary cusps (protoconid, metaconid, hypoconid, and entoconid) are robust. The anterior cingulid is a thick, short cristid extending from the anterior edge of the protoconid to the labial base of the metaconid. A moderate metastylid crest extends posteriorly from the metaconid. The metalophulid II (posterior arm of the protoconid) is incomplete, extending lingually from the protoconid to terminate near the posterolabial base of the metaconid. The mesoconid is isolated from the protoconid and hypoconid and is doubled (two swellings or incipient cusps). The entoconid is prominent with a very short hypolophid extending posteriorly and at an oblique angle from it. The posterior cingulid is robust extending lingually in an arc from the hypoconid and terminating at the posterolingual corner of the tooth, where it is separated from the entoconid by a shallow, but distinct, valley.

Remarks. Tillomys? parvidens was originally assigned to Sciuravus by Marsh (1872), but Troxell (1923b) and Wilson (1938b) noted that the m2 of the holotype is more similar in occlusal morphology to those of T. senex than to those of Sciuravus and questionably referred it to Tillomys (see also Walton and Porter, 2008). The lower molars of Tillomys? parvidens are very similar to those of T. senex , but differ primarily by the following: 1) slightly smaller; 2) a tendency for a doubled mesoconid (two-cusped) that is less transversely elongate; and 3) a shorter hypolophid. Whether these differences rise to the level of specific separation is debatable, but the four lower molars from the TBM exhibit the putative diagnostic characters of T.? parvidens and are tentatively assigned to the species.

Figures

FIGURE 7. Larger Sciuravidae from TBM. 1, Sciuravus nitidus, RP4, UCM 68430. 2-4, Taxymys lucaris: 2, RM1 or 2, UCM 68307; 3, LM1 or 2, UCM 95762; 4, LM3, DMNH 75336. 5-7, Tillomys senex: 5, Lm1 or 2, DMNH 75328; 6, Rm1 or 2, DMNH 75279; 7, Rm1 or 2, SDSNH 110363. 8-9, Tillomys? parvidens: 8, Rm1 or 2, SDSNH 110379; 9, Lm3, DMNH 75288. All occlusal views. Scale bar equals 1 mm.

FIGURE 8. Smaller Sciuravidae from TBM. 1, Pauromys sp., cf. P. perditus, Lm1, SDSNH 110414. 2, sciuravid sp. B, RM1 or 2, SDSNH 110412. 3-4, Sciuravid sp. C: 3, RM1 or 2, SDSNH 110443; 4, Lm3, SDSNH 110441. 5-7, Sciuravid premolars: 5, LP4, SDSNH 110462; 6, partial Rp4, SDSNH 110454; 7, partial Rp4, SDSNH 140176. All occlusal views. Scale bar equals 1 mm.

FIGURE 9. Sciuravid sp. A from TBM: 1, RM1 or 2, UCM 709102; 2, RM1 or 2, UCM 68903; 3, LM1 or 2, UCM 71413; 4, LM1 or 2, UCM 95763; 5, Rm1, UCM 95694; 6, Lm1 or 2, UCM 95757; 7, Rm1 or 2, SDSNH 110459; 8, Rm1 or 2, UCM 95699; 9, Lm3, UCM 95756. All occlusal views. Scale bar equals 1 mm.