Thisbemys corrugatus Wood, 1959a

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 : 15-16

Thisbemys corrugatus Wood, 1959a

Figure 6.1 , Table 2

Referred specimen. From UCM Locality 92189: m1, UCM 70671.

Description. Differentiation of m1 from m2 can be difficult when dealing with isolated ischyromyid teeth, but in Thisbemys the m1 differs from the m2 by being slightly smaller in size and having the trigonid narrower relative to the talonid. For example,

in T. corrugatus the mean tra/mean ap ratio for m1 is 0.93 and that of m2 is 0.99 ( Wood, 1962). UCM 70671 has a tra/ap ratio of 0.93, indicating that it most likely represents m1. UCM 70671 is relatively large with robust primary cusps (protoconid, metaconid, hypoconid, and entoconid). The trigonid valley is moderately wide and open anterolingually. The metalophid is incomplete with a distinct valley bisecting it about one-third of the way from the protoconid to the metaconid. The ectolophid is incomplete between the protoconid and hypoconid, separated by a narrow transverse valley. The talonid is wide with a deep lingual gorge (valley) between the metaconid and entoconid that extends into the central basin. The hypoconulid is relatively robust and centrally positioned on the posterolophid (= posterior cingulid). The enamel is heavily corrugated (crenulated) throughout the talonid basin.

Remarks. Five species of Thisbemys are recognized from the Bridger Formation ( Gunnell, 1998; Gunnell et al., 2009; Anderson, 2015): T. corrugatus Wood, 1959a ; T. plicatus Wood, 1962 ; T. nini Wood, 1962 ; T. perditus Wood, 1962 ; and T. brevicrista ( Ostrander, 1986) . All five species coexisted during the middle Bridgerian (biochron Br2) in the Bridger Basin ( Wood, 1962; Gazin, 1976; Anderson, 2015). Anderson's (2015) figure 2 showed T. plicatus occurring in the Twin Buttes Member (late Bridgerian, biochron Br3), but in her conclusions section ( Anderson, 2015: p. 329) she stated that "only T. brevicrista and T. corrugatus continued into the latest Bridgerian (Br3)." Although apparently not recorded from the Twin Buttes Member of the Bridger Formation, T. plicatus has been recorded from the lower Bridgerian (biochron Br1b = Bridger A of Matthew [1909]) of Wyoming ( Gunnell, 1998; Gunnell et al., 2009) and in early Uintan faunas (biochrons Ui1a and Ui1b) from the Devil's Graveyard Formation of Texas ( Wood, 1962, 1973; Campisano et al., 2014).

Thisbemys nini and T. perditus can be easily distinguished from T. plicatus , T. corrugatus , and T. brevicrista by their significantly smaller size and by having the crenulations on the talonid enamel much less developed, along with certain other minor occlusal differences ( Wood, 1962, 1973; Anderson, 2015). The lower molar occlusal morphology of T. plicatus is quite similar to that of T. corrugatus , but differs by having the talonid slightly less open with slightly weaker enamel crenulations and being slightly smaller ( Wood, 1962, 1973; Campisano et al., 2014; Anderson, 2015). Thisbemys brevicrista is known from the middle through late Bridgerian (Br2-Br3) and is distinguished from T. corrugatus and T. plicatus by its slightly larger size and by having complete m1-3 metalophids and complete m1-2 ectolophids ( Wood, 1962; Anderson, 2015). Thisbemys brevicrista further differs from T. corrugatus by having m1 with the trigonid narrower relative to its length (mean tra/mean ap = 0.86), m1-2 with a relatively narrower and less open trigonid, and m2 with more rhomboid occlusal outline ( Anderson, 2015).

UCM 70671 is referred to Thisbemys corrugatus because it is well within the observed size range for the m1 of the species, and its occlusal morphology is indistinguishable from that of the species, including an incomplete metalophid, an anteriorly incomplete ectolophid, a relatively unconstricted and open trigonid, and an identical ratio of the trigonid width to tooth length.

Figures

FIGURE 6. Thisbemys and Rodentia, family undetermined from TBM. 1, Thisbemys corrugatus, Rm1, UCM 70671. 2-3, Rodentia, family undetermined, Rm1 or 2, UCM 95700. 1-2, occlusal views. 3, oblique labial view. Scale bar equals 1 mm.