Omomys carteri Leidy, 1869

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 : 10-11

Omomys carteri Leidy, 1869

Figure 4.7-9 , Table 1

Referred specimens. From SDSNH Locality 5841: M3, SDSNH 110357; partial m1, SDSNH 110355. From DMNH Locality 4672: m3, DMNH 75326. From UCM Locality 92189: p4, UCM 68564; m3, UCM 95722.

Description. The M3 is complete, but its enamel surface is somewhat abraded. Its occlusal outline is transversely elongate and triangular. The paracone is larger than the metacone and a mesostyle is lacking between these cusps. The protocone is large and positioned anterior of the midline of the tooth. The preprotocrista and postprotocrista extend labially to join a small, but distinct protoconule and metaconule, respectively. The preprotoconule, postprotoconule, premetaconule, and postmetaconule cristae are very short and extend labially from the protoconule and metaconule to join the labial base of the paracone and metacone, respectively. The ectocingulid is moderately robust, extending from the posterolabial base of the metacone to join the base of the paracrista (preparacrista). The precingulum and postcingulum are moderately strong and joined labially, forming a distinct, continuous lingual cingulum. The pericone is a distinct cusp on the labial cingulum, but a hypocone is lacking on the lingual cingulum.

A single p4 was recovered from the TBM. Its protoconid is the largest and tallest cusp. The paracristid descends rapidly in a gentle arc from the protoconid apex to terminate at a small, shelf-like paraconid. The protocristid is short, extending posterolingually from the protoconid apex to join a small, but distinct metaconid. The cristid obliqua is narrow and descends posteriorly from the posterior wall of the trigonid, below the center of the protocristid, to terminate at a very small hypoconulid. The talonid is relatively shallow and transversely narrow with a slight swelling at its posterolingual corner (incipient entoconid). The lingual and anterolabial cingulids are distinct.

The partial m1 (SDSNH 110355) has the labial bases of the protoconid and hypoconid along with the anterolingual edge of the trigonid broken away. The two m3s are complete and differ from the m1 by having a more anteroposteriorly elongate occlusal outline due to the presence of a large hypoconulid that extends posteriorly from the talonid, a relatively smaller paraconid that is positioned less lingually (just lingual of the midline of the trigonid), and relatively narrower trigonid and talonid basins. Although the ectocingulid (labial cingulid) morphology for the m1 cannot be determined because the basal labial margin of the tooth is missing, the m3 ectocingulid extends from the anterolabial base of the protoconid to the anterolabial base of the hypoconid and then continues from the posterolabial base to the hypoconid to terminate at the labial base of the hypoconulid. The m1 and m3 exhibit the following additional characters: 1) a three cusped trigonid (protoconid, metaconid, paraconid) that is moderately taller than the talonid; 2) a widely basined talonid, especially on m1; 3) an anteroposteriorly wide hypoconid that is larger than the entoconid; and 4) a cristid obliqua extending anterolingually from the hypoconid apex to terminate at the base of the protoconid, below and in line with its apex.

Remarks. Omomys carteri is common throughout the upper and lower Bridger Formation (e.g., Gazin, 1958; West, 1969, 1973; Szalay, 1976; Gunnell, 1998; Gunnell et al., 2009; Murphey, 2001; Tornow, 2005; Cuozzo, 2008). The TBM lower premolar and molars are indistinguishable in size and occlusal morphology from those of O. carteri ( Gazin, 1958; Szalay, 1976; Cuozzo, 2008) and can be confidently assigned to the species.

SDSNH 110357 has a complete labial cingulum that continues across the labial base of the protocone, a distinct pericone on the lingual cingulum, and is lacking a protocone fold and mesostyle, characters typical of the upper molars of Omomys ( Szalay, 1976) . In all other occlusal characters, SDSNH 110357 agrees well with the M3 of Omomys . However, compared to the M3 of O. carteri , its length is at the largest recorded measurement, and its width is slightly larger than the largest recorded measurement for the species, whereas its length and width are well within the observed ranges for the M2 of O. carteri ( Tornow, 2005; Cuozzo, 2008). So, the question arose when examining SDSNH 110357, could it possibly represent M2? Tornow (2005) and Cuozzo (2008) recently documented a significant amount of individual variation in samples of upper molars of O. carteri from the Bridger Formation. In Omomys , certain characters can be used to distinguish the M3 from the M2 ( Gazin, 1958; Szalay, 1976; Tornow, 2005; Cuozzo, 2008). The M3 metacone is smaller than the paracone, wherein the anterior portion of the labial margin is more expanded labially than the posterior portion of the labial margin resulting in posteriorly inclined plane along the labial occlusal outline, whereas on M2 the metacone is equal in size or larger than the paracone, resulting in a straighter labial occlusal margin. A hypocone is always lacking on M3, whereas on M2, a hypocone is usually present on the postcingulid (95% in Tornow's [2005] sample, 67% in Cuozzo's [2008] sample). A pericone is variably present on the M3 lingual cingulum, whereas it is always present on M2 (100%). The M3 has little to no inflection along the posterior margin of the tooth from the posterior margin of the metacone to the level of the protocone, whereas on M2 there is a distinct inflection. SDSNH 110357 has a posteriorly inclined labial occlusal margin (metacone smaller than paracone) and a distinct pericone, but is lacking a hypocone and a distinct inflection along the posterior margin of the tooth. The variable presence of a pericone on M3 and a hypocone on M2 does not allow use of these characters for identification of tooth position. The presence of the other two characters on SDSNH 110357 plus the fact that a distinct anterior appression facet is present along the anterolabial border of the tooth where it contacted another molar, but a posterolabial appression facet is lacking, strongly supports its identification as M3. Therefore, SDSNH 110357 is identified as M3 and apparently represents the largest M3 recorded for O. carteri .

Figures

FIGURE 4. Primates from TBM. 1-3, Notharctus robustior: 1, RM1, DMNH 75300; partial RM1 or 2, UCM 78457; 3, partial RM3, UCM 69054. 4-5, Trogolemur myodes: 4, Rm2, UCM 78097; 5, Rm3, UCM 67883. 6, Washakius insignis, LM1 or 2, UCM 68541. 7-9, Omomys carteri: 7, LM3, 110357; 8, Lm1, SDSNH 110355; 9, Rm3, DMNH 75326. All occlusal views. Scale bar equals 1 mm.

FIGURE 5. Microparamys from TBM. 1-5, Microparamys minutus: 1, LdP4, UCM 66310; 2, LP4, UCM 95695; 3, LM1 or 2, UCM 95697; 4, Rp4, SDSNH 110374; 5, Lm1 or 2, DMNH 74140. 6-9, Microparamys sp.: 6, LdP4, SDSNH 110395; 7, Lp4, SDSNH 110360; 8, Lm1, SDSNH 110361; 9, Lm2, SDSNH 110362. All occlusal views. Scale bar equals 1 mm.

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.

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.