Apatemys bellulus Marsh, 1872b

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 : 2-5

Apatemys bellulus Marsh, 1872b

Figure 2.1-6 , Table 1

Referred specimens. From UCM Locality 92189: RM 1, UCM 70967; Lm1, UCM 95804; Lm2, UCM 68923; Rm3s, UCM 68424, 79676; Lm3, UCM 95807.

Description. The M1 paracone, metacone, and protocone have relatively tall, sharp apices with the protocone positioned anteriorly. The hypocone is distinct and positioned posterolabially of the protocone apex. The parastyle is robust and forms an anterior extension at the anterolabial corner of the stylar shelf. The labial border along the stylar shelf between the paracone and metacone (ectoflexus) is moderately concave. The postmetacrista extends posterolabially in a gentle arc from the metacone to terminate at a distinct metastyle on the posterolabial corner of the stylar shelf. The anterior and posterior cingula are weak.

The m3 differs from m1-2 by having a more anteroposteriorly elongated talonid, and the m1 differs from the m2 by having a more anteroposteriorly expanded trigonid with the paraconid positioned slightly further from the metaconid. The lower molars have an accessory cuspid present at the anterolabial corner of the trigonid, giving it a trapezoidal occlusal outline. The metaconid is larger than the other cusps of the trigonid and positioned posterior of the protoconid apex. The entoconid varies from a weakly expressed bump to a small distinct cuspid. The hypoconid is broad with the cristid obliqua extending anterolabially from its apex to join the posterior wall of the trigonid just labial of the notch between the protoconid and metaconid. A moderately distinct anterior cingulid is present that extends from the accessory cuspid to the paraconid.

Remarks. Apatemys is a relatively rare taxon in the middle Eocene ( West, 1973b). Three species have been described from the Bridger Formation; A. bellus Marsh, 1872b , A. bellulus Marsh, 1872b , and A. rodens Troxell, 1923b . These species are differentiated primarily by size and minor differences in dental occlusal morphology ( Marsh, 1872b; Troxell, 1923b; Jepsen, 1934; McKenna, 1963; Gazin, 1976). However, West (1973b), in a revision of the North American Apatemyidae , placed A. bellulus , A. rodens and a number of other apatemyid taxa including Labidolemur kayi Simpson, 1929 , in synonymy with A. bellus , resulting in a geochronologic range for the species from the late Paleocene to the middle Eocene. This excessive taxonomic lumping also resulted in extreme dental size ranges with high coefficients of variation for A. bellus . Most recent investigators have not accepted most of West's (1973b) synonymies (e.g., Gazin, 1976; Gunnell and Bartels, 1999; Koenigswald et al., 2005; Gunnell et al., 2008a; Silcox et al., 2010) and recognize the three originally described species from the Bridger Formation.

The Apatemys specimens from the TBM at UCM Locality 92189 are represented by two distinct dental size groups. The smaller sized group of teeth ( Figure 2.1-6 ) are well within the observed ranges of those of Apatemys bellulus and are referred to the species. Although both groups exhibit similar occlusal morphology, the A. bellulus specimens differ by the following: 1) a more concave labial border along the M1 stylar shelf between the paracone and metacone; 2) a slightly less developed M1 hypocone; 3) a slightly more weak and slightly more lingually positioned m3 paraconid; and 4) a slightly more squared off posterior border of the m3 talonid (less pointed).

Figures

FIGURE 2. Apatemyidae from TBM. 1-6, Apatemys bellulus: RM1, UCM 70967 (1); Lm1, UCM 95804 (2); Lm2, UCM 68923 (3); partial Rm3, UCM 68424 (4); Lm3, UCM 95807 (5); Rm3, UCM 70676 (6). 7-12, Apatemys rodens: LM1, UCM 68580 (7); LM3, SDSNH 110342 (8); LM3, UCM 68970 (9); Rm3, UCM 95805 (10); partial Lm3, UCM 66324 (11). Rm3, 95801 (12). All occlusal views. Scale bar equals 1 mm.

FIGURE 3. Sespedectidae from TBM. 1-8, Scenopagus curtidens: LM1 (UCM 68966) in occlusal view (1); Ldp4 (UCM 110344) in occlusal (2) and labial (3) views; Lm1 or 2 (UCM 95685) in occlusal (4) and labial (5) views; Rm1 or 2 (UCM 95684) in occlusal (6) and labial (7) views; partial Rm3 (UCM 68968) in occlusal view (8). 9-11, Scenopagus priscus: Rm1 or 2 (UCM 95683) in occlusal (9) and labial (10) views; Rm3 (SDSNH 110345) in occlusal view (11). 12- 13, Sespedectidae, genus undetermined, partial Lm1 or 2 (SDSNH 110343) in labial (12) and occlusal (13) views. Scale bar equals 1 mm.

FIGURE 4. Entomolestes westgatei new species from TBM. 1-2, partial dentary with Lp4-m1 (UCM 95687, holotype) in labial (1) and occlusal (2) views. 3-5, partial dentary with partial Rm1 and m2-3 (UCM 67884) in occlusal (3), labial (4), and lingual (5) views. Scale bar equals 1 mm.

FIGURE 5. Entomolestes westgatei new species from TBM. Specimens showing position of mental foramen (MF) and alveoli for antemolars anterior of p4. See text for proposed alveolar homologies. 1-2, partial dentary with Lp4-m1 (UCM 95686, m1 not shown) in occlusal (1) and labial (2) views. 3-4, partial dentary with Lp4-m1 (UCM 95687, holotype, m1 not shown) in occlusal (3) and labial (4) views. Scale bar equals 1 mm.

FIGURE 6. Geolabididae from TBM. 1-4, Centetodon bembicophagus: LP4 (UCM 68681) in occlusal view (1); partial Lm1 or 2 (UCM 68899) in occlusal view (2); Lm1 or 2 (DMNH 75256) in occlusal view (3); Lm3 (DMNH 75257) in occlusal view (4). 5-10, Centetodon pulcher: partial dentary with Lp4-m1 (UCM 95767) in occlusal (5) and labial (6) views; Rm1 or 2 (UCM 95768) in occlusal view (7); partial dentary with Lp1 and p4-m3 (UCM 68963) in occlusal (8), labial (9), and lingual (10) views. Scale bar equals 1 mm.