Nyctitherium velox ( 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 : 22

Nyctitherium velox ( Marsh, 1872b)

Figure 8.1-6 , Table 2

Referred specimen. From UCM Locality 92189: RM2s, UCM 68581, 95771; Lm1, UCM 68967; partial dentary with partial Lm2 and m3, UCM 67899. From SDSNH Locality 5787: partial Lm2, SDSNH 110409. From DMNH Locality 4672: RM2, DMNH 75292; partial RM1 or 2, DMNH 75290; RM3, DMNH 75291. From DMNH Locality 4673: partial Lm2, DMNH 75317.

Description. The M2 paracone and metacone are tall with sharply tapered apices (sectorial condition of Robinson, 1968a) and the protocone is robust. The preprotocrista is well defined and extends anterolabially from the protocone apex to terminate at a distinct V-shaped protoconule. The postprotocrista is also well defined and extends posterolabially from the protocone apex to terminate at a distinct, V-shaped metaconule. The hypocone is very large and expanded, giving it a protruding, circular occlusal outline. A small mesostyle (or stylocone) is present along the stylar shelf posterior of the paracone apex. The parastyle is distinct and positioned at the anterolabial corner of a prominent parastylar lobe. The ectoflexus is moderately concave between the parastylar lobe and the robust, labially projecting metastylar lobe. The anterior cingulum is moderately robust.

The M3 paracone is taller and slightly larger in size than the metacone with a long preparacrista extending labially to join a small, but distinct, parastyle. The stylar shelf extends anterolabially as a long rapidly ascending projection from the labial base of the metacone to the anterolabial corner of the tooth. A very small mesostyle (or stylocone) is present on the labial edge of the stylar shelf that is positioned above the paracone apex. The protocone is robust, but a hypocone is lacking. The paraconule and metaconule are well developed with distinct parastylar and metastylar wings. The anterior and posterior cingula are weak.

The m1 and m2 trigonids are anteroposteriorly compressed with a large protoconid and a cuspate paraconid that is slightly smaller and lower in height than the metaconid. The talonid is wider than the trigonid with a large hypoconid and a distinct entoconid that is positioned close to a distinct hypoconulid (twinned condition). The cristid obliqua extends anterolingually from the hypoconid towards the metaconid and terminates just lingual of the notch between the metaconid and protoconid, resulting in a deep ectoflexid. The anterior, labial, and posterior cingulids are joined as a continuous, well-developed cingulid that extends from the anterior base of the protoconid across the labial bases of the protoconid and hypoconid to terminate near the posterior base of the hypoconulid.

The occlusal morphology of the m3 is very similar to that of the m1 and m2, but differs by having a narrower talonid and smaller hypoconid.

Remarks. Prior to the discovery of Nyctitherium gunnelli , only two other species of Nyctitherium , N. serotinum and N. velox , were previously recognized from the Bridger Formation. Nyctitherium velox is easily distinguished from N. serotinum by its larger size and the presence of a complete, well-developed labial cingulid on the lower molars ( Robinson, 1968a; Krishtalka, 1976b). Nyctitherium velox can also be easily distinguished from N. gunnelli by its significantly smaller size ( Figure 9 ) and certain other dental characters listed above. The referred specimens from the TBM agree well in their occlusal morphology and size to those of N. velox and are referred to the species.

Figures

FIGURE 8. Nyctitheriidae from TBM. 1-6, Nyctitherium velox: RM2 (DMNH 75292) in occlusal view (1); RM2 (UCM 95771) in occlusal view (2); RM2 (UCM 68581) in occlusal view (3); RM3 (DMNH 75291) in occlusal view (4); partial Lm2 (DMNH 75317) in occlusal view (5); partial Lm2 (SDSNH 110409) in occlusal view (6). (7), Pontifactor bestiola, RM1 (UCM 71129) in occlusal view. 8-10, Nyctitherium gunnelli new species, LP4 (UCM 95769) in occlusal (8), labial (9), and lingual (10) views. Scale bar equals 1 mm.

FIGURE 9. Chart comparing m1-2 anteroposterior length (ap) versus m1-2 talonid width (trp) in mm of specimens of Nyctitherium velox (including two referred specimens from TBM, SDSNH 110409 and UCM 68967) and holotype m1- 2 (SDSNH 110393) of Nyctitherium gunnelli new species. Black solid filled squares/circles are m1s and white filled squares/circles are m2s. Except for SDSNH 110409 and UCM 68967, measurements of other specimens of N. velox from Robinson (1968a) and Krishtalka (1976b).