Triconodontidae Marsh, 1887

Cifelli, Richard L., Davis, Brian M. & Sames, Benjamin, 2014, Earliest Cretaceous mammals from the western United States, Acta Palaeontologica Polonica 59 (1), pp. 31-52 : 39

? Triconodontidae Marsh, 1887

Gen. et sp. indet.

Fig. 6D .

Material.—OMNH 62855, right mesial lower premolar from OMNH 1254, Fuson Member, Lakota Formation (Lower Cretaceous: upper Valanginian–lower Barremian), Meade County, South Dakota, USA.

Description.—The crown of OMNH 62855 is comparatively long mesiodistally (L = 1.04 mm) and low-crowned, as is the case with mesial premolars of triconodontids (see Simpson 1928). Both roots of the tooth are broken and appear to have been buccolingually compressed, but the distal root was clearly larger than the mesial root. Between the roots, the inferior margin of the crown has a pronounced dorsal flexure. The crown is well preserved, with light apical wear on cusps a and c; cusp b appears to be damaged, but was clearly placed lower on the crown than cusp c. Cusp a is slightly recumbent distally, with sharp crests descending mesially and distally from its apex. The buccal face of cusp a is convex. Mesially, the crown is broad buccolingually; a faint bulge is present near the mesiobuccal base of the tooth, suggesting the presence of cusp f, but this region of the tooth is abraded and no further details are available. Distally, the crown tapers buccolingually; a stronger bulge is present at the distal base of the crown, though no cusp is developed. There is no evidence of a buccal cingulid.

Remarks.—OMNH 62855 is embedded in a rock fragment; it is fully exposed in lateral view, and the crown is visible in occlusal view. The principal cusp is slightly asymmetrical in lateral view, with the apex tilted toward the presumed distal end (a supposition supported by the relative heights of the flanking cusps: presumed cusp c is placed higher on the flank of the central cusp than presumed cusp b, as is generally the case in mesial premolars of triconodontids). In this view, the central cusp has a convex lateral face, but a cingulid is entirely lacking. This supports our conclusion that this tooth represents a lower right premolar. Though larger than the species represented by OMNH 62800, this premolar represents another species smaller than all previously described triconodontid taxa.

Figures

Fig. 6. Triconodontid eutriconodontan mammals from OMNH locality V1254, Lakota Formation (Lower Cretaceous: upper Valanginian–lower Barremian), South Dakota, USA. A–C. Camera lucida drawings (A 1–C1) and interpretive illustrations (A2–C2) of OMNH 62800, right lower molar in occlusal (A), lingual (B), and buccal (C) views. D. OMNH 62855, right lower premolar in buccal view. A–C courtesy of Nicholas J. Czaplewski.