Dentition of the Early Eocene primates Niptomomys and Absarokius

Abstract

The mandibular dentition of Niptomomys doreenae was previously known only from an edentulous mandible preserving alveolae for all teeth, and jaw fragments preserving P4 and M1-3. A new mandible of Niptomomys is described here which preserves an enlarged, lanceolate lower incisor and a small, blunt, single-rooted P3. The incisor morphology confirms placement of Niptomomys in the Family Microsyopidae. The presence of a single-rooted P3 invalidates the previous interpretation of the lower dental formula. Comparison with the related early primates Navajovius, Palaechthon, Plesiolestes and Uintasorex shows the lower dental formula of Niptomomys to be 1.1.3.3. The total number of teeth in the mandible of Absarokius was previously determined to be eight (except for a single specimen of Absarokius near A. abbotti which Gazin, 1962, suggested might possibly have nine). Two mandibles of A. abbotti described here clearly had nine teeth and a lower 2 dental formula of 2.1.3.3. The upper canine and P2 of this species are also described here for the first time. Comparison of the new specimens of A. abbotti with the later A. noctivagus noceri demonstrates that the tooth previously interpreted in the latter taxon as P2 is in fact the canine, thus the lower dental formula of A.n. noceri is 2.1.2.3, not 1.1.3.3. Absarokius abbotti, with a dental formula of I2?2, C11, P33, M33 seems clearly to be derived from a species of Tetonius