New Mexico Geological Society Annual Spring Meeting — Abstracts

Triisodontids are an archaic group of mammals that possess dental adaptations for carnivory, but are not members of the order Carnivora. They were probably apex mammalian predators for much of the early Paleocene. The wolf-sized species of Triisodon were the largest triisodontids and were also among the largest mammals of their time. A new specimen of the poorly known species Triisodon crassicuspis clarifies the diversity and evolutionary history of triisodontids and provides new morphological information that helps to untangle triisodontid interrelationships and aids in establishing the place of triisodontids on the mammalian tree of life.

Triisodon crassicuspis is one of the most poorly known mammal taxa named from the lower Paleocene Nacimiento Formation. The type specimen (AMNH 3178) consists of a lower jaw fragment with a partial m2-m3. Another species, Triisodon rusticus Cope, 1885, may be a synonym of T. crassicuspis, and is also poorly known, represented only by the holotype (AMNH 3225), a lower concreted jaw fragment with an m1-m2. Both specimens were collected by David Baldwin and lack precise provenance information. Some workers have suggested that T. crassicuspis should be transferred to the triisodontid genus Goniacodon. Williamson and Lucas (1997) described briefly some upper teeth (NMMNH P-19319) from the Chico Springs locale, a P3 and M1 that they referred to G. crassicuspis, and argued that the specimen possesses features that support its referral to Goniacodon. A new specimen of T. crassicupsis allows a reappraisal of this enigmatic taxon.

During the 2015 field season, a partial skull and skeleton (NMMNH P-72096) was recovered from basal Torrejonian age strata of the Nacimiento Formation in Kimbeto Wash. The specimen includes a nearly complete upper dentition (left C, P2-M3 and right C, P3, M1-3) a partial lower dentition (left c, p2, m3 and right c, partial m1, partial m2, m3) and fragments of the postcranial skeleton including a partial humerus, partial radius, and partial ulna.

The preserved lower molars are nearly identical to those of the type specimen of Triisodon crassicuspis. The upper teeth are essentially the same as Triisodon quivirensis, but about 20 percent smaller. NMMNH P-72096 thus clearly establishes that T. crassicuspis is a valid taxon that is referable to Triisodon rather than Goniacodon. It is also distinctly different from the Chico Springs taxon, which may represent a large species of Goniacodon. NMMNH P-72096 is similar in size and closely resembles the holotype specimens of T. crassicuspis and G. rusticus in preservation and possibly they also were collected from basal Torrejonian strata, which are not generally fossiliferous and poorly sampled.

References:

1. Cope, E.D., 1882. Synopsis of the Vertebrata of the Puerco Eocene epoch. American Philosophical Society Proceedings 20, 461-471.
2. Cope, E.D., 1884. Second addition to the knowledge of the Puerco epoch. Proceedings of the American Philosophical Society 21, 309-324.
3. Williamson, T.E., Lucas, S.G., 1997. The Chico Springs locality, Nacimiento Formation, San Juan Basin, New Mexico. New Mexico Geological Society Guidebook 48, 259-265.