New Mexico Geological Society Annual Spring Meeting — Abstracts

The Torrejonian North American Land-Mammal “age” (NALMA) was subdivided into “subages” named To1-To3 based largely on the succession of mammals in the Nacimiento Formation, San Juan Basin, New Mexico. The To2 interval is based primarily on the faunas of the “Deltatherium zone.” The To3 interval is based on the faunas of the Mixodectes pungens zone (=”Pantolambda zone”). The To2-To3 boundary is marked by the extinction of several genera including Deltatherium, Triisodon, Haploconus, and Ellipsodon. Several species make there first appearance include Mixodectes pungens and Pantolambda bathmodon. Some of the differences between the two faunas can be attributed to a collecting bias as the M. pungens zone has been sampled extensively for microvertebrates and the “Deltatherium zone” has not. Also, there is usually a distinct difference in preservation resulting in a taphonomic bias between the two zones. One aspect of this manifested on the abundant preservation of fossil mollusks in the M. pungens zone and their near absence in the underlying “Deltatherium zone.”

For part of the West Flank of Torreon Wash, however, the distinctive redbed facies of the M. pungens zones extends downward to include strata that fall within the “Deltatherium zone.” This results in the unusual association of typical “Deltatherium zone” fossils including Triisodon and Deltatherium with gastropods. Also, the rare pentacodontid Pentacodon occultus is reported from the “Deltatherium zone” for the first time based on NMMNH P-44597, a specimen consisting of the lower jaws and teeth of this taxon, from this area.