EARLIEST CENOZOIC MAMMALIAN FAUNAL SUCCESSION IN THE SAN JUAN BASIN, NEW MEXICO

The Nacimiento Formation of the San Juan Basin (SJB) contains the most diverse, best sampled, and best documented middle (Pu2) and late (Pu3) Puercan mammal faunal assemblages in North America. Middle Puercan assemblages are from the Hemithlaeus Zone (="Ectoconus Zone") fossil horizon and late Puercan fossil assemblages are from the Taeniolabis Zone. Each zone is thin (< 10 m) and from a relatively limited geographic extent but found in direct superposition in the De-na-zin Wash area. Also, both zones are found in the same normal polarity zone (magnetochron 29n) and are thus separated by less than 0.7 Ma. We have recovered microvertebrate fossils using screenwashing from both zones (NMMNH localities 646 and 4723, respectively) that helps to correct for sample biases that favor larger, surface-collected specimens. Both middle and late Puercan assemblages of the SJB are similar (Simpson index=0.75), but middle Puercan assemblages are distinctly uneven and are dominated by two periptychids “condylarths,” Hemithlaeus kowalevskianus and Conacodon entoconus. Both of these taxa are absent from late Puercan assemblages. By contrast, the large multituberculate Taeniolabis taoensis is present and relatively common in late Puercan assemblages but absent in the middle Puercan. Differences between the middle and late Puercan small-bodied mammal components are minor.

The middle and late Puercan have not been distinguished in assemblages of the northern Western Interior. However, this may be attributed to poor temporal resolution, absence of index taxa, and resulting from reworking and mixing of middle and late Puercan faunas. Middle and late Puercan assemblages can be differentiated in the Ferris Formation of the Hanna Basin, southern Wyoming, but sample sizes are small. Other probable middle or late Puercan sites in the western interior are poorly known but suggest a high degree of provinciality during the middle and late Puercan.

The earliest Cenozoic has been characterized as a time of “explosive” diversification among mammals. However, the middle to late Puercan transition in the SJB shows a moderate drop in mammal diversity, especially among periptychid “condylarths” and therefore does not support a high rate of diversification for mammals during middle to late Puercan time.