SUCCESSIVE MULTITUBERCULATE MAMMAL ASSEMBLAGES IN THE CAMPANIAN – EARLY PALEOCENE OF THE SAN JUAN BASIN, NEW MEXICO

The San Juan Basin of Northwestern New Mexico preserves Campanian through Paleocene faunas in a single stratigraphic section and geographic area. We have sampled Campanian, Maastrichtian, and Paleocene faunas from this area by means of underwater screening. By using a consistent screen mesh aperture of .05 mm, we have minimized collecting biases that may result from surface collection or from usage of different screen mesh sizes by various scientists.

Among other results, our work enables a comparison of multituberculate mammal assemblages through the K-T boundary interval in this area. Localities dated between 74.56 and 74.11 Ma in the Fossil Forest and Bisti Badlands produce a Campanian multituberculate assemblage similar to that of the Kaiparowits Plateau, although with less diversity within the genus Paracimexomys. Temporal constraint of the Naashoibito Member of the Kirtland Formation is less precise, but the presence of the multituberculate Essonodon browni and the probable presence of the dinosaur Tyrannosaurus rex suggest that it is correlative with the Lancian Land Mammal “Age” (~68-65.5 Ma). Essonodon is very common in this stratum; Mesodma, a cimolodontid, and a fourth multituberculate are also present. Middle and late Puercan (earliest Paleocene) mammal-bearing localities reflect greater neoplagiaulacid diversity, including endemic Neoplagiaulacidae. When compared to northern Puercan assemblages, the multituberculate Stygimys is conspicuously absent. Also, the genus Cimexomys is absent from both the Maastrichtian and Paleocene of the San Juan Basin.

On the basis of our work we draw the following broad conclusions: (1) The Campanian, Maastrichtian, and earliest Paleocene multituberculate faunas of the San Juan Basin are distinct from one another. The genus Mesodma may be the only taxon they have in common; (2) The Maastrichtian multituberculates do not appear to be ancestral to those in any Puercan multituberculate assemblages in North America, including those of the San Juan Basin; (3) Intriguingly, the Maastrichtian and Puercan faunas lack many multituberculates with broadly spaced, pyramidal molar cusps and short lingual cusp rows on the upper first molar. This molar morphology, exemplified by Cimexomys and Stygimys, is common in correlative localities at higher latitudes.