LATE CRETACEOUS THROUGH EARLIEST TERTIARY EVOLUTION OF MULTITUBERCULATE MAMMALS IN THE SAN JUAN BASIN OF NEW MEXICO, AND THEIR BIOGEOGRAPHIC CONTEXT

The San Juan Basin of New Mexico is significant to studies of mammalian evolution in North America for at least two reasons: first, although not all strata produce mammalian fossils and erosional disconformities are present, mammalian assemblages of Campanian through Paleocene age, spanning a major mammalian extinction event, are present in superposition in a restricted geographic area. Also, there are few latest Cretaceous mammalian faunas in the southern portion of the Western Interior, so the San Juan Basin assemblages are key data.

Late Campanian multituberculates, dated to between 74.56 and 74.11 Ma, include an unpublished collection from KU, as well as preliminarily published collections at MNA and both published and substantial new collections at the NMMNH. The fauna is similar to those of the Kaiparowits and Aguja Formations, but includes new and possibly endemic species.

The Naashoibito Member of the Kirtland Formation has not yielded a date independent of its vertebrate assemblage. The presence of the uniquely Lancian taxa Essonodon and Glasbius indicate a latest Cretaceous age, however. Both presence/absence comparisons and relative abundance comparisons show a mammalian fauna that differs from those of the Lance and Hell Creek Formations of Wyoming and Montana. The faunal samples are of such unequal size, however, that statistical results are questionable. It is possible that provinciality in the Lancian was less marked than in the Late Campanian.

Puercan (earliest Paleocene) multituberculate assemblages from the Nacimiento Fm. include endemic taxa, one of which is morphologically startling. Presence/absence and relative abundance comparisons again set them apart from northern faunas.

The Campanian, Lancian, and Puercan multituberculate assemblages are distinct from one another, which is unsurprising as they are separated by hiatuses in the rock record. The Lancian and Puercan assemblages include taxa from lineages present lower in the section, but the Puercan assemblages also include several “alien” taxa that do not have sister taxa represented in the Late Cretaceous of the San Juan Basin – nor do any sister taxa appear to be present in other southern faunas. This pattern is similar to that previously documented in Montana, even though the faunal composition differs.