EARLY PALEOCENE CLIMATE CHANGE AND BIOTIC RECOVERY OF PLANTS AND MAMMALS OF THE SAN JUAN BASIN, NEW MEXICO: A PRELIMINARY REPORT

The early Paleocene was a critical time in the evolution of mammals and the modernization of ecological communities. This interval marks the initial biotic recovery following the Cretaceous-Paleogene (K-Pg) mass extinction. It also represents the beginning of the explosive radiation of mammals, when mammals radiated and filled newly opened ecological niches. Large questions remain about the importance of external influences, such as climate, on recovering ecosystems following mass extinctions. Using fossils from the early Paleocene Nacimiento Formation of the San Juan Basin, New Mexico, we are testing the relationship between species diversity and turnover in mammals, and climate following the K-Pg mass extinction. The Nacimiento Formation is the ideal place to test these relationships because it contains the most complete, diverse, and longest record of early Paleocene mammal evolution known anywhere in the world, spanning nearly four million years from about 65.5 – 62 million years ago.

Within the lower Paleocene Nacimiento Formation we find that the therian mammal record contains several episodes of turnover (elevated appearance and/or disappearance rates). The highest levels of turnover coincide with the Puercan-Torrejonian boundary at ~64.6 Ma and the middle-late Torrejonian (To2-To3) boundary at ~62.6 Ma. The timing of the Puercan-Torrejonian boundary turnover correlates with a cooling trend observed in deep sea marine record. Mean annual temperature estimates using several Puercan and Torrejonian fossil floras also suggest a slight cooling roughly coincident with this mammalian turnover event. Stable isotopes from the tooth enamel of two closely related periptychids (archaic ungulate mammals), Carsioptychus coarctatus (Puercan) and Periptychus carinidens (Torrejonian), show stable or slightly increasing δ¹³C values in the late Torrejonian, when atmospheric δ¹³C values appear to have been decreasing as inferred by the marine benthic record. This suggests drying in the late Torrejonian. Overall, these results suggest that climate change in the early Paleocene may have had significant impacts on the local environments and on mammalian evolution in western North America during the early phase of the Paleogene mammal radiation.