EARLY PALEOCENE PLANT AND MAMMALIAN COMMUNITIES FROM THE SAN JUAN BASIN (NEW MEXICO, USA) RECORD LONG TERM ECOSYSTEM INSTABILITY FOLLOWING THE CRETACEOUS-PALEOGENE BOUNDARY

There are few high-resolution early Paleocene records of both plants and mammals in the same location inhibiting our ability to assess ecosystem response and plant and animal community change following the K-Pg boundary. The San Juan Basin (SJB), located in northwestern New Mexico, preserves nearly complete and well-dated early Paleocene plant and mammalian record making it an ideal location to study patterns of turnover in early Paleocene floral and faunal communities. Here we investigate early Paleocene ecosystem change using a coupled high-resolution record of plant and mammalian communities from the SJB.

Plant and mammalian fossils were collected from the lower Paleocene Ojo Alamo Sandstone and lower Nacimiento Formation spanning the initial ~1.5 Myr of the Paleocene. Two intervals of high floral turnover, the first from ~65.6 – 65.3 Ma and the second from ~65.2 – 64.7 Ma, were identified associated with increased rates of extinction and decreased rates of origination, respectively. The mammalian fossil record also documents two turnover events contemporaneous with floral turnover - an interval of elevated extinction at ~65.3 Ma associated with the Puercan (Pu) 2 – Pu3 North American Land Mammal Age (NALMA) turnover and a turnover in community composition from ~65.2-64.6 Ma associated with the Pu3 – Torrejonian (To) 1 NALMA transition - suggesting shared mechanisms. The driver of the first turnover event (Pu2-Pu3) is unclear due to no evidence for significant change in climate, basin evolution, or other extrinsic factors, suggesting that this turnover event may be a coupled ecosystem response to the K-Pg boundary possibly driven by intrinsic factors. The second turnover event (Pu3-To1) was plausibly driven by extrinsic factors, primarily decreasing mean annual temperature and precipitation coupled with evidence of accelerated sediment accumulation rates which altered local environments. Additionally, there is evidence for the migration of plant and mammalian taxa from northern North America into the SJB contemporaneous with the second turnover event. Taken together, these results indicate linked the plant and mammal community response in the SJB following the K-Pg boundary driven both by intrinsic and extrinsic factors leading to long-term ecosystem instability in early Paleocene.