EARLY PALEOCENE TERRESTRIAL ECOSYSTEMS IN THE SAN JUAN BASIN, NEW MEXICO, USA

The San Juan Basin (SJB) in New Mexico, USA contains one of the best early Paleocene records of mammalian evolution, making it an ideal location to examine ecosystem response and change following the Cretaceous-Paleogene (K-Pg) extinction. This record is punctuated by intervals of substantial faunal turnover; however the timing of some of the turnover events is relatively poorly constrained. Also, little is known about the early Paleocene plant communities, paleoenvironments or paleoclimate. This makes it difficult to assess the timing of faunal stasis vs. change and to discern if the intervals of faunal turnover were driven by extrinsic factors such as climate change or intrinsic factors such as rapid evolution following the K-Pg extinction. We present new age constraints and an assessment of plant communities and paleoclimate for early Paleocene fossil bearing intervals in the SJB.

Magnetostratigraphy and ⁴⁰Ar/³⁹Ar sanidine dates indicate that in the SJB the middle Puercan land mammal age (Pu2) began within ~350 ka of the K-Pg boundary and only ~150 ka separated the type Pu2 and Pu3 faunas. Floral collections from Puercan and early Torrejonian strata indicate that these early Paleocene floras were dominated by angiosperms and were relatively diverse. Interestingly, these floras are comprised of taxa that appear to be endemic to the SJB and are significantly more diverse than contemporaneous floras from the Northern Great Plains. Puercan floras also appear to be considerably different from Torrejonian floras, suggesting the possibility of synchronous turnover of plant and animal communities. Analyses from fossil leaves and from stable carbon and oxygen isotopes from mammalian tooth enamel suggest little variability in paleoenvironment or paleoclimate between the Puercan and Torrejonian. These results indicate that ecologically complex mammal and plant communities were present locally within less than a few hundred thousand years after the K-Pg boundary and that the earliest Paleocene faunas, and possibly floras, were relatively short-lived and turned over rapidly. Further, similar climatic conditions in the Puercan and Torrejonian suggest that plant and mammal turnover may be related to factors intrinsic to the SJB and/or immigration, rather than significant environmental or climatic change.