EARLY PALEOCENE FLORAS FROM THE SAN JUAN BASIN, NEW MEXICO, USA: IMPLICATIONS FOR LOCAL AND REGIONAL RESPONSES TO THE CRETACEOUS-PALEOGENE EXTINCTION EVENT

The San Juan Basin (SJB) of New Mexico, USA preserves arguably the best record of early Paleocene mammalian evolution, spanning the Puercan and Torrejonian North American land mammal ages (NALMA). The lack of floral data from the SJB limits paleoclimatic and paleoenvironmental reconstruction and prevents an assessment between the mammalian and plant records. Further, although early Paleocene fossil floras are well known from the Northern Great Plains of North America, relatively little is known about floras from southern North America. The lack of southern North American data limits regional comparisons of floral diversity, composition, and plant community responses to environmental change. We present a description of early Paleocene floras from the SJB and a comparison of SJB floras to floras from the Northern Great Plains.

Fossil leaves were collected from the lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation, with a focus on localities correlative to the early Puercan (Pu1) to early Torrejonian (To1) NALMAs which have been dated to ~65.8-64.0 Ma in the SJB. These floras are dominated by angiosperms, diverse (average site diversity = 32.5), dominated by species that appear to be endemic to the SJB, and display variability in morphotypes occurrences between facies. The taxonomic composition of Puercan floras appears to be distinct from Torrejonian floras suggesting a possible large scale change in plant communities in the earliest Paleocene that may be correlative with a major turnover in the mammalian fauna. Paleoclimate analyses indicate the Puercan and Torrejonian floras sample a warm and relatively wet climate, which remained similar throughout the early Paleocene. Interestingly, the SJB floras are significantly more diverse than age equivalent Northern Great Plains floras and indicate a warmer and wetter climate. These results indicate that a diverse and largely endemic plant community existed in the SJB shortly after the Cretaceous-Paleogene (K-Pg) boundary and that these floras may have experienced a rapid turnover coincident with the Puercan-Torrejonian turnover in mammalian faunas. Further, these results suggest a significant difference between northern and southern floras and a large north-south diversity gradient, which may suggest a variable response to the K-Pg extinction event.