DIVERSE EARLY PALEOCENE FOSSIL FLORA FROM THE OJO ALAMO SANDSTONE, SAN JUAN BASIN, NEW MEXICO, USA: IMPLICATIONS FOR LOCAL AND REGIONAL RESPONSES TO THE CRETACEOUS-PALEOGENE EXTINCTION EVENT

Earliest Paleocene fossil floras from across North America have been characterized as being low diversity and dominated by long lived, cosmopolitan, mire adapted species. These patterns in floral composition have been hypothesized to be a response to the Cretaceous-Paleogene (K-Pg) mass extinction. However, this pattern of response to the K-Pg extinction is based primarily on studies from terrestrial basins in Northern Great Plains of North America, and relatively little is known about floras from more southern basins. This lack of data from southern North American limits regional comparisons of floral diversity, composition, and plant community response to the K-Pg boundary. The San Juan Basin (SJB), located in northwest New Mexico, contains a nearly complete record of the earliest Paleocene with abundant fossil leaf localities making it an ideal place to fill this in the gap in the record. Here we present a description of earliest Paleocene floras and paleoclimate from the SJB, and then compare the SJB floral record to those from the Northern Great Plains.

Fossil leaves were collected from the earliest Paleocene Ojo Alamo Sandstone, which corresponds with the Puercan 1 (Pu1) NALMA and was deposited in polarity C29r, indicating an approximate age of 66.0-65.7 Ma for these leaf localities. Fifteen leaf localities were collected, and census collections were made at 5 of the sites. The Ojo Alamo flora is dominated by angiosperms, relatively diverse, dominated by species that appear to be endemic to the SJB, and displays variability in morphotypes occurrences between facies. Leaf physiognomic paleoclimate analyses indicate the Puercan 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 within <~300 kyr of the K-Pg boundary. Further, these results demonstrate significant differences in species composition of northern and southern floras and a potentially large north-south diversity gradient, which suggests variable responses to the K-Pg extinction event across North America.