Diverse early Paleocene megaflora from the Ojo Alamo Sandstone, San Juan Basin, New Mexico, USA

Abstract

Earliest Paleocene megafloras from North America are hypothesized to be low diversity and dominated by long-lived cosmopolitan species following the Cretaceous-Paleogene (K/Pg) mass extinction. However, megafloras used to develop this hypothesis are from the Northern Great Plains of North America, and relatively little is known about floras from southern basins. Here, we present a quantitative analysis of a diverse earliest Paleocene megaflora (<350 kyr after K/Pg boundary) from the Ojo Alamo Sandstone in the San Juan Basin (SJB), New Mexico. The megaflora, comprised of 55 morphotypes, was dominated by dicotyledonous angiosperms, with accessory taxa composed of ferns, monocotyledonous angiosperms, and conifers. The majority of morphotypes were restricted to the SJB indicating the presence of an endemic flora in the earliest Paleocene. Diversity analyses indicate that the flora was species rich, highly uneven, and laterally heterogeneous. Paleoclimate estimates using multivariate and univariate methods indicate warm temperatures and high precipitation. When compared to contemporaneous floras from the Denver Basin (DB) of Colorado and the Williston Basin (WB) of North Dakota, the SJB flora had significantly higher species richness, site rarefied richness, and basin rarefied richness, but was significantly less even. Paleoclimate estimates from the SJB were 7-14 °C warmer than the DB and WB, indicating a shift from a temperate deciduous forest biome in the Northern Great Plains to a tropical seasonal forest biome in the SJB. These results demonstrate that the SJB flora is markedly different from floras in the Northern Great Plains, that there were latitudinal differences in species composition and diversity, and that there was a variable floral response to the K/Pg boundary in North America during the earliest Paleocene. We hypothesize that the warm, wet conditions in the earliest Paleocene SJB drove rapid rates of speciation following the K/Pg boundary resulting in a heterogenous, endemic flora.