Reconstruction of Latitudinal Mammalian Diversity Gradients In the Mid-Paleocene of Western North America

Decreasing species richness with latitude is a major feature of North American mammal biogeography today and is linked to climatic gradients, but the long-term history of the relationship is not well documented. Mammalian diversity during the Paleocene in North America has been examined at basinal and continental scales, but not the regional scale necessary to examine latitudinal gradients. We compiled mammalian taxonomic data from published literature, online museum collection databases, and the Paleobiology Database for roughly 160 Torrejonian (To) and Tiffanian (Ti) (middle Paleocene, ~63-57 Ma) North American fossil mammal localities within a zone from 29-54°N and 102-115°W. Most localities have been extensively collected, some for several decades. Rarefied abundance data were plotted against paleolatitude to examine richness gradients.

The zone ranging from southern Wyoming to southern Montana is both the most species rich and the most even, although this region also has the greatest exposure of mid-Paleocene terrestrial sediments and number of To-Ti mammalian localities, many with long collecting histories. A negative relationship between latitude and species diversity is observed for the Torrejonian, while the opposite pattern is observed for the Tiffanian. However, the predictive power of latitude for determining number of species is extremely low in both cases (r² of 0.018 and 0.0005, respectively). The San Juan Basin saw a decrease in diversity across the To-Ti boundary while diversity increased across the boundary in southern Wyoming. Diversity did not change significantly over this interval north of about 50° latitude. Using generic data, both the Torrejonian and Tiffanian exhibit negative gradients with no significant increase in the r² values. If estimates of mid-Paleocene species richness are accurate, it suggests a different relationship between richness and latitude compared to the modern pattern in western North America, perhaps reflecting a shallower latitudinal temperature gradient related to a difference in climate.