34 MILLION YEARS OF CONSISTENCY IN FUNCTIONAL DIVERSITY OF NORTH AMERICAN MAMMAL PALEOCOMMUNITIES

Identifying and understanding community assembly rules is a central goal of ecology and is critical to predicting the effects of anthropogenic impacts on future biodiversity. Although mammals are well-studied, general rules underlying their community assembly are as yet not well understood. Functional diversity has emerged as a useful method to quantify and compare the structure of communities across time and space because it uses ecological traits rather than taxonomic identity. Moreover, functional diversity allows for inferences about a community’s health and resiliency to disturbances. Modern mammalian functional diversity varies by region with the highest values in New World temperate regions and at the equator in the Old World. However, modern communities are affected by climate change and other anthropogenic impacts, which may be influencing our interpretations. Here, we use the fossil record to quantify the functional diversity of North American mammal communities prior to large scale human impacts. We compiled a dataset that includes 167 North American mammal paleocommunities spanning the Cenozoic. Four species traits (locomotion, diet, body mass, and life habit) that are commonly used in modern studies were compiled for the fossil taxa in our communities and used to evaluate functional diversity and identify periods of transition in community structure. Segmented Regression analysis reveals 3 breakpoints in community structure associated with major events in mammalian evolution history: 1) during the rapid immigration and diversification of the early Cenozoic (~56 Ma), 2) coincident with the saturation of mammalian body size niche space (~42 Ma), and 3) near the point of archaic order extinction (~30 Ma). A prolonged period of consistency in functional diversity begins ~34 Ma and lasts until 5000 years ago. That levels of functional diversity remained similar for 34 million years suggests that there are fundamental rules underlying mammal community assembly. Global climate and the community proportion of archaic orders are not significantly related to paleocommunity functional diversity on this scale. Our work provides an important baseline for understanding how modern anthropogenic disturbances may be affecting the functional diversity and assembly of modern mammal communities.