Evolution and ecology of body size in North American terrestrial mammals during the Paleocene and Eocene transition

One of the largest turnovers in mammalian history occurred during the transition from the Paleocene to the Eocene. Paleocene communities in North America were comprised of small endemic archaic mammals. In the late Paleocene and early Eocene modern mammals immigrated into North America from Asia. The arrival of modern immigrants was followed by a decline in archaic mammals. Numerous studies have argued for climate and/or competition with modern mammals being potential causes behind the decline and eventual extinction of archaic mammals. We examined the effects of the changing climate and faunal turnover on the structure of North American mammalian communities. To represent structure we use the distribution of body sizes, estimated using lower first molar areas. Using maximum likelihood model fitting we identified periods of stasis and change points in the community structure, taxonomic composition, and climate. We found that an initial change in community structure correlated with the change in taxonomic composition. The other changes points in structure are correlated with changes in climate suggesting climate played a role in the restructuring of communities. We used two non-parametric statistical tests; the Mann-Whitney U-test and Kolmogorov-Smirnov test, to confirm the placement of change points. The tests also showed that the multiple significant changes in structure occurring during the arrival of modern mammals suggesting a gradual change in community structure. Non-Metric Multidimensional Scaling (NMDS) was used to characterize communities. NMDS showed that the communities of the Paleocene were stable for most of the Paleocene while the communities of the Eocene were gradually changing. We specifically looked at the effects of the transition in one archaic group, Plesiadapiformes. Plesiadapiformes’ decline has been hypothesized to be caused by competition with the modern immigrants. Maas et al 1988 found that within plesiadapiformes the superfamily Plesiadapoidea was in competition with rodents but found no evidence to support the same for a second superfamily, Microsyopoidea. Soligo 2006 found a trend towards increasing body size which he found to not be a result of changing climates. To understand how competition may have affected plesiadapiformes evolution we tested the evolutionary dynamics of body size for plesiadapiformes as a whole and for the superfamilies Plesiadapoidea and Microsyopoidea separately. We found that plesiadapiformes show higher extinction of larger bodied species after the arrival of rodents. Within Plesiadapiformes, Plesiadapoidea and Microsyopoidea show differences in their rates and mode of evolution suggesting different pressures acting upon them. The transition from the Paleocene to the Eocene represents a major faunal reorganization within mammalian communities. Using body sizes we were able to better understand the ecology and evolution of mammals during the Paleocene and Eocene