South America was an island continent from the end of the Cenozoic until 2.7 million years ago, when the Isthmus of Panama connected South America with North America, leading to the most dramatic biotic interchange in the fossil record. The interchange was not balanced between the two continents; half of modern South American mammals are of North American decent, but only ten percent of modern North American mammals are of South American decent. Hypotheses to explain this pattern include phylogenetic superiority of North American mammals, easy invasion of South America due to its long isolation, and differential timing of dispersal north during glacial minima and dispersal south during glacial maxima. We examined the body size and ecological characteristics of mammals that descended from participants and non-participants in the interchange. We compare body size distributions directly, and use decision tree modeling to evaluate the importance of different factors. 

Results/Conclusions

We find that South American species are significantly less likely to cross, but factors related to dispersal vary between continents. For North American species, small-bodied species are more likely to disperse, as are certain feeding guilds and orders. For South American species, body size is by far the most important factor in dispersal, with dispersing species mostly having larger body size. South America has a lower proportion of carnivores in the mammalian community, which may have contributed to the mammal species assemblage. In ongoing work, we hope to compare the characteristics of the original dispersers with the characteristics of the mammal communities descended from them, in order to differentiate between dispersal and in situ speciation in the data.