A TALE OF TWO CONTINENTS: ECOLOGY, PHYLOGENY, AND THE GREAT AMERICAN BIOTIC INTERCHANGE

South America was an island continent from the end of the Cenozoic until 3 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. The most prominent hypothesis to explain this pattern, proposed by G. G. Simpson, is phylogenetic superiority of North American mammals and easy invasion of South America due to its long isolation. Other hypotheses include superiority of placental mammals, predator naiveté among South American mammals, 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. We use data for genera rather than species in the decision trees to dampen the effects of speciation since the interchange.

The most important factors determining whether genera crossed the land bridge were continent of origin and order, strongly supporting the phylogenetic superiority hypothesis. However, not all North American genera were more likely to disperse, and not all South American genera were unlikely to disperse. Body mass was also an important factor in dispersal, suggesting an interaction of ecological factors such as range size and dispersal ability with order. We therefore conclude that certain orders were more likely to disperse, but not strictly for the reasons that Simpson envisioned when he proposed the phylogenetic superiority hypothesis. We hope in further research to elucidate the ecological factors leading to the differences between orders.