LYELLIAN ANALYSIS OF THE GREAT AMERICAN FAUNAL INTERCHANGE IN FLORIDA

By forming a narrow land bridge between two previously long-isolated continents, the tectonic uplift of the Isthmus of Panama some three million years ago opened a biogeographic corridor that resulted in a remarkable intermingling of North and South American mammals. The dense Plio-Pleistocene fossil record of Florida offers an exceptional test of several important aspects of this dramatic evolutionary experiment in temperate North America. Lyellian curves were plotted from over thirty temporally well-constrained fossil mammalian faunas (also including the huge flightless terror bird Titanis) of North and South American ancestry in Florida; because of the famous terminal Pleistocene mass extinction that primarily affected the terrestrial vertebrate megafauna, the beginning of the late Wisconsin glacial interval was used to conclude the curves instead of the Recent. Estimates derived from these plots suggest that South American stocks have a shorter average species longevity (2.6 MY) than their North American counterparts (3.4 MY), and therefore a higher mean rate of extinction. As the Lyellian curve for North American natives generally lies above that for South American immigrants, these findings likely reflect genuine termination of lineages rather than artifacts of pseudoextinction. Furthermore, eight out of twelve faunas common to both plots have higher Lyellian percentages of native species, which is significant by the Sign Test (P = 0.039). Such a disparity in extinction rates is consistent with the classic double-wedge pattern of competitive displacement. Multiple lines of evidence, however, favor alternative explanations for elevated extinction among the immigrants. These include the extremely specialized nature of the edentates (e.g., the heavily armored glyptodonts and the giant ground sloths) stemming from their low basal metabolism, the restricted geographic ranges of most immigrant families, and the documentation from the well-studied late Pleistocene Rancholabrean faunas that taxa of South American origin tend to be relatively large-bodied.