The Plio-Pleistocene Ground Sloth Megalonyx (Xenarthra: Megalonychidae) : an Investigation of Variation, Evolution, and Taxonomic Implications

Access restricted to the OSU CommunityVariation in specimens of extinct taxa complicates attempts to interpret the fossil record. The factors that contribute to morphological plasticity are numerous and sometimes difficult to discern, especially when attempting diagnosis of fossil species. An extensive review of the literature and examples are presented from the late Cenozoic fossil record of the southeastern United States with emphasis on ground sloths. One particularly productive fossil locality is the late Irvingtonian "Camelot" site in the Coastal Plain of South Carolina. Over 250 teeth and postcranial elements of the ground sloth Megalonyx have been recovered. Morphometric data from this sample suggest that the population was in a state of transition towards the larger size that characterized the later Rancholabrean forms. Quantitative characters exhibit considerable overlap, and qualitative characters are not adequate to distinguish species. One character that remained unchanged in the Plio-Pleistocene history of Megalonyx was hypsodonty, or crown height of the teeth. Although there are many factors that contribute to hypsodonty and affect its interpretation, statistical analysis revealed that there was no significant change in this character during the ontogeny or phylogeny of Megalonyx during the time studied. The primary change in Megalonyx was a general increase in size. Patterns of evolution were assessed by calculating ratios of size increase and rates of evolution for various skeletal elements over a ca. two-million year period. To minimize the possible effects of Bergmann's Rule, only samples from the southeastern United States were used. Results demonstrated that Megalonyx underwent variable rate phyletic change during the Plio-Pleistocene. While most skeletal elements displayed an isometric increase in size, some characters of limb bones showed more dramatic changes and at higher rates of evolution, most likely to support the increasing body mass. Many of the ground sloth species of the genus Megalonyx that were proposed over the last two centuries were relegated to junior synonym status by McDonald (1977), who suggested that there were three valid species of Megalonyx during the late Pliocene and Pleistocene epochs. However, a recent analysis suggests that quantitative data are insufficient to distinguish Irvingtonian and Rancholabrean forms, and proposed qualitative characters are highly subjective. Graphic representations of morphometric data and Principal Components Analysis revealed no significant differences between M. wheatleyi and M. jeffersonii, and it is therefore recommended that the two forms be recognized as a single species, M. jeffersonii