Paper No. 2
Presentation Time: 8:20 AM


ORCUTT, John D., Department of Geology, Cornell College, 600 1st St. SW, Mt. Vernon, IA 52314,

The Pleistocene fossil record is an invaluable tool for observing how organisms have responded to past climate change and for predicting how they might react to similar change in the future. While the Pleistocene record is remarkably complete, it is not without bias, and recognizing and accounting for this bias is necessary in any paleoecological analysis. In the case of North American mammals, understanding the influence of caves on the fossil record is especially important. Caves can preserve extraordinarily rich faunas that often differ from those found above ground nearby, in particular by preserving large numbers of small taxa. The fidelity of cave faunas to local mammal communities tends to be high relative to surface sites, but the importance of caves to our understanding of Pleistocene ecology makes it crucial to recognize not only any preservational differences that do exist between the two but the degree to which our understanding of the fossil record as a whole is biased by these differences. As a case study to address these questions, I have calculated relative abundances of different mammal taxa from Rancholabrean sites in California and Nevada based on specimens in museum collections. My results confirm the finding that small mammals are more frequently preserved in caves, which contain large numbers of rodents and mustelids. However, body size alone cannot account for all the differences between cave and surface faunas, as several large taxa (e.g. caprine artiodactyls, bears, mastodons, and the ground sloth Nothrotheriops) are significantly more abundant in cave faunas, while many small taxa (e.g. lagomorphs, talpids, and the rodents Microtus and Thomomys) are more abundant in surface deposits. These preservational discrepancies can be explained by ecological and behavioral differences between taxa (for example, animals that used caves for dens, such as bears and Nothrotheriops, are much more likely to be preserved underground than are their surface-dwelling relatives). The record of the region addressed in this study is heavily influenced by data from caves, underscoring the importance of taking into account the effect of paleobiological variables on the Pleistocene fossil record when drawing conclusions about paleoecological change on a large scale.