Climate and Ecological Change in Oligo-Miocene Mammals
Orcutt, John D.
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Orcutt, John D.
Whether or not a causal relationship exists between climate and mammal body size is one of the longest-standing and most intractable questions in ecology. The classic model of body size evolution (Bergmann's Rule) holds that body size is driven by temperature, but more recent hypotheses have suggested that other climatic variables or biotic interactions may play a more important role. The use of paleoecological data to address this question allows variables that are tightly correlated in modern ecosystems to be teased apart and allows body size patterns to be observed through time, adding an extra dimension to analyses. This dissertation details the findings of two paleoecological tests of Bergmann's Rule in the Oligo-Miocene (30-5 Ma), one tracking body size and climate through time in the northwestern United States and another tracking geographic body size trends through time along the west coast of North America. In both cases, body size was analyzed in three representative families of mammals: equids, canids, and sciurids. Such large-scale analyses are dependent on fossils that can be placed in a reliable taxonomic, geologic, and temporal context, and this dissertation also focuses on a reevaluation of the canid fauna of Oregon's Juntura Formation that places a critically important Late Miocene carnivore fauna in just such a context. Two genera of canids - Epicyon and Carpocyon - are described from the fauna for the first time, with important implications for regional biostratigraphy. The body size analyses show no consistent relationship between body size and any climatic variable. Further, body size patterns vary widely between taxa at several levels, suggesting that one universal driver of body size evolution does not exist. Not only is there no evidence for Bergmann's Rule in Oligo-Miocene mammals, but comparative analyses of geographic body size patterns in the modern genera Odocoileus, Canis, and Spermophilus fail to show the latitudinal gradients upon which Bergmann's Rule is predicated. The apparent existence of such trends in some taxa may be the result of anthropogenic extirpation at low latitudes, further underscoring the importance of including paleontological data when formulating models predicting the response of biotic variables to environmental change.