Research has shown that there was dramatic climate change, specifically a significant drop in temperature, across the Eocene – Oligocene (Chadronian – Orellan) boundary. However, few studies have looked at the effects of this climatic cooling on the terrestrial vertebrate fauna. My study focused on changes in mammalian body size across the Chadronian – Orellan transition, in particular within the small artiodactyl Leptomeryx and the large rhinocerotine perissodactyls, based upon fossils from the White River Group in the Northern Plains (Colorado, Wyoming, Nebraska, and South Dakota). Specifically, I tested whether Bergmann's Rule (which suggests that in a given taxon, those individuals inhabiting colder climates will be larger-bodied than those living in a warmer climate) occurred with climatic cooling across the Chadronian-Orellan boundary. With regard to the rhinocerotines, I used Trigonias, Subhyracodon, and specimens identified as Rhinocerotinae gen. indet. Rhinocerotineae gen.indet. specimens are missing their anterior dentition (which is what differentiates teeth of Trigonias from Subhyracodon), and consequently were simply identified only as Rhinocerotinae genus indet. Length and width measurements of upper and lower molars were used as a proxy for body size of both Chadronian and Orellan representatives of two lineages of Leptomeryx, (L. speciosus – L. evansi lineage and L. yoderi – L. mammifer – L. exilis lineage). The two lineages of Leptomeryx showed a statistically significant decrease in tooth size in the Orellan. The Rhinocerotinae show a significant decrease in tooth size across the Chadronian – Orellan transition. Although other researchers have noted that only one Leptomeryx lineage survived the Eocene-Oligocene transition, this research suggests that both lineages survived this boundary. The decrease in body size with decrease in temperature in Leptomeryx and Rhinocerotinae is opposite of the expected pattern of Bergmann's Rule. My data suggest that other factors, besides climatic cooling, also need to be considered in mammalian body size evolution across the Eocene-Oligocene boundary.
|Commitee:||McCain, Christy, Stucky, Richard|
|School:||University of Colorado at Boulder|
|School Location:||United States -- Colorado|
|Source:||MAI 48/03M, Masters Abstracts International|
|Subjects:||Evolution and Development, Paleontology, Museum studies|
|Keywords:||Bergmann's rule, Eocene-Oligocene transition, Leptomeryx, Subhyracodon, Trigonias|
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