Dietary ecology and community paleoecology of early Tertiary mammals
An understanding of the relationships and lineage ranges among Early Tertiary mammals is established, but a quantitative characterization of community-level changes as been slower to develop. I focused on the dietary ecology of mammals as the major criterion to describe their communities. Dietary hypotheses were proposed using stereoscopic observation of enamel microwear. I applied this method to 23 species of living carnivorans in order to relate those diets to their microwear. I recognized diets in microwear state space that are different from those known for herbivores. This method reliably discriminates diets of larger species. Microwear of the slicing carnassial teeth has to be evaluated in the light of masticatory differences among species. Styles of "omnivory" can also be identified with microwear.
I next applied this microwear method to a group of almost 90 species of Paleogene mammals. Most were browsers or mixed feeders, but some show divergence toward grazing. Microwear indicators of durophagy was recovered in some groups. Most archaic groups described were omnivorous, but their microwear reflects different types of omnivory.
The Eocene-Oligocene transition (37-30 Mya) was a critical period in mammalian history characterized by climatic cooling and drying. The White River Group (WRG) in North America is a long-term record of this transition. I combined my dietary characterizations with body size to establish feeding guilds Eo-Oligocene faunas. I found that the early Eocene faunas were dominated by browsing guilds with only a few species apparently specializing on grass. By the Chadronian, both grazing and browsing guilds were established, but both were dominated by a large mixed feeder group, in keeping with the open woodland of the time. This basic structure was sustained through the Orellan and Whitneyan of the WRG in a surprisingly consistent form. However, a contemporaneous Chadronian fauna in southwestern Montana is known to contain very different proportions of herbivores; I found that this fauna was dominated by browsers. The surprising degree of stasis in the WRG though this substantial climate change interval is probably the result of the fauna's initial assembly from the survivors of pre-Chadronian extinctions.