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My research focuses on vertebrate paleoecology and evolution, though I always try to put my studies of vertebrates in environmental context through analysis of ancient ecosystems and climates. My work often includes isotopic or biochemical analysis of animal tissues (teeth, bones, fur, skin, etc.) or environmental samples (soil minerals, fossil plants, etc.).

In recent years, I've continued to work on Cenozoic environmental evolution, something I began as a post-doc. Working with students and colleagues around the globe, I've done a great deal of work on the chronology and ecological consequences of the Paleocene-Eocene boundary thermal maximum (PETM) and other less hyped climatic events in the early Cenozoic. Since 2000, much of this work has been in Asia, where we've shown that the first appearance of many "modern" orders of mammals (e.g., primates, perrisodacyls) is essentially coincident with the PETM, as is the case on other northern continents. My former post-doc David Fox and I studied the origin of the prairie ecosystem in North America. My students and I have begun to explore the topographic history of the western United States using a variety of different isotopic tools. Finally, we have also begun to reconstruct Holocene oceanographic changes along the California coast through biogeochemical records from mollusk shells.

Another major focus has been detailed reconstruction of the paleoecology of different groups of vertebrates. My students and I have studied the diets, habitat preferences, and migratory behaviors of many taxa. In some cases, we've focused on evolutionary ecology, for example investigating the stages in the transition from terrestrial to aquatic habitats in cetaceans (dophins, whales, and their kin) and sirenians (manatees, sea cows, etc.). Another example is our ongoing work on the evolution of salt-water tolerance in crocodilians. In other cases, we've worked at an ecosystem scale, exploring how different members of a community exploit food resources or space. Much of this latter work has focused on Pleistocene communities. We've studied diet breadth or habitat preferences in many taxa (teratorns, dire wolves, horses, hippos, proboscideans, bison, mole rats, etc.) and the migratory patterns of mammoths and mastodons. My original motivation was to study the ecology of these systems to understand the factors leading to their late Pleistocene collapse, but in recent years, I've focused more on how these ancient communities "worked" prior to their restructuring by this extinction.

An important related thread has been reconstruction of the prior ecological roles of extant, endangered species through study of their Pleistocene or Holocene fossils. My students and I have a number of ongoing projects on seals and sea lions, lemurs, condors, and wolves. In some cases, our work has shown that species behaved very differently in the recent past. We've tried to understand why behaviors have changed, and whether or not species might be able to recover their lost ecological potential.

Finally, a great deal of work in my lab is now devoted to ecological studies of modern animals. In some cases, these studies are designed as "ground-truthing" excercises to set the stage for paleoecological studies. In a growing number of cases, however, there are no "paleo" connections. Rather, ecologists studying modern species have discovered that the biogeochemical methods developed to study ancient animals can provide insights into the ecology of living species, especially ones with behaviors or habitats that make them hard to observe or track (e.g., migratory birds, marine mammals, deep forest species). This began with my early work on the ecology of living elephants, but has expanded greatly in recent years, especially in the work of my graduate students and colleagues, to include African carnivores, lemurs, sharks, bears, and many different kinds of seals and sea lions.