Relations of Eocene volcaniclastic rocks and Eocene fossils date the sedimentary and structural evolution of the Southeast Absaroka Range, clarify correlations with adjacent areas, an aid in timing large-scale events of gravity faulting.

Initially, this work was designed to document and study the diversification of modern mammalian groups and was quite successful and satisfying. However, as field and laboratory work continued, there began to develop a suspicion that not all of the Eocene story was being told. It became apparent that most fossil samples, especially those from the American West, were derived from similar preservational circumstances and similar depositional settings. A program was initiated to look for other potential sources of fossil samples, either from non-traditional lithologies or from geographic areas that were not typically sampled. As this program of research grew it began to demonstrate that different lithologies and different geographic areas told different stories from those that had been developed based on more typical faunal assemblages. This book is conceived as an introduction to non-traditional Eocene fossils samples, and as a place to document and discuss features of these fossil assemblages that are rare or that come from rarely represented habitats.

This book places into modern context the information by which North American mammalian paleontologists recognize, divide, calibrate, and discuss intervals of mammalian evolution known as North American Land Mammal Ages. It incorporates new information on the systematic biology of the fossil record and utilizes the many recent advances in geochronologic methods and their results. The book describes the increasingly highly resolved stratigraphy into which all available temporally significant data and applications are integrated. Extensive temporal coverage includes the Lancian part of the Late Cretaceous, and geographical coverage includes information from Mexico, an integral part of the North American fauna, past and present.