Book excerpt: 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.