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.

The author describes forty-two fossil taxa recovered during a study of the San Timoteo Badlands that used magnetobiostratigraphy to develop a temporal framework for addressing the tectonic evolution of southern California over the last 6 million years. For the Pliocene, small mammals are an effective means of correlating a magnetostratigraphy to the Geomagnetic Polarity Time Scale when radioisotopic dates are unobtainable.

Microtine history indicates mosaic evolution and complex dispersal patterns around the Northern Hemisphere ; by reflecting this history and evaluating stage of population evolution, microtine biochronology can discriminate time periods as brief as 5,000 years.