The Green River Formation is known for its well-preserved fossil fishes. The best known vertebrate remains have been collected from the fossil lagerstätten of the Fossil Basin over the last 130 years. However, these fossils exist largely as a chronologically isolated fauna, biostratigraphically unconstrained within the Early Eocene. Although very rare components of the fauna, mammals can provide significant biostratigraphic constraint for dating the units within the Green River Formation using the framework of North American Land Mammal Ages (NALMA). In general, mammal fossils (both from within the Green River Formation itself and from the fluvial deposits of the adjacent units) allow approximate correlation of components of the Green River Formation to the established terrestrial biostratigraphic framework of the NALMA. More specifically, recently identified remains of Lambdotherium from the Fossil Butte Member of the Green River Formation firmly places the majority of the Fossil Basin quarries within the Lambdotherium-interval zone which corresponds to the Lostcabinian subage of the Wasatchian NALMA (Late Wasatchian, Wa₇; Early Eocene). Using radiometric and paleomagnetic data from correlative units allows us to date these quarries as having been deposited between 52.2 and 52.7 mya. This period of Cenozoic time is generally characterized by the most equable Tertiary climate in North America (the Cenozoic Global Climate Optimum) and the turnover of the mammalian communities from archaic to modern groups. In addition, these mammal fossils can provide important information on the paleoenvironments and taphonomic processes of the Green River lakes and their peripheries.