Smith et al. (2003, hereafter referred to as Smith et al.) present seven new isotopic ages from the Green River Formation in the greater Green River Basin. These data provide welcome new information in the ongoing effort to develop a coherent and reliable geochron ological framework for this large and stratigraphically complex basin. Several of their main conclusions are based on a revised age estimate for the Wasatchian-Bridgerian North American Land Mammal Age boundary. For instance, Smith et al. use the revised age estimate for the Wasatchian-Bridgerian boundary to argue that the onset of maximum temperatures during the early...

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