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This study documents diversity decline in a once-speciose rodent clade, the family Aplodontidae, and evaluates the potential influence of three commonly suggested controls on diversity: climate change, floral change, and competitive interactions. Aplodontids first appeared in the late Eocene, diversified during the early Oligocene, declined precipitously at the end of the Oligocene such that standing diversity was only about 5 species during the early Miocene, peaked again in the early middle Miocene, then declined through the late Miocene, and are entirely absent from the Pliocene and early Pleistocene fossil record. This long term pattern culminated in the survival of a single extant species, Aplodontia rufa, the mountain beaver. The species’ richness and body size distribution through time were compared with the timing of climatic changes as inferred from global oxygen isotope curves, with the rise of grasslands as inferred from phytolith and other stable isotope studies, and with fluctuating diversity of potential competitors as inferred from published stratigraphic and geographic distributions. The timing of global climate change is decoupled from the diversity fluctuations and seems not to have been a proximate cause. Rise of grasslands and the increasing dominance Of C-4 vegetation correlates with diversity decline in the late Oligocene and late Miocene, but data are sparse, and more work will be required to determine the mechanism driving this relationship. Examination of potential mammalian competitors (sciurids and castorids) finds no evidence for competitive replacement of aplodontids. It is difficult to ascribe the fluctuations in aplodontid diversity to a single cause. The explanation likely involves vegetation changes associated with the spread of grasslands, but there is some variation in diversity that cannot be explained by the vegetation, at least using the proxies employed here. Climate and competition are less consistent with the available data. The reasons for the decline of aplodontids in the late Oligocene and the late Miocene apparently involved the interaction of multiple physical and biological causes, coupled with the chance events that underlie any evolutionary process. (c) 2006 Elsevier B.V. All rights reserved.