Evolution and the fossil record: patterns, rates, and processes

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Abstract
Mammals have an unusually good Cenozoic fossil record providing evidence of their evolutionary diversification. We view this record in hindsight, which biases our perception in many ways. Overall worldwide diversity appears to increase exponentially through time, while intensive sampling in local areas indicates that modern levels of diversity were achieved early in the Cenozoic. The evident significance of Pleistocene extinctions depends critically on how extinction rates are quantified. Our taxonomic hierarchy probably reflects the number of major faunal turnovers a group has survived rather than declining intensity of successive turnovers. Morphological innovation and taxonomic diversification appear following intervals of climatic cooling, suggesting that major features of evolution are extrinsically controlled. Favorable stratigraphic settings yield detailed records of gradual anagenesis and cladogenesis in mammals, with intermediates present as evidence of transition. The apparent dichotomy between high evolutionary rates measured by neontologists over short intervals of time and low evolutionary rates measured by paleontologists over long intervals of time disappears when rates are measured on intermediate scales of time. Microevolution and macroevolution are manifestations of common underlying processes expressed on different time scales.

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