Since the birth of modern geology, Cenozoic deposits have commonly been dated according to Lyellian percentages (proportions of living species within fossil faunas), but this approach has never been tested. To remedy this situation, we have calculated Lyellian percentages for gastropod and bivalve faunas of Japan and western North America. When plotted against geologic age (determined by independent methods), the percentages for each class form a narrow band that reveals similar patterns of extinction for the two regions and an absence of major pulses of extinction. The plots thus seem to represent useful “standard” curves but display enough spread of data to cast doubt on the value of Lyellian percentages to chronostratigraphy.

Our plots do have many other useful applications: (1) They reveal differences in regional patterns of extinction; in particular, they document a Pleistocene mass extinction of Mollusca in the confined seas of the tropical Americas and an unusually low rate of extinction in the enormous tropical Indo-Pacific region. (2) They indicate that gastropods are characterized by higher rates of extinction than bivalves. (3) They provide a mechanism for evaluating differences in rates of extinction between discrete subgroups of taxa, such as tropical species and temperate species, or species with planktic larvae and species with nonplanktic larvae.

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