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Abstract.-The fossil record has been used to show that in some geologic intervals certain traits of taxa may increase their survivability, and therefore that the risk of extinction is not randomly distributed among taxa. It has also been suggested that traits that buffer against extinction in background times do not confer the same resistance during mass extinction events. An open question is whether at any time in geologic history extinction probabilities were randomly distributed among taxa. Here we use a method for detecting random extinction to demonstrate that during both background and mass extinction times, extinction of marine invertebrate genera has been nonrandom with respect to species richness categories of genera. A possible cause for this nonrandom extinction is selective clustering of extinctions in genera consisting of species which possess extinction-biasing traits. Other potential causes considered here include geographic selectivity, increased extinction susceptibility for species in species-rich genera, or biases related to taxonomic practice and/or sampling heterogeneity. An important theoretical result is that extinction selectivity at the species level cannot be smoothly extrapolated upward to genera; the appearance of random genus extinction with respect to species richness of genera results when extinction has been highly selective at the species level.
Introduction
In recent decades increased attention has been paid to extinction in the fossil record. This research on extinction has been inspired by numerous factors including the availability of global, synoptic fossil databases (e.g., Sepkoski's unpublished genus compendium; the Paleobiology Database [Alroy et al. 2001]). The publication of the Alvarez et al. (1980) hypothesis of an impact-induced, catastrophic cause of extinction at the K/Pg boundary and the widespread belief that we are in a "sixth mass extinction" (e.g., Thomas et al. 2004), have increased the attention focused on mass extinction events. While there has been a dramatic increase in publications associated with the "Big Five" mass extinctions (Twitchett 2002), the community has debated whether mass extinctions form a distinct mode separate from "background" extinction (Bambach et al. 2004), or if they form part of a continuous distribution distinguished only by an arbitrary cut-off (Raup 1994; "continuity of magnitude" in Wang 2003). The general acceptance that at certain intervals of time mass extinctions occur, suggested by the weight of research on these intervals, prompts the...