Variance and taxonomic selectivity were studied as functions of time across the Phanerozoic for duration of genera and families in the fossil record and for habitat of fossil families. The variance of duration increases temporarily before mass extinctions but otherwise decreases across the Phanerozoic, and the variance of habitat increases to an asymptote. For both duration and habitat, the percentage of variance explained by differences among orders and classes shows no temporal trend, whereas the percentage of variance explained by differences among phyla decreases across the Phanerozoic. The latter decrease depends upon the time elapsed since the origination of classes within phyla in the early Paleozoic; a similar decrease appears in differences among classes if the analysis is restricted to orders that originate before the Silurian. The observed patterns can be described by bounded random walks that include some infrequent but large steps.

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