Consideration of the Phanerozoic fossil record, made up predominantly of level-bottom, marine, benthic organisms, indicates that it may be subdivided into twelve ecologic-evolutionary units. Within each of these units the varied community groups of level-bottom organisms maintain their generic integrity from beginning to end of the time interval. But, the species of the more endemic, more stenotopic genera tend to evolve in a manner most easily interpreted as phyletic, and time-sympatric. Using these data one may then provide for better evaluations of diversity changes through geologic time that are less biased by measurements because they do not always accord with ecologic-evolutionary unit boundaries. Correlations with a number of classes of geologic variables and the boundaries of the ecologic-evolutionary units at present do not appear to be strong enough to be termed causal with much confidence. Further work might change this situation.--Modified journal abstract.

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