Species richness in marine benthic habitats through the Phanerozoic

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Abstract
The distribution of numbers of species and the median number of species from 386 selected fossil communities are tabulated for high stress, variable nearshore, and open marine environments during the Lower, Middle, and Upper Paleozoic, the Mesozoic and the Cenozoic. The number of species always increases from high stress to variable nearshore to open marine environments. Within-habitat variation in number of species is small for long intervals of the Phanerozoic. The median number of species in communities from high stress environments remains fixed at about 8 from the Cambrian to the Pleistocene. In open marine environments, the median is near 30 for the Middle and Upper Paleozoic and almost the same for the Mesozoic. Increases of 50% in median number of species between the Lower and Middle Paleozoic and 2 times between the Mesozoic and Cenozoic occur in open marine environments with parallel, but less pronounced, increases in variable nearshore environments. Conditions controlling overall within-habitat species richness changed at those times. These changes do not correlate directly with evolution of new major taxa, change in physical conditions, predation, space availability or oxygen supply. They may be related to changes in resource availability influenced by factors such as the developing terrestrial flora, to lag-time inherent in the evolutionary process of diversification, or to as yet undetermined factors. Although provinciality determines total species richness for the biosphere, the within-habitat data suggest that the number of marine invertebrate species in the world has increased since the Middle Paleozoic, contrary to Raup's (1976b) contention, but possibly only by about 4 times, not the order of magnitude or more suggested by Valentine (1970).