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American Zoologist 1975 15(2):391-404; doi:10.1093/icb/15.2.391
© 1975 by The Society for Integrative and Comparative Biology
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Adaptive Strategy and the Origin of Grades and Ground-Plans

JAMES W. VALENTINE
Department of Geology, University of California Davis, California 95616

The ground-plans of higher metazoans seem to have originated chiefly in two waves, one near 700 million, the other near 580 million years ago. The first wave, involving the origin of the coelom, was probably associated with invasion of the substrate and the evolution of an infaunal community, while the second involved a reinvasion of the sea-floor surface and the development of an epibenthic fauna, for which skeletonization was a common adaptation. Each of these waves seems to represent adaptations to patterns of environmental variability—that is, they originate as adaptive strategies. Later waves of diversification tend to involve lower taxonomic categories but nevertheless appear to have been associated with changes in adaptive strategies.


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