Patterns of speciation in JurassicGryphaea

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Abstract
It is shown that the abundant Jurassic oysterGryphaeaprovides material adequate to test biogeographic and evolutionary hypotheses involving speciation. Phyletic size increase is widespread if not characteristic and tends to be marked by a sharp initial increase followed by a longer period of stability. The larger size of phyletically younger forms is apparently because of greater age of individuals rather than a higher growth rate. Species durations fall within the range of the majority of Jurassic bivalve species. The detailed study of EuropeanGryphaeaindicates a pattern of punctuated equilibria allied with morphological trends, some of which are paedomorphic. Gradualistic, species selection and cladistic models of change are not supported. World-wide analysis ofGryphaeaspecies supports centres of origin and migration rather than vicariance, and a relationship exists between migrational and evolutionary events and changes in the physical environment.