Biotic crises in the history of Upper Silurian graptoloids: A Palaeobiological model

Four major biotic crises are recognized in the history of the Upper Silurian graptoloids. While the causes of extinction are largely unknown, the scenarios of the crises are fairly well ascertained. A model of recovery proposed here implies two alternatives: (1) some of the relic species exhibit great abundance frequently associated with a subnormal phenotype (the post‐event syndrome) and the ensuing rediversification involves indigenous speciation or (2) as a result of the lack of evolutionary response on the part of the local survivors, vacant habitats are filled by immigrants. Numerous cryptic and Lazarus taxa among the immigrants indicate the presence of pelagic refuges and centers of evolution. They may be identified as Central Water masses bounded by gyres.