Abstract

Suturing of Laurussia and Africa, elimination of easterly tropical currents, and consequent equatorial diversion of cold and possibly dysaerobic waters are suggested as prime causes of the Frasnian/Famennian (Late Devonian) extinction. These are explained by using a new paleogeographic model. Evidence for stepdown extinctions that took place from Givetian through Frasnian time is taken particularly from atrypoid brachiopods, one of the most abundant representatives of the global tropical shelly marine benthos during the Middle and Late Devonian. Only 5 of 17 Frasnian genera or subgenera of atrypoids are known to be present in late Frasnian sediments, and possibly only a single genus, Spinatrypa, was present at the Frasnian/Famennian boundary. Evidence for stepdown extinction from other invertebrate and vertebrate groups in general corroborates that provided by brachiopods.

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