Differentiation of generic extinction rates among Upper Ordovician-Devonian articulate brachiopods

Abstract

The data on the longevity of genera of articulate brachiopods for the Upper Ordovician-Devonian interval (Boucot 1975) are averaged for the taxa of each order. The generic longevity-frequency distributions form three significantly different clusters. The orthids have the highest mean generic longevity value and display a semi-log survivorship profile that is more convex than the other orders. The spiriferid and strophomenid orders have mean generic longevity values higher than and slopes in the survivorship curves less steep than the pentamerids, rhynchonellids and terebratulids. The appreciable differences in the inferred generic extinction rates for the clusters are attributed to the heterogeneity of the ecologies of the orders. The pentamerids had a proportionately higher number of genera concentrated in narrow-niched reefs wherein higher extinction rates depressed the mean generic longevity of this order relative to the orthids, strophomenids and spiriferids. Enhanced longevity of many offshore (non-reef) genera possibly explains the greater mean generic longevity of the orthids, spiriferids and strophomenids relative to the rhynchonellids and terebratulids which included a proportionately higher number of genera concentrated in nearshore habitats. Alternatively, the depressed mean generic longevity values for the rhynchonellids and terebratulids may reflect systematic oversplitting and temporal bias, i.e., origination and initial diversification of order.