Global Ordovician faunal transitions in the marine benthos: proximate causes

-During the Ordovician Radiation, domination of benthic marine communities shifted away from trilobites, toward articulate brachiopods, and, to a lesser degree, toward bivalves and gastropods. In this paper, we identify the patterns in origination and extinction probabilities that gave rise to these transitions. Using methods adapted from capture-mark-recapture (CMR) population studies, we estimate origination, extinction, and sampling probabilities jointly to avoid confounding patterns in turnover rates with temporal variation in the quality of the fossil record. Not surprisingly, higher extinction probabilities in trilobites relative to articulate brachiopods, bivalves, and gastropods were partly responsible for relative decreases in trilobite diversity. However, articulate brachiopods also had higher origination probabilities than trilobites, indicating that relative increases in articulate brachiopod diversity would have occurred even in the absence of between-class differences in extinction probabilities. This contrasts with inferences based on earlier Phanerozoic-scale, long-term averages of turnover probabilities, and it indicates that a major cause of this faunal transition has been overlooked. Sean R. Connolly.* Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Arnold I. Miller. Department of Geology, Post Office Box 210013, University of Cincinnati, Cincinnati, Ohio 45221-0013 *Present address: Department of Marine Biology, James Cook University, Townsville, Queensland 4811, Australia. E-mail: [email protected] Accepted: 3 April 2001