Latest Ordovician extinctions: one disaster after another

Abstract

Abstract Between the end-Cambrian crisis and the next major mass extinction in the latest Ordovician, trilobites continued to flourish. However, their predominance in benthic communities was usurped by the increasingly diverse groups that belong to Sepkoski’s (1981) ‘Palaeozoic fauna’. These included articulate brachiopods, bryozoans, graptolites, condones, echinoderms, and the rugose and tabulate corals. Various mollusc orders(such as the nautiloids, bivalves, and gastropods) also diversified, although to a lesser degree than the Palaeozoic fauna. Peak Ordovician diversity coincided with the mid-Caradoc sea-level high stand which appears to have marked an all-time Phanerozoic high; both basinal black shale and platform carbonate deposition occurred over vast areas at this time (Thickpenny and Leggett 1987; Cocks and Fortney 1988; Brenchley 1989; Barnes et al. 1995). Ordovician palaeogeography was undoubtedly a contributory factor to this diversity maximum because the continents each contain their own endemic fauna (Fig. 3.1).