There are extinctions and extinctions: examples from the Lower Palaeozoic

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Abstract
The extinction events at the Cambrian-Ordovician and Ordovician-Silurian boundaries are compared and contrasted. A simple theoretical model shows that times of increased cladogenesis produce elevated rates of taxonomic pseudoextinctions, according to the recognition of paraphyletic groups. Taxonomists have traditionally placed stratigraphically early and morphologically primitive members of clades into paraphyletic groups. The Cambrian-Ordovician boundary coincided with such a period of cladogenesis. Extinctions occurred among shelf taxa: deeper-water taxa were mostly unaffected. The various explanations that have been proposed to explain Cambrian-Ordovician extinctions are evaluated. The Cambrian-Ordovician boundary event was probably similar to ‘biomere-type’ events that preceded it in the Cambrian and followed in the Ordovician. However, the rapid, but apparently staggered appearance of major new taxa at this time elevated taxonomic pseudoextinctions. In contrast, the Ordovician-Silurian extinction event terminated many major clades. An important ‘oceanic’ event (or events) profoundly affected outer- to off-shelf taxa (including plankton), some having had long and stable histories. The late Ordovician glaciation produced changes in shelf taxa, but changes in brachiopod faunal composition were spread over a long time compared with that for oceanic events. The likely role of anoxia in explaining deeper water end-Ordovician extinctions at the time of deglaciation is discussed.