Article: Patterns of diversification and extinction in early Palaeozoic echinoderms

It has been claimed that Cambrian and early Ordovician echinoderms show two phases of diversification as recognized by Sepkoski for marine taxa in general. However, a more critical assessment of the record, using cladistic analyses of all determinable echinoderm genera to identify sister group relationships, allows sampling deficits to be taken into consideration. It is shown that the Upper Cambrian dip in diversity amongst echinoderms is likely to be the result of preservation failure and that diversification more closely approximates to a continuous process of expansion during the Cambrian and early Ordovician. Taxonomic diversity cannot be used as a measure of morphological diversity because rank has been applied for a number of different and incompatible reasons. There is no evidence to support the claim that morphological evolution was occurring significantly faster during this period compared with later periods.