Morphological diversification of Paleozoic crinoids

Abstract

Several metrics, including average difference among species, range of occupied morphological space, and number of character-state combinations, are used to investigate morphological diversification in Paleozoic crinoids. Despite several phases of taxonomic diversification, the maximal level of disparity reached in the Ordovician remained essentially unsurpassed. Although new regions in morphological space were occupied after the Devonian, these were not as extensive as those that had been evacuated prior to the Carboniferous. This discordance between extensive total morphological change and limited net change further supports previous arguments for the importance of morphological constraints in crinoid evolution. Major changes in the occupation of morphological space correspond with changes in taxonomic diversity within certain higher taxa. The extent to which advanced cladids (Poteriocrinina) appear to expand into new morphological space is exaggerated by the large number of very similar species in this group. If fewer species are sampled, by considering only those forms that differ from each other by at least some prescribed amount, poteriocrines appear to be less extreme morphologically. In contrast, other groups that seem to occupy unique regions in morphological space continue to do so even if fewer of them are sampled. Major crinoid clades—Camerata and Cladida+Flexibilia—do not show the same evolutionary pattern as Crinoidea, but instead exhibit a more gradual diversification of morphology. This observation provides additional support for the existence of qualitative differences among taxa of different rank.