ECOLOGICAL AND MORPHOLOGICAL DISPARITY BOTH INCREASE DURING THE EARLY PALEOZOIC DIVERSIFICATION OF ECHINODERMS

The early Paleozoic was a pivotal interval in the evolution of most major phyla. Echinoderms were major components of early Paleozoic ecosystems and diversified during both the Cambrian Explosion and the Great Ordovician Biodiversification Event, ushering in a large number of higher taxa, most of which are extinct today. Although the pattern of taxonomic diversification is well established, it remains unclear to what extent the evolutionary diversification was accompanied by ecological and morphological changes. Here, we use independent compilations of the morphology and life habits of approximately 370 early Paleozoic echinoderm genera to test how the diversification played out across these complimentary dimensions of diversity.

Ecological and morphological disparity both increase significantly from the Cambrian to the Ordovician. The post-Ordovician sample is small, but suggests continuing increases in morphological and ecological disparity into the Silurian and possibly Devonian. These trends are robust to sample standardization and the use of different metrics of morphological disparity and functional diversity. The disparity trends also generally coincide with similar increases in genus diversity. Individual echinoderm classes (including crinoids, eocrinoids, and edrioasteroids) generally exhibit similar increases, implying a shared cause across clades rather than simple replacement of clades. Important ecological changes during the interval include increases in body size and new foraging and diet habits but few changes in microhabitat usage and mobility. Taken together, these patterns suggest that early Paleozoic echinoderm diversification involved parallel increases in taxonomic, morphological, and ecological diversity associated with novel evolutionary and ecological specializations.