PHYLOGENETIC CONSTRAINT AND ECOLOGICAL OPPORTUNITY IN THE CAMBRIAN AND ORDOVICIAN RADIATION OF ECHINODERMS

The Cambrian and Ordovician radiations were the most important events in echinoderm history, producing all major classes, many with peculiar and now-extinct morphologies. Although the taxonomic pattern 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 early Paleozoic echinoderm genera within a phylogenetic framework to test how the diversification played out across these complimentary perspectives of evolutionary history.

Ecological and morphological disparity both increase substantially from the Cambrian through the Ordovician, and their trends are strongly correlated. Increasing disparity persists in spite of dramatic changes in diversity during the two radiations, although the rate of increase in ecological disparity is dampened during the Ordovician.

Despite such overall similarities, resulting morphospace and ecospace structure suggests differences in how these diversifications unfolded. Morphospace structure is strongly phylogenetically constrained, with each taxonomic group clustering in distinct regions of morphospace and related higher taxa occurring in adjacent morphological regions. In contrast, these same taxa are over-dispersed in their life habits: clusters within the ecospace typically include members of distantly related genera. In other words, ecological convergence is more common than morphological convergence. In both data sets, the earliest echinoderms originate in central portions of their eventual morphospace and ecospace, expanding progressively throughout their history. With the exception of the morphologically distinctive but ecologically unremarkable homalozoans, now-extinct lineages also occur in more central portions of the echinoderm morphospace and ecospace, with the major extant classes occurring in more extreme locations.

Taken together, these patterns suggest that the earliest echinoderm diversifications involved concomitant increases in taxonomic, morphological, and ecological diversity. At the same time, the diversifications persisted in the face of important phylogenetic constraints and ecological opportunities that set the seed for later echinoderm evolution.