THE MACROEVOLUTION OF SIZE IN ECHINOIDS: CLADE SORTING AND INNOVATION

Size is a potentially important factor across scales of time and hierarchical levels, but broad empirical studies on the relationship between size and macroevolutionary patterns and processes have only recently appeared. This study explores this issue in the echinoderm class Echinoidea (sea urchins). Maximum size estimates for approximately 1000 species spanning 400 million years were collected from the literature and surveys of museum collections. All major groups were sampled, and robust estimates of ordinal-level phylogenetic relationships were used as a guide in data analysis.

Regularities were found in the evolution of the overall size distribution. During the Paleozoic, the distribution expands with increase in the mode, range, and variance, but it later contracts in the Permian and Triassic. The Triassic recovery happens in the small size range, with return of the mode to the smallest size class. The mode persists in the early Jurassic as the distribution expands. From the middle Jurassic to the Neogene, the shape of the distribution and the mode remain relatively stable.

Despite these regularities, substantial discordance between size variance and diversity accrues through time and within major clades. Size also shows no clear correlation with clade sorting between the two major sister clades of the post-Paleozoic radiation. Despite marked differences in diversity, size statistics are very similar. Higher diversity in the clade that includes irregular echinoids might be related to the higher post-metamorphic allometric variability, and dissociability, that the evolution of more localized differential plate growth allowed. Major innovation, in contrast, appears to be associated with small size in both regular and irregular echinoids. Among 12 orders with good preservation potential, 10 originate at small size. Within orders, a tendency is also found for the founders to be smaller than the median size in the descendant pool. Collectively, these results suggest that microevolutionary expectations may not uniformly apply to macroevolutionary patterns. Instead of linearly deploying the size spectrum for ecological advantage alone, echinoids seem to have nonlinearly explored the potentialities of contingency and constraint in the production of diversity and disparity.