EVOLUTION OF ECHINODERM BODY SIZES THROUGHOUT THEIR PHANEROZOIC HISTORY

Body size has increased in many animal lineages, but our knowledge of body-size evolution is less well understood for echinoderms, one of the major marine phyla. In this poster, we document trends in body size evolution for 932 echinoderm genera spanning the Phanerozoic, representing all major taxa (more than 20 classes, 90 orders, and 350 families). Body size is measured in three dimensions for the metabolically active body parts (e.g., theca, calyx, test) for intact specimens. In cases where a body length measure is unpreserved or not available, a taxonomically-based algorithm is used to estimate the missing length based on the shape of the closest taxonomic relative with that length. Where possible, separate measurements are compiled for stem length, arm/brachiole/aulacophore length, and other ecologically meaningful body sizes. The three length measurements are converted to body volume using an allometric equation established for marine invertebrates. Trends in body size are analyzed using maximum likelihood methods to test whether the trends are consistent with common evolutionary models.

Throughout the Phanerozoic, body size tends to increase significantly across echinoderms, a pattern most consistent with a driven (general random walk) trend. The trend is relatively unaffected by mass extinctions. Increasing trends are also evident within individual classes (asteroids, rhombiferans, and Paleozoic crinoids) and most orders with large sample sizes (primarily crinoid orders), although the magnitude of statistical support for driven models declines within these smaller taxa. Edrioasteroids show a significant decreasing trend in body size. Eocrinoids increase in size, ophiuroids fluctuate substantially, and stylophorans and diploporitans are relatively unchanged through time, but these trends are most consistent with random walks. Echinoids appear to show a trend of stasis during the post-Paleozoic. Collectively, these results suggest that the net increase in echinoderm body size throughout the Phanerozoic results from the accumulation of a diverse set of body size dynamics within individual echinoderm taxa.