THEORETICAL AND REALIZED MORPHOLOGY IN EARLY PALEOZOIC ECHINODERMS

Exploring the patterns and mechanics involved in the establishment of animal body plans is fundamental in understanding long-term evolutionary dynamics. Given the numerous intrinsic and extrinsic variables involved in body form evolution, testing hypothesized patterns is difficult. One method to explore these patterns is to construct a theoretical morphospace, examine how it changes with the addition of constraints, and determine what proportion has been explored by a given lineage.

A novel character suite was constructed that encompasses the diverse features present within Early Paleozoic echinoderms. In addition, the character contingencies were mapped in order to better capture morphological patterns. This character suite has been previously used to examine trends in morphology in 110 fossil echinoderms. Characters were randomly generated to build a theoretical morphospace of approximately 10,000 individuals with similar proportions of present, absent, non-applicable, and missing data found in fossil data. These modeled organisms occupied a large area in morphospace that far exceeded the range of morphologies seen in fossil echinoderms. When morphological constraints, based on the character contingencies, were added to the theoretical morphspace, the area occupied (defined by the convex hull) decreased by sixty percent. Large areas of this theoretical morphospace were not explored by Paleozoic echinoderms. However, multiple lineages occupied areas at the fringe of plausible morphologies given the character suite. This result is consistent with the early exploration of morphospace and exhaustion of character combinations, which would predict an increased level of convergence as younger taxa are included within the analysis.