Measuring changes in articulate brachiopod morphology before and after the Permian mass extinction event: do developmental constraints limit morphological innovation?

The pattern of decreasing disparity has been observed in both the metazoans and metaphytes throughout the Phanerozoic. The pattern is manifest as a decreasing trend in the origination of higher taxa. Currently, two competing evolutionary hypotheses have been proposed to explain this phenomenon: the empty ecospace hypothesis and the developmental constraint hypothesis. To empirically distinguish between these hypotheses, the change in disparity before and after the end-Permian mass extinction event was measured in the articulated brachiopods. The assumption is that ecospace-limiting constraints are removed after mass extinctions revealing the effect of developmental constraints. For each taxon within the group, both continuous and discrete character sets were analyzed. Four different measures of disparity were used to analyze each character suite. Additionally, a separate analysis was performed on a subset of the articulated brachiopods, the rhynchonellids and terebratulids. In most cases investigated, disparity rebounded to comparable levels, with the rhynchonellids and terebratulids showing the largest increase in disparity after the end-Permian extinction, a clear example of an increase in disparity without a significant increase in taxonomic diversity. The results indicate that developmental constraints may not be responsible for the decreasing disparity in this group. The more likely scenario is that increasingly structured ecological guilds have made it much more difficult for large increases in disparity to occur.