Evolution and Development at the Origin of a Phylum.

Quantifying morphological evolution is key to determining the patterns and processes underlying the origin of phyla. We constructed a hierarchical morphological character matrix to characterize the radiation and establishment of echinoderm body plans during the early Paleozoic. This showed that subphylum-level clades diverged gradually through the Cambrian, and the distinctiveness of the resulting body plans was amplified by the extinction of transitional forms and obscured by convergent evolution during the Ordovician. Higher-order characters that define these body plans were not fixed at the origin of the phylum, countering hypotheses regarding developmental processes governing the early evolution of animals. Instead, these burdened characters were flexible, enabling continued evolutionary innovation throughout the clades' history.