MORPHOLOGIC EVOLUTION AND DISPARITY IN CAMBRIAN ECHINODERMS

Documenting and interpreting the establishment of body plans is pivotal in unraveling the long-term evolutionary history of a clade. Unfortunately, quantifying morphology is difficult especially at higher taxonomic levels. This difficulty is further exaggerated given a wider array of morphologies, increased taphonomic differences, and expanded sample size at the phylum rather than class or order-level. A novel character suite that encompasses the features and body plans found in Early Paleozoic echinoderms was constructed. This character suite was used to analyze 246 Early Paleozoic echinoderms including a comprehensive sampling of Cambrian genera. As expected, a Principal Coordinate Analyses of this dataset produced a morphospace that recovered the three major body plans found within echinoderms (i.e. pelmatozoans, homalozoans, and eleutherozoans). Pelmatozoans and eleutherozoans occupied large areas within morphospace, while homalozoans appear morphologically conservative. The occupation of morphospace and the disparity of echinoderms changed substantially through the Cambrian. The area occupied in morphospace and disparity (measured as average squared distance) increased dramatically from Cambrian Series 2 to 3, coinciding with large ecological shifts including a decrease in the proportion of genera attached to hard substrates along with an increase in those feeding at the sediment water interface. Disparity continued to rise in the Furongian, which is the result of an increase in the stereotypy of body plans rather than the exploration of new forms. The variance in form during the Furongian was not exceeded during the Ordovician despite the large increase in biodiversity, although the area occupied in morphospace expanded with the origination of new classes of echinoderm. It may be surprising that disparity plateaued before the appearance of iconic Paleozoic echinoderm classes (e.g. crinoids, blastoids, or echinoids). However, major components of those body plans such as stalks composed of columnals, organized theca, and arms all appeared during the Cambrian.