THE CONTRIBUTION OF CRINOID SUBCLASSES TO ECHINODERM DISPARITY DURING THE ORDOVICIAN RADIATION

Macroevoltuionary patterns can be interpreted through the use of morphological diversity (disparity). Disparity illustrates shifts in body plans through the arrival of new forms, changes to earlier forms, and the loss of features due to extinction. Tracking morphological patterns at the phylum level is difficult considering the large variety of forms. However, investigating patterns at high taxonomic levels allows a broader view of body plan evolution. Echinoderms are a diverse phylum ecologically, taxonomically, and morphologically. Within echinoderms, crinoids are the most distinctive and common group within the Paleozoic.

Approximately 400 genera of Cambrian and Ordovician Echinoderms were coded using a character suite that focuses on the features found within the phylum. This was used to explore trends in disparity through time, in particular the relative contribution of different echinoderm groups to overall morphological diversity. Crinoids were a major contributor to Early Paleozoic echinoderm disparity representing approximately one third of the morphological diversity in the Ordovician. Without the presence of crinoids, the disparity of echinoderms would have plateaued during the Ordovician radiation. The lack of morphological expansion in other echinoderm groups may be the result of either developmental constraints or delayed morphological diversification (e.g. after the Ordovician radiation).

The partial disparity of Ordovician crinoids was further divided into the major subclasses. Following the origin of crinoids in the Early Ordovician, protocrinoids and monobathrids contributed over half of the morphological disparity of crinoids. The short-lived but high contribution of protocrinids is expected given their complex and unorganized theca. The disparity of monobathrids contracted through the Ordovician, which was then replaced by that of diplobathrid camerates mirroring their ecological dominance in the Late Ordovician. The disparity of cladids, disparids, and the other minor groups remained fairly consistent throughout the Ordovician radiation. Further research will examine how patterns of disparity change during the macroevolutionary transition in crinoids after the Late Ordovician Mass Extinction.