DECIPHERING THE ROLE OF CRINOIDS IN LATE ORDOVICIAN ECHINODERM DISPARITY

Morphological diversity is a useful tool in analyzing evolutionary patterns. Disparity quantifies changes in biological form including appearances of new traits, alterations to earlier features, and the loss of characters through extinction. Studying disparity at a higher taxonomic level is complicated because of the vast array of forms to be characterized. However, these studies can provide a broader understanding of body plan evolution. Echinoderms are ideal for this purpose in that that they are taxonomically, morphologically, and ecologically diverse.

Approximately 370 Cambrian and Ordovician echinoderm genera were examined using a character suite that encompasses the features found within the phylum. Echinoderms showed a steady rise in disparity through the Cambrian followed by a plateau during the Ordovician. In the Late Ordovician, crinoids represent 30% of the overall echinoderm disparity despite high class-level diversity. Without crinoids, echinoderm disparity would have peaked in the Late Cambrian and declined through the Ordovician. Additional analyses were performed to determine what aspects of crinoid morphology led to their large contribution to echinoderm disparity.

We hypothesized that the morphological distinctiveness of crinoids was associated with characters from specific body regions, such as circlets, ray structures, tegmen and anal structures, arms, and stalks. To test this, we sequentially reduced the number of characters for each of the above body regions to determine if the de-emphasis of body regions resulted in diminished crinoid contribution to overall disparity. The de-emphasis of stem and arm characters showed results that were similar to random character loss. However, the removal of characters associated with rays, anal structures, and to a lesser extent circlets reduced the partial disparity of crinoids. Collectively, the removal of calyx characters reduced the crinoid proportion of overall echinoderm disparity by over 40%. The remaining crinoid contribution to echinoderm disparity is likely from unique combinations of characters that are applicable for other echinoderm groups rather than the presence of crinoid specific characters. To further explore this pattern, we will explore which crinoid groups or genera are influential to Late Ordovician echinoderm disparity.