EXPLORING PHYLOGENIC RELATIONSHIPS AMONG ORDOVICIAN CRINOIDS

Application of Universal Elemental Homology on stellate echinoderms has demonstrated that crinoids are nested within the blastozoans; thus blastozoans are used to root crinoids and test among competing hypotheses for the early divergence of crinoids. Phylogenetic analyses were run on 40 Tremadoc to Darriwilian (Ordovician) crinoids using more than 100 unweighted characters (in excess of 470 character states) with blastozoans as the outgroup. Analyses with taxa from only the Tremadocian to Floian and Tremadocian to Dapinigian were also completed. All analyses confirm five basic groupings of taxa: camerates, “normal” cladids, disparids, hybocrinids, and a grouping of cladids and other crinoids with non-standard plating. Relationships among these groupings indicate that camerates are basal from which cladids arose, and disparids and hybocrinids originated from within cladids. Contrasting analyses indicate that protocrinids were either primitive or more derived. Additional, separate analyses of Sandbian crinoids with well-preserved oral regions corroborate the basic crinoid groupings listed above. During their initial divergence crinoids experienced morphological experimentation leading to a significant amount of homoplasy before the morphological disparity among clades became more diagnostic for each group.