THE MORPHOLOGIC EXPLORATION OF THE PELMATOZOAN ORAL REGION

The pelmatozoan oral region shows a large array of morphologic features (2-1-2 symmetry with fixed orals forming the peristome and moveable cover plates in ancestral pelmatozoans to derived crinoids with a rigid tegmen above a decalcified oral frame). However, these vastly different morphologies share an underlying homologous framework. The evolutionary pathways that Early Paleozoic pelmatozoans explored can be examined quantitatively in order to begin to formulate adaptive and constructive hypotheses for shared evolutionary patterns. Features of the oral region were coded for edrioasteroids, crinoids, and various blastozoan echinoderms (including eocrinoids, rhombiferans, diploporans, and coronoid blastoids) and used to construct a morphospace as well as a phylogenetic hypothesis based exclusively on features of the oral surface. These two methodologies were combined by modeling character state evolution and projecting the position of ancestral nodes onto the morphospace so that the evolutionary pathways in the oral region of pelmatozoans could be visualized. The primary axes in the morphospace reflect changes in the structure of the oral frame and the interambulacral areas, the types of respiratory structures, and extensions of the oral surface (i.e. feeding or anal structures). Three major groups can be identified within morphospace: edrioasteroids and eocrinoid, blastozoans, and coronoid blastoids and crinoids. All three groups show convergent evolution as well as crossing evolutionary trajectories indicating a mixture of common responses to selective pressures as well as morphologic divergence that is paired with ecological separation.