THE EVOLUTION OF BLASTOZOAN MORPHOLOGY: EXPLORING THE DISPARITY OF EOCRINOIDS

The Cambrian represents a pivotal time in early blastozoan echinoderm evolution. Stalked echinoderms originated and diversified during Cambrian Series 2 and 3 and subsequently expanded to a number of distinctive and successful Paleozoic classes. The early evolution of this group is contained within eocrinoids, which are brachiole-bearing stalked echinoderms. Eocrinoids are a morphologically varied group and represent a paraphyletic stem-lineage. Therefore, understanding the evolutionary pathways explored by eocrinoids is vital to understanding the morphology of Paleozoic blastozoans.

To explore this evolutionary progression, a character suite was modified from a previous study that encompasses the features observed in eocrinoids. This character suite also included characters from sister groups to place eocrinoids into a broader morphological prospective. The data set included 133 characters that focused on features associated with the ambulacral system, body wall, pelma, feeding structures, and respiratory structures. The character suite was used to analyze 13 genera along the blastozoan stem lineage including lepidocystoids, gogiids, derived eocrinids, and early rhombiferans along with an edrioasteroid. The resulting morphospace was combined with a recent phylogenetic analysis to produce a phylomorphospace which visualizes the range in morphologies with regard to their phylogenetic relationships. As expected, this created an evolutionary progression from the edrioasteroid and lepidocystids to the more derived blastozoans along the primary axis. The gogiids and derived eocrinids explored a large area within Cambrian Series 3 representing a surge in new forms bridging the gap toward later blastozoans (e.g. Ubaghicystis), expanding into new areas of morphospace (e.g. gogiids), and regressing toward more basal forms (e.g. Akadocrinus). Overall, this preliminary analysis indicates that eocrinoids spanned an area of morphospace that matched the distance between edrioasteroids and basal glyptocystitids. This methodology highlights the morphological diversity within eocrinoids as well as the evolutionary pathway establishing later blastozan evolution.