BLASTOID (ECHINODERMATA) PHYLOGENY AND IMPLICATIONS FOR BLASTOZOAN CLASSIFICATION

Quantitatively assessing clade defining characters in Paleozoic groups is becoming increasingly common as statistical methods are adapted to support morphological data. To begin testing complex questions such as rate of character change, biogeographic patterns, and environmental parameters, we must first understand the evolutionary relationships of the group. Blastozoan echinoderms have traditionally been separated into major groups by the presence of specific respiratory structures. Recent work suggests that this may be an inappropriate means to classify taxonomic groups of echinoderms. In order to further examine this assumption, we aim to generate a comprehensive phylogenetic analysis of Blastoidea, a Paleozoic group of blastozoans that have complex internal respiratory structures.

Traditionally, blastoids have been separated into two major groupings based on the external expression of their internal respiratory structures. Fissiculata possessing slits running parallel to the ambulacra and Spiraculata possessing pores along the ambulacra with large openings at the summit. It has been suggested on multiple occasions that spiraculates are an evolutionary grade within the larger fissiculate clade, but this has never been tested in a rigorous phylogenetic framework. Herein, we reassess and reexamine species with a better understanding of homologous elements and infer evolutionary relationships of blastoids. Our results indicate that fissiculates and spiraculates cannot be separated on such simple characters, as it has been determined that some species possess slits, pores, and large openings at the summit. This indicates that the features are not homologous and should not be treated as such. Results of the inferred phylogeny provide the basis for revision of classification.