PHYLOGENETIC AND MORPHOLOGIC ANALYSES ELUCIDATE RELATIONSHIPS IN PARACRINOIDEA (ECHINODERMATA)

One of the great challenges facing paleobiology is understanding aberrant morphologies from the combined perspectives of phylogeny and morphological disparity. Among echinoderms, the evolution of numerous body plans during the late Cambrian and early Ordovician lead to high level taxonomic diversity seen in few clades. This study examines the evolution of disparity among the extinct echinoderm clade, Paracrinoidea. Paracrinoidea is a well-constrained clade with a set of easily identifiable synapomorphies that characterize members of the group including asymmetry in mouth and stem position, asymmetrical ambulacral construction, and unusual placement of the anal opening. However, the relationships of species within this clade have never been rigorously examined in a quantitative framework. The complex morphologies exhibited by paracrinoids makes them an ideal focus group for assessing evolutionary change and morphological disparity analyses.

Within clade relationships are unclear, as describing homologous characters has proven difficult because of the high morphological disparity. To better assess potentially homologous characters, morphometric analyses are conducted prior to phylogenetic analyses to determine what characters are driving separation in morphospace. This analysis suggests respiratory structures, oral plate arrangement, and ambulacral morphology are important in separating specific subclades of paracrinoids, which is consistent with the characters that are interpreted as phylogenetically important in other blastozoan clades. Subsequent phylogenetic analysis supports these characters as clade defining. A better understanding of elemental homology within paracrinoids will further refine evolutionary relationships. Initial analyses suggest ambulacral configuration, thecal plate reduction, and structure of respiratory structures are clade defining, however, continued analyses will further elucidate the enigmatic relationships within Paracrinoidea.