ROOTING THE EARLY CRINOID DIVERSIFICATION WITH THE EOCRINOID RHOPALOCYSTIS

Competing hypotheses on crinoid origination and early diversification differ based on the closest outgroup. For example, should tracing of one or a few characters, many characters, arguments based on ontogeny, or stratigraphic position be used to establish the crinoid outgroup? To date, phylogenetic analyses have only used calyx and column characters. Here, we propose extensions of the echinoderm extraxial-axial theory (EAT) and the blastozoan universal elemental homology scheme (UEH) to establish hypotheses on the early diversification of crinoids. With the recognition of oral region homologies between crinoids and blastozoans, a new suite of characters can be utilized to evaluate homology between crinoids and blastozoans and within crinoids.

Based on UEH arguments, Rhopalocystis shares pleisomorphic characters with basal crinoids so that it is an strong candidate for an outgroup. Further, Rhopalocystis has four circlets of plates, similar to aethocrinid crinoids, and “fixed brachials” and “fixed interradial” plates, identical to early camerate crinoids. Thus, based on both oral region (axial) and calyx and column (extraxial) characters, the eocrinoid Rhopalocystis shares the greatest number of potential homologies with crinoids and is used here as the outgroup for an analysis of Ordovician crinoids. Parsimony-based phylogenetic analyses were completed on Ordovician crinoids with well-documented oral surfaces and calyx characters. 185 characters were coded using 21 taxa. Preliminary phylogenetic analysis produced a single most parsimonious tree. Hybocrinid crinoids are basal for crinoids from which primitive cladids, disparids, and diplobathrid camerates were derived. Monobathrid camerates were derived from diplobathrid camerates. If included in analyses, aethocrinids are linked to disparids.