NEW EVIDENCE OF EARLY HYBOCRINID TEGMENS; PHYLOGENETIC IMPLICATIONS

Discovery of an early hybocrinoid oral surface alters our understanding of the origin and early history of these unusual crinoids, and factors into our understanding of early crinoid phylogeny. Recently, authors have proposed that the few-plated oral surface of Hybocrinus represents a basal crinoid condition, linking crinoids to blastozoan (cystoid, eocrinoid) echinoderms. Rigidity is accomplished by tight suturing of thick oral to radial plates. Here, we report a new Late Floian (Early Ordovician) hybocrinid with a thin many plated oral surface whose cup plating is essentially that of Hybocrinus. Many non-differentiated interambulacrals transition into plate webs extending up the arms, much like other Early Ordovician crinoids. The whole oral surface is collapsed and shifted laterally in the critical specimen, supporting pliancy. Presence of orals is uncertain. The early Middle Ordovician Hoplocrinus, although incompletely known, shows similar, though thicker-plated construction. The record of hybocrinids and related clades now suggests Early Ordovician tegmen-bearing forms are basal to rigid arrangements which first appeared during the Middle Ordovician. Only rigid patterns occurred during a modest Late Ordovician diversification. Evolution of crinoids from rigidly constructed blastozoans is not supported by the hybocrinid record.