Feeding postures as indicators of mutable collagenous tissue in extinct echinoderms.

Abstract

Echinoderms are a diverse phylum with a rich fossil record. The five extant classes of echinoderms are characterised by a pentameral (or pseudo-pentameral) symmetry, a water vascular system, a mesodermal skeleton of calcite stereom, and Mutable Collagenous Tissue (MCT), a unique type of connective tissue. Difficulties in tracing the geologic history of these traits complicates phylogenetic analyses of echinoderms. We present evidence herein of MCT in an extinct class of echinoderms, the Blastoidea. Blastoids have composite hair-like structures, brachioles, which formed a feeding filtration fan. Rare specimens from the Devonian of Germany demonstrate the presence of MCT by preserving brachioles as long rigid structures making a feeding fan with MCT in a rigid state. Specimens show brachioles in different configurations in the same specimen, which may indicate nervous control of MCT in individual brachioles. Other specimens appear to indicate the transition of MCT from a rigid to a compliant state as rigid brachioles begin to curve. Still other specimens show a majority of brachioles as limp hair-like structures swept by currents while a minority of brachioles remain rigid. These remarkable specimens could capture MCT transitioning from its rigid to compliant states in individual specimens indicating rapid burial and preservation.