EDRIOASTEROIDS AS EPIBIONTS: A COMMON HARD SUBSTRATE TYPE FOR ENCRUSTING ECHINODERMS

Throughout the Paleozoic, edrioasteroid echinoderms were obligate encrusters inhabiting a variety of environments. Although they are typically considered encrusters of hardgrounds and shell pavements, many examples have recently come to light of edrioasteroids as epibionts. Live organisms provide islands of hard substrate in an otherwise soft bottom environment, analogous to discarded shells. Documented live fauna used by edrioasteroids as hard substrate include: trilobites, conulariids, articulate brachiopods, and blastozoan echinoderms. Epibiotic edrioasteroids on trilobites are recognized by full articulation of encrusted trilobites, with edrioasteroid attachment restricted to individual sclerites. Because edrioasteroids do not cross body segments, trilobite movement is not greatly restricted. Epibiotic edrioasteroids on conulariids are recognized by edrioasteroid attachment to all sides of the conulariid indicating the cone was in life position during attachment. On articulate brachiopods, epibiotic edrioasteroids can be hard to diagnose. However, in some smothered hardgrounds, brachiopods preserved articulated, in life position, spar-filled, and not abraded are consistent with live attachment. Edrioasteroids have been found attached to such brachiopods where their footprint is restricted to a single valve. Epibiotic edrioasteroid on blastozoan echinoderms are recognized by the attachment of edrioasteroids to fully articulated substrate organisms. Echinoderm taphonomy experiments show that delicate structures such as stems and fine feeding appendages disarticulate in days to weeks. Articulation of these structures show the substrate echinoderm was alive while the edrioasteroids attach as larvae and grew to maturity prior to burial. A remarkable specimen from the Devonian of Iowa preserves an edrioasteroid overgrowing the theca of a rhombiferan, and the ambulacrum of the rhombiferan, in turn, overgrowing the edrioasteroid, unequivocally demonstrating live attachment. Several edrioasteroid species are known only as epibionts on specific species suggesting a commensal relationship. It is not clear what, if any, benefit edrioasteroids gain from epibiotic attachment, but feeding higher in the water column and in some cases mobility are possibilities.