THE HITCHHIKER'S GUIDE TO EDRIOASTEROIDS: EDRIOASTEROIDS AS EPIBIONTS

Edrioasterid echinoderms are obligate encrusters that inhabited a variety of environments during the Paleozoic. Typically, they are thought of as encrusters of hardgrounds and shell pavements, but many examples have recently come to light of edrioasteroids encrusting as epibionts. Live organisms that edrioasteroids use as hard substrate include: large trilobites, conulariids, articulate brachiopods, and blastozoan echinoderms. Evidence for edrioasteroid epibionts on trilobites includes full articulation of the trilobites with edrioasteroid attachment restricted to individual sclerites. Because edrioasteroids do not cross body segments, movement of the trilobite is not greatly restricted. Evidence for edrioasteroid epibionts on conulariids includes attachment to all sides of the conulariid indicating the cone was in life position during attachment. Edrioasteroid epibionts on articulate brachiopods is generally hard to diagnose, however in some smothered hardgrounds, brachiopods that are in life position, articulate, spar-filled, and not abraded can be interpreted as alive at the time of burial. Edrioasteroids have been found attached to such brachiopods where their footprint is restricted to a single valve. Evidence for edrioasteroid epibionts on blastozoan echinoderms comes from the attachment of edrioasteroids to theca with the small delicate structures still attached. Taphonomy experiments on echinoderms show that such structures disarticulate in days to weeks. This would not allow enough time for edrioasteroids to attach as larvae and grow to maturity before the substrate organism disarticulates unless the substrate was alive. A remarkable specimen from the Devonian of Iowa has an edrioasteroid overgrowing the theca of a rhombiferan, but the ambulacrum of the rhombiferan, in turn, is 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.