Sequential, multi-taxon encrustation of an orthoceratid cephalopod by stalked blastozoan echinoderms in the middle Silurian (Wenlock Series) of southeastern Indiana, USA

Abstract

An orthoceratid cephalopod serving as a substratum for rare encrustation relationships is herein described from the middle Silurian (Wenlock Series, Sheinwoodian Stage) Massie Formation of southeastern Indiana, USA. The partial orthoconic cephalopod phragmocone is preserved as a flattened internal mold with some remnant shell material. The more damaged (presumably upward­facing) side is encrusted by the thecal attachment structure of a trematocystinid holocystitid diploporitan (probably Paulicystis ), which is, in turn, encrusted by a dendritic attachment structure attributable to the hemicosmitid rhombiferan Caryocrinites . This represents an unusual form of multigenerational encrustation by blastozoan pelmatozoans; an encrustation that did not culminate in overgrowth of the cephalopod substratum. More importantly, this occurrence demonstrates preferential use of echinoderm remains as settling sites for later encrusting echinoderms, despite class­level taxonomic differences, and represents an additional example of utilization of a discrete macrofossil – and a rarely reported example of a coeval echinoderm attachment structure – as a substratum for Caryocrinites attachment. Collectively, this material indicates that pelmatozoan encrustation of bioclasts in otherwise softground substrates is controlled by more complex factors than mere occurrence of available biomineralized material. •