SEQUENTIAL ENCRUSTATION OF A MIDDLE SILURIAN NAUTILOID CEPHALOPOD (MICHELINOCERAS?) BY THE BLASTOZOAN ECHINODERMS PAULICYSTIS (DIPLOPORITA) AND CARYOCRINITES (RHOMBIFERA)

A nautiloid cephalopod (Michelinoceras?) serving as a substratum for rare encrustation relationships is herein described from the middle Silurian (Wenlock: Sheinwoodian) Massie Formation of southeastern Indiana, USA. The partial orthoconic cephalopod phragmocone is preserved as a flattened internal mold surrounded by some remnant shell material. The more damaged (presumably upward-facing) side is encrusted by the thecal attachment structure of a trematocystinid holocystitid diploporitan (seemingly a minute Paulicystis), which is, in turn, encrusted by a dendritic attachment structure attributable to the hemicosmitid rhombiferan Caryocrinites. Taphonomic evidence indicates that this sequential encrustation did not result from limited shell space being available for occupation. Examples of a representative of one blastozoan class being encrusted by a representative of another coeval blastozoan class are rare, as are examples of diploporitans utilizing large, intact cephalopods as substrata rather than small fragments that are completely overgrown by the basal region of the echinoderm. The encrusting diploporitan attachment structure displays a morphology that is typically seen on laterally continuous hardgrounds rather than isolated bioclasts in softground settings, demonstrating previously unrecognized diploporitan morphological plasticity. More importantly, the preferential encrustation of a diploporitan attachment by Caryocrinites in spite of the fact that bare, hard space was readily available elsewhere on the cephalopod suggests that this taxon requires initial settlement upon a discrete macrofossil. Preferential use of echinoderm remains as settling sites for later encrusting echinoderms, despite class-level taxonomic differences, illustrates the commonly overlooked concept that pelmatozoan encrustation of bioclasts in otherwise softground substrates is controlled by more complex factors than mere occurrence of available biomineralized material.