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

 

Authors: Thomka JR, Bantel TE

Published in: Bulletin of Geosciences, volume 96, issue 1; pages: 53 - 59; Received 18 August 2020; Accepted in revised form 6 November 2020; Online 27 November 2020

Keywords: Diploporita, Rhombifera, Massie Formation, Sheinwoodian, biotic interactions, attachment structures,

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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 upward-facing) 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 class-level 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.

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