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Taphonomy of diploporite (Echinodermata) holdfasts from a Silurian hardground, southeastern Indiana, United States: palaeoecologic and stratigraphic significance

Published online by Cambridge University Press:  19 September 2013

JAMES R. THOMKA  and
CARLTON E. BRETT
JAMES R. THOMKA*
Affiliation:
Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221, USA
CARLTON E. BRETT
Affiliation:
Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221, USA
*
*Author for correspondence: [email protected]
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Abstract

A microbioherm-bearing hardground within the middle Silurian (Wenlock) Massie Formation near Napoleon, southeastern Indiana, United States is encrusted by the attachment structures of numerous pelmatozoan echinoderms. Among the most common of these holdfasts are multi-plated discoidal structures representing the thecal attachments of diploporite ‘cystoids’. This large population of holdfasts permits the first detailed taphonomic and palaeoecologic study of hardground diploporite attachments, allowing for increased morphological understanding of these rarely studied structures and facilitating identification of holdfasts in deposits where they might have been overlooked or misidentified. The biostratinomic sequence commences with detachment of thecae, followed by weathering of isolated discoidal holdfasts to bring out radiating canal structures and plate sutures, eventually leading to removal of the interior floor to expose the underlying substrate. Continued exposure can result in separation of component holdfast plates, though cementation to the substrate prevents scattering of plates. Diagenetic precipitation of pyrite occurred after burial; the large size of crystals suggests late diagenesis, perhaps seeded by early diagenetic pyrite crystallites produced by decay of ligamentary tissue. Extrinsic taphonomic factors include overgrowth of holdfasts by laminar stenolaemate bryozoans and other echinoderm attachment structures. Diploporite holdfasts are not bored and are absent on microbioherms. Taphonomic data indicate the time-averaged nature of this hardground and its diploporite assemblage and permit prediction of similar occurrences at major flooding surfaces.

Keywords

Diploporitaattachment structuresencrustationWenlockHolocystites fauna
Type
Original Articles
Information
Geological Magazine , Volume 151 , Issue 4 , July 2014 , pp. 649 - 665
Copyright
Copyright © Cambridge University Press 2013 

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