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Abstract: The long-ranging, plug-shaped ichnofossil Conostichus, attributed to solitary polypoid cnidarians, is most commonly described in the Paleozoic from fine-grained siliciclastic sediments, with few descriptions from carbonate settings. The few [...]

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. [ABSTRACT FROM AUTHOR]

The long-ranging, plug-shaped ichnofossil Conostichus, attributed to solitary polypoid cnidarians, is most commonly described in the Paleozoic from fine-grained siliciclastic sediments, with few descriptions from carbonate settings. The few described examples of putative Conostichus preserved in carbonate sedimentary rocks of Paleozoic age are essentially conical masses lacking recognizable external radial or longitudinal structures. Herein, we describe the occurrence of well-preserved examples of this ichnogenus within middle Silurian (Wenlock: Sheinwoodian) carbonate sedimentary rocks of the Massie Formation from southeastern Indiana, USA. These specimens represent isolated apical discs of Conostichus with prominent radiating physal impressions displaying duodecimal symmetry. Interestingly, well-preserved specimens co-occur with relatively poorly preserved ('typical carbonate') specimens. The factors responsible for this unusual taphonomic state are unclear, but the most likely explanation is that exceptionally preserved specimens represent burrows that were somehow infilled, at least in their apical terminations, with fine-grained carbonate sediment, enhancing preservational fidelity. In contrast, other burrows had their apical regions passively filled with larger carbonate particles that could not preserve fine details. This occurrence indicates that early Paleozoic carbonate sediments are capable of preserving Conostichus, and potentially other ichnofossils, in similar modes to later Paleozoic siliciclastic deposits. [ABSTRACT FROM AUTHOR]

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