UNUSUAL PRESERVATION OF CONOSTICHUS IN CARBONATE SEDIMENTS (SILURIAN, SOUTHEASTERN INDIANA)

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 Paleozoic-age putative Conostichus preserved in carbonate sediments are essentially conical masses lacking recognizable internal radial or longitudinal structures. Herein we describe the occurrence of well-preserved examples of this ichnogenus within middle Silurian (Wenlock: Sheinwooodian) carbonate sediments 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 argillaceous 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.