Interpreting the Origins of Some Conical Body and Trace Fossils from Upper Ordovician and Middle Silurian Strata of Southern Indiana

Noteworthy conical or plug-shaped macrofossils are herein described from the Upper Ordovician and middle Silurian of Ripley County, southeastern Indiana. The Upper Ordovician (Katian, Richmondian) Whitewater Formation produced an enigmatic specimen that is circular in transverse view, completely smooth-sided, and strongly tapering, terminating in a blunt, round apex. It resembles both a conostichoid burrow and the infilling of the calyx of a camerate crinoid, but the most likely identity is the internal mold of a nautiloid cephalopod body chamber. The middle Silurian (Wenlock, Sheinwoodian) Massie Formation yielded two conostichoid burrows (Conostichus? isp.) worth description. One specimen is exceptionally large, being approximately 138 % wider at top, 132 % wider at apex, and 114 % deeper than the largest measured example from a sample of 200 specimens. The other conostichoid specimen contains a long, intact crinoid pluricolumnal that is oriented vertically within the center of the burrow. This must have washed into an open burrow and become fortuitously embedded upright, a hitherto undocumented ichnological event.