Noteworthy Taphonomic Attributes of a Nautiloid Cephalopod from the Middle Silurian (Wenlock) Massie Formation of Southern Indiana

Abstract

A specimen of the nautiloid cephalopod Dawsonoceras annulatum from the middle Silurian (Wenlock) Massie Formation of southeastern Indiana, USA, is herein described because of its suite of uncommon taphonomic characteristics.  Three noteworthy features are observable.  First, an articulated calymenid trilobite is preserved in the sediment infill of the cephalopod, having been either swept into the open shell post-mortem or rapidly buried alive while sheltering inside the hollow cavity.  Second, a series of small (pinhole-sized) borings, potentially produced by ctenostomate bryozoans, are present on the upward-facing and lateral surfaces of the cephalopod shell, a feature that is otherwise unreported in this fauna.  There appears to be a preference for boring into raised portions of the shell.  Third, a number of tightly coiled microconchids are encrusting the exterior of the shell, an association not previously documented in this unit.  Some of these phenomena represent first reports from a fossil fauna in a region strongly affected by dolomitization and are worthy of description on that basis alone.  However, their co-occurrence on a single specimen highlights the fact that relatively common taxa such as cephalopods are still capable of yielding rare and/or significant taphonomic and paleoecological data.