Pseudo-Borings and Predator Traces: Artifacts of Pressure-Dissolution in Fossiliferous Shales

Well-preserved brachiopods from two Paleozoic deposits (Ordovician Richmond Group, Indiana; Devonian Silica Formation, Ohio and Michigan) occasionally contain skeletal fragments embedded in their exterior surfaces. Cross-sections of the shells reveal that the fragments are the remains of brachiopods that cleanly pierce one or both valves of the host specimen. A sharp contact between fragment and host, a coincident film of insoluble residue, and, in some samples, a thin zone of recrystallization, all suggest that the fragments were emplaced via pressure-dissolution during post-burial sediment compaction. Diagenetically emplaced fragments may disintegrate during weathering and leave grooves (pseudo-borings) that resemble the traces of shell-boring organisms. Apparent teeth or rhyncholites found embedded in brachiopod shells may arise from pressure-dissolution between fortuitously associated particles. If pressure-dissolution artifacts, like these, are not distinguished from true biological traces, an overestimation of boring and predation intensities in paleoecological studies may result. Several simple criteria for distinguishing artifacts from traces are proposed, including regular plicate or linear shape of groove, random orientation and lack of host response. This study documents that even in exceptionally well-preserved fossil deposits, pressure-dissolution can be an important taphonomic process.