Pseudoborings in Ammonite Molds: The Combined Result of Predation and Taphonomic-reworking (Bajocian, Normandy, N. France)

Unusual structures, observed on taphonomically-reworked ammonite molds collected in the Oolithe Ferrugineuse de Bayeux Formation (Bajocian, Normandy), are described. They appear as depressions in the mold, located below “holes” in the shell, and they are filled with a sediment plug texturally different from the mold. These structures are called “pseudoborings” because of their similarity in both external shape and size (from 2 mm to over 10 mm in diameter) to bivalve borings. Detailed analyses of polished surfaces and thin sections cut through the mold show that the depth of penetration of pseudoborings and the irregularity of the plug-mold boundary exclude a bioerosional origin. The origin of pseudoborings is hypothesized to be due to erosion of unlithified sediments and exhumation of sediment-filled ammonite shells, previously damaged by attacks by cephalopod predators, resulting in a partial emptying of the conch.