Pseudoborings in ammonite molds: The combined result of predation and taphonomic-reworking (Bajocian, Normandy, N. France)

Unusual structures, observed on taphonomically-reworked ammonite molds coll ected 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 differe nt from the mold. These structures are called "pseudoborings" because of th eir 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 th in sections cut through the mold show that the depth of penetration of pseu doborings and the irregularity of the plug-mold boundary exclude a bioerosi onal origin. The origin of pseudoborings is hypothesized to be due to erosi on of unlithified sediments and exhumation of sediment-filled ammonite shel ls, preciously damaged by attacks by cephalopod predators, resulting in a p artial emptying of the conch.