Predation on the Pennsylvanian ammonoid Gonioloboceras and its implications for allochthonous vs. autochthonous accumulations of goniatites and other ammonoids

Damage displayed by 18 of 954 relatively mature (> 35 mm diameter) specimens of Gonioloboceras goniolobum (Meek) from North American Midcontinent Upper Carboniferous sediments is interpreted to be bite marks caused by condrichthyans and other fish, particularly the symmoriid shark Symmorium reniforme. It is likely that these and other predators regularly preyed on Gonioloboceras goniolobum because all of the analyzed specimens of Gonioloboceras exhibit some evidence of unrepaired damage, including broached camerae and missing body chambers. Two new characteristics utilized with caution to detect predatory events on ammonoids are raised oval or circular pedestals on internal molds and the breakage and termination of septa in conjunction with a broken external shell. When sufficient damage to the Gonioloboceras conchs occurred during predatory attacks, the broached camerae flooded, the conch became negatively buoyant, and it sank in the vicinity of the attack. When preserved, these conchs form a variable component of an autochthonous fossil accumulation that can include numerous other cephalopod taxa, which also were probably subjected to predatory attacks. Other, but not all, ammonoid accumulations in the fossil record form similar autochthonous deposits, or even autochthonous portions of mixed deposits.