Alleged mosasaur bite marks on Late Cretaceous ammonites are limpet (patellogastropod) home scars

Abstract

New data contradict the mosasaur-bite hypothesis for the origin of holes seen in Placenticeras ammonites from the Late Cretaceous Pierre Shale and Bearpaw Formation of the western interior of North America. Observations of a limpet-infested Placenticeras ammonite and of several Placenticeras specimens with radular scratch marks reveal that the limpets dwelled on floating ammonites, and their homing activity produced circular depressions. When altered during diagenesis, the depressions mimic tooth punctures. Crushing experiments on fresh Nautilus shells using a mosasaur robot show that mosasaur bites could not have produced holes resembling those in the fossils. Furthermore, sectioning of “bitten” ammonites reveals that sepia are sometimes intact under the holes, an observation irreconcilable with penetration by a tooth. We present an alternative interpretation that the alleged “bite marks” in Placenticeras ammonites are really limpet home scars that were altered after burial. While predation of mosasaurs on ammonites remains a possibility, it would have to be demonstrated by other criteria. Our findings do not support mosasaur-ammonoid coevolution in the Late Cretaceous.