Were limpets or mosasaurs responsible for the perforations in the ammonite Placenticeras?

Abstract

We present evidence defending the hypothesis that specimens of the ammonite Placenticeras from the Upper Cretaceous Bearpaw Formation of southern Alberta were preyed upon by mosasaurs. Perforations linked to diagenetic modification of limpet home scars are anomalous features that are rare in comparison to true tooth marks. Point loading of the ammonite phragmocone wall by mosasaur teeth produced distinct perforations without the development of long, angular fractures. This was due to low pressure and rate of loading permitted by the unusually loose articulation of the lower jaw of mosasaurs, combined with the nearly uniform internal support of the shell wall by complexly fluted septa and their sutures. In contrast, the body-chamber, which lacked septal support (except at its posterior end), tended to shatter. Remnants of the body-chamber preserve distinctive notches that resemble tooth marks and sometimes show evidence of healing. Thorough reassessment of the perforations and associated features indicates that the large majority of perforations in shells of Placenticeras, at least those from southern Alberta, are best explained by predatory activities of mosasaurs. The currently popular hypothesis that all perforations preserved in shells of Cretaceous ammonites were produced by sediment loading and collapse of limpet home scars fails to explain many diagnostic features of the perforations. These include the shape, siting and arrangements of perforations as well as the equal puncturing of all size classes of alleged limpet home scars. The limpet hypothesis also fails to consider structural differences between shells of ammonoids and Nautilus and erroneously assumes point loading by sediment compaction. Furthermore, the extreme rarity of limpets that had the potential of excavating home scars in Placenticeras from the Upper Cretaceous Bearpaw Formation of Alberta, Canada, cannot be attributed to preservational bias and is at odds with the large number of perforated specimens known from this formation. The mosasaur-bite hypothesis, on the other hand, accommodates all of these features.