Predation on bellerophontiform molluscs in the Palaeozoic

Abstract

Shell repair assumed to result from failed predation is documented in 66 specimens of Ordovician-Carboniferous bellerophontiform tergomyan and gastropod molluscs to examine the relationship between the distribution and appearance of injuries, shell morphology and the internal anatomy of the molluscs, as well as the attack strategies of the presumed predators. Furthermore, the distribution of repaired injuries from failed attacks along the apertural margin as a reflection of the nature of the margin and emarginations is investigated. Bellerophontiform molluscs are ideal for this study because of their distinctive isostrophic morphology and the possibility to directly compare broad and narrow conchs with either deep or shallow medial emarginations. The results show that taxa with a deep medial emargination in the form of a slit have significantly more medial injuries than lateral ones. Near-equal frequencies of lateral and medial injuries in specimens with a shallow emargination (slit or sinus) suggest random distribution. Shell form (narrow or broad) does not exert overall control on the distribution of injuries except, perhaps, in some broad explanate shells with an insignificant medial emargination. While this suggests that it is the type of medial emargination that governs distribution of injuries in these forms, it is not clear if this is a result of passive selection due to structural geometry or preferential targeting by predators (i.e. site-specific mode of attack). Predation strategies on bellerophontiform molluscs thus seem to be dependent on the morphological features of the shells rather than their interpretation as tergomyan or gastropod.