Sublethal injuries in Middle Jurassic ammonite shells from Poland

Sublethal injuries, the effects of which are seen as regeneration patterns, are described from Late Bajocian and Bathonian (Middle Jurassic) ammonites from Poland (Polish Jura area) for the first time. The total number of ammonite shells bearing signs of sublethal injuries is small (only 11 specimens, which constitute ∼1.2% of all ammonites investigated), and this value is even smaller (∼0.3 to 0.8%) when analysing a large sample of a particular ammonite species. Specimens under consideration represent ten species, belonging to six genera and five families. All the healed injuries are represented by only one type, referred to as the ‘forma verticata’ of Hölder. This type of regeneration, very common in ammonite shells in general, is an effect of a puncture injury of the shell-secreting mantle-epithelia at the apertural margin. Although many different extrinsic (mechanical) factors may be responsible for such healed injuries, here it is most plausible they are an effect of either competitive or predatory activities. Other causes, like collision of the ammonite shells with the substrate in a high-energy environment, are excluded because the sea-bottom was soft and situated below the storm wave-base. From many potential predators inhabiting the Polish Basin during the Bajocian and Bathonian, the most likely to have caused these injuries are other ammonites, belemnites and nautiloids. Crabs, which are cited in the literature as a probable perpetrator of the ‘forma verticata’ injuries, appear unlikely here, as the ammonites under discussion were not purely benthic.