Parasite-induced teratologies in the Pliocene bivalve Isognomon maxillatus

In a sample of the bivalve Isognomon (Hippochaeta) maxillatus from a single Upper Pliocene outcrop in NW Italy, over half of the specimens show teratologies produced as a reaction to an associated, probably endoparasitic organism. This organism settled inside the hinge region, causing a detachment of the mantle from the shell and ligament and the subsequent appearance of abnormal characters. These include four instances of teratologic lithodesmata, morphologically comparable to the non-pathologic lithodesma present in anomalodesmatan bivalves. This shows that the morphogenetic potential for the construction of a lithodesma is present also in bivalve groups that apparently never evolved functional lithodesmata. This also suggests that morphologic evolution is more likely to occur if the evolving feature is the product of a self-organising morphogenetic programme, rather than of a set of independent and rigid programmes controlling each component character.