Megalodontids demise – rudists rise? On the origin and affinities of the Pachyrismatidae (Bivalvia)

Although supposed to be placed at a prominent position with regard to phylogeny, i.e. the evolutionary bottleneck between the Megalodontidae and the Hippuritida, the Pachyrismatidae, or “Jurassic megalodontids” have received little scientific attention for almost a century. Presumably, one of the reasons for the lack of treatment is their scarcity, with several taxa being reported from a single outcrop or formation. The rediscovery and restudy of types and topotypic material has revealed inconsistencies in the generic concepts and specific assignments, mostly caused by erroneous drawings and misinterpretation of hinge elements. Facing the upcoming revision of the bivalve Treatise a proposal is put up for discussion, suggesting a major split of the “family” into true Pachyrismatidae and several genera to be assigned to the Cardioidea. Furthermore, megalodontid and hippuritid affinities of the remaining genera are reviewed and discussed in light of the revised timing of evolutionary “steps”, as provided by the fossil record. Last not least, the palaeoecologic preferences of the type species of the genus and family, Pachyrisma grande from the Bathonian of the UK, have been found strikingly similar to those perceived for Upper Jurassic Epidiceratidae (basal Hippuritida), fostering ideas of close relationship.