Tracing the Devonian Roots of the Parallelodontidae (Bivalvia): Origin and Evolution

The Parallelodontidae is one of six families of the Arcoidea that is assumed to range from the Devonian to the Recent. Despite over 150 years of research, the origins and early history of the family remain largely unresolved. We reviewed the Devonian taxa that match the typical parallelodontid morphology regarding hinge details or external shell characteristics, leading to a list of some 50 species. However, most of them can only be vaguely attributed to Parallelodontidae, as poor preservation can obscure diagnostic features, causing potential confusion with contemporaneous genera showing comparable characteristics.

Here, we present details of the Devonian species that show parallelodontid morphology in the strict sense. All Ordovician and Silurian taxa previously attributed to Parallelodon or presumed to possess parallelodontid characteristics turned out to be misclassified. Parallelodon mandelensis from the upper Emsian Rhenotypic Facies of Germany undoubtedly has a parallelodontid hinge and seems to be the earliest member of the family. Next unequivocal parallelodontids have been noted in the Middle Devonian, primarily in the shelf region deposits of the Rheic Ocean in Central Europe and eastern North America, where up to ten species were identified. The relatively high sea level and, accordingly, wide shelf areas during this period provided ideal conditions for the diversification of parallelodontids. The Late Devonian evolution of the group is mainly documented in North America, as facies conditions in Europe seem to have been unfavorable. With the transition from the Devonian to the Carboniferous, a remarkable radiation of parallelodontids led to significantly higher parallelodontid diversity.