New tools for taxonomy: a revised terminology for Palaeozoic and Mesozoic arcoid bivalve shells

The Arcida – vulgo ‘ark shells’ – is one of eight orders in the bivalve subclass Pteriomorphia and has a story of remarkable evolutionary success. Originating in the Ordovician, the Arcida have a continuous and comprehensive fossil record and occur with a global diversity of some 600 species in eleven families today. However, their route to success remains poorly understood. This is partly a result of the unresolved phylogeny of the superfamily Arcoidea in the late Palaeozoic and early Mesozoic, where a vast number of species are still assigned to a single, rather poorly diagnosed waste-basket genus, Parallelodon, which is the genotype of the Parallelodontidae. Thus, as a first step towards establishing a robust phylogeny of the group, we aim to revise the taxonomy and systematics of key Palaeozoic and Mesozoic Arcoidea. This will require an improved and expanded set of terms to describe the variability currently contained in only a handful of genera. Here, we present an updated terminology for key characters of the Parallelodontidae/Arcoidea. These include (1) the characteristic ‘parallelodontid’ hinge, comprising a set of short, oblique antero-pseudolateral teeth, followed by a second set of hinge line-parallel, horizontal postero-pseudolateral teeth; (2) a simple, amphidetic, duplivincular ligament; (3) a variety of common shell outlines; (4) and several basic patterns of shell ornamentation, generally composed of radial and commarginal elements. As is commonly the case in the strongly substrate-related Bivalvia, diverging shell morphologies in the Arcoidea are thought to relate to disparate modes of life. We anticipate that, along with revised taxonomy, the ecology of parallelodontid and more generally arcoid taxa can be inferred from a thorough morphological assessment.