Is Cyclocardia (Conrad) a wastebasket taxon? Exploring the phylogeny of the most diverse genus of the Carditidae (Archiheterodonta, Bivalvia)

The carditid genus Cyclocardia is currently the most diverse genus of the family, including nearly 180 nominal species from a wide stratigraphical (Cretaceous–Recent) and geographical range (Antarctica, South and North America, Europe, Africa, Alaska, Russia, Japan, and New Zealand). Due to the lack of autapomorphies in the diagnosis of the genus and its large account of species, we re-evaluated the systematic and phylogenetic status of Cyclocardia. We carried out three approaches: bibliographic revision, phylogenetic analysis, and morphological disparity exploration. We used a shell-character matrix including 65 taxa (two outgroups, 29 non-Cyclocardia carditids, and 28 species of Cyclocardia) for phylogenetic and disparity analyses. The morphological disparity analysis was conducted using Maximum Observable Rescaled Distances for calculate distance matrix, and comparing Cyclocardia species and other carditid groups. According to our results, Cyclocardia is a 'wastebasket taxon', chiefly because its diagnosis was based mainly on plesiomorphic characters. Also, Cyclocardia represents a non-monophyletic taxon. The European species C. kickxi and C. chameformis are placed within Scalaricardita, and the previously proposed genus Crassicardia is monophyletic (including C. crassidens, C. crebricostata, C. isaotakii, and C. rjabininae). Three new genera are proposed for new groups identified by the phylogenetic analysis: South American Oesterheldia nov. gen. (including O. cannada and O. dalek), Western North American Coanicardita nov. gen. (including C. ventricosa and C. occidentalis), and North Pacific Hippocampocardia nov. gen. (including H. barbarensis, H. hamiltonensis, and H. yakatagensis). The newly defined monophyletic Cyclocardia is restricted to the Atlantic Ocean species C. borealis, C. novangliae, and C. compressa.