Phylogenetic affinities of the poorly known bivalve genus Xenocardita Vokes, 1946 (Bivalvia: Carditidae): new data on the evolution of the tripartite radial ribs

Abstract

The Mesozoic record of the Carditidae is sparse and poorly known, and the genus Xenocardita constitutes a significant record on account of its early Aptian age, but lacks a proper phylogenetic placement within the family. Based on new observations of specimens and a revision of external sculpture characters, we perform a phylogenetic analysis of the Carditidae including all subfamilies and lineages, with a total of 72 species as ingroup and 153 morphological characters of the shell. Our results show Xenocardita to belong to the tribe Venericardiini (formerly known as ‘alticostate’ carditids). In most runs, Xenocardita is recovered as an early diverging Venericardiini, sister taxon of all remaining members. In consequence, the origin of the clade is moved downwards from the Campanian–Maastrichtian to at least the lower Aptian. The new phylogenetic position of Xenocardita and the re-evaluation of the morphology of the radial ribs, now recognized in this taxon, move the first record of the tripartite radial rib pattern back in time and re-write its evolutionary history. Tripartite radial rib pattern begins with a central cord sided by little pronounced slopes during Early Cretaceous and becomes strongly marked with pronounced paracostal ribs in the Late Cretaceous (represented by the ‘Baluchicardia’ Clade), and survives toward the Miocene, but experiences simplifications in some lineages. A second Cenozoic tripartite pattern appears in the Eocene with strongly pronounced slopes (represented by the ‘Venericardia’ Clade). Other Cretaceous taxa and their possible relationships with the Venericardiini are also discussed.