A global perspective of the Trigoniida (Bivalvia: Palaeoheterodonta), with a focus on their Mesozoic and Cenozoic representatives

The Trigoniida Dall, 1889 are a globally important group of dominantly marine Bivalvia, which had their acme during the Mesozoic, when they showed diversity equal in magnitude to extant cardioids, and played a similarly influential role in shallow marine communities. The anticipated revision of the bivalve volumes of the Treatise on Invertebrate Paleontology has fostered the authors’ attempt on a global genus-level systematic arrangement for the Mesozoic and Cenozoic Trigoniida. Currently, classification is not based on rigid phylogenetic analysis, but on qualitatively defined apomorphies. Nevertheless, it allows for identification of several large palaeogeographically and stratigraphically well-constrained clades, e.g. The Myophoriidae, Minetrigoniidae or Nototrigoniinae. Other clades, in particular the Myophorellidae and Pterotrigoniidae are considerably less resolved and their subdivision is controversially discussed, mainly due to conflicting genus-level concepts. The previously reported circum-Pacific origin seems obvious for most of the larger Triassic clades of the Trigoniida. Subsequently, the break-up of Pangaea and later Gondwana created space for global expansion across shelf seas. At the same time, isolation forced more localized radiations of several groups. We present chronostratigraphic range charts for more than 100 genera currently recognized as valid, supplemented by palaeogeographic distribution maps for selected taxa. We further exemplarily discuss difficulties of generic assignment in the Myophorellidae, which affect species and genera that are highly abundant in southern Gondwana. Finally, we propose a revised classification for the Trigoniida that is put up for discussion.