Generic demarcation of Permo-Triassic Claraia-like species and their biogeographic significance

Three types of byssal notch are present in Permo-Triassic Claraia-like species, i.e. Claraia-type, Pseudoclaraia-type and Claraioides-type. At present, there is no consensus in the use of the generic names. The preferred classification should be both useful in practice and compatible with the rules of taxonomy. For the genera in question, details of the byssal notch are important in assessing relationships because the morphology of this structure may closely reflect phylogenetic affinities. However, a particular feature of the notch may have great significance in one taxon but low significance in a related taxon. In order to seek the best taxonomic characters for classification of Permo-Triassic Claraia-like species, we should follow empirical evidence rather than preconceived opinions. Pseudoclaraia Zhang, 1980 should be considered a junior synonym of Claraia Bittner, 1901 because the Pseudoclaraia-type and Claraia-type byssal notches form a continuum within populations. In contrast, all known Claraioides species possess the Claraioides-type byssal notch, which is their synapomorphy, indicating monophyly. The notch is characterized by ventral enlargement due to ventral dissolution and by its contracted anterodorsal outlet due to the accretion of shell material. This is a unique character in Palaeozoic bivalves. It implies that the Claraioides byssal notch must have had a different mode of development than the other two types of byssal notch. It is thus unreasonable to place Claraioides in synonymy with Claraia. A review of the biogeographic patterns of Permo-Triassic Claraia-like species indicates that trans-oceanic dispersal of Claraia from the Boreal Realm to the rest of the world may have occurred during the latest Permian. It is possible that volcanic-winter effects triggered the trans-oceanic dispersal and the tremendous change in Permian-Triassic global biogeographical patterns.