Article: Revision of the Alatoconchidae: a remarkable family of Permian bivalves

Genera and species in the family Alatoconchidae are described and reillustrated, including a major revision of the type genus, and a family concept is established that transfers them from the Megalodontacea to the Ambonychiacea. Saikraconcha n. gen., with S. tunisiensis n. sp. as type species, and Dereconcha n. subgen., with S. (D.) kamparensis n. sp. as type species, are described. Two lineages are present in the family, with one including Shikamaia and its subgenera Alatoconcha and Tanchintongia, and the other including Saikraconcha and its subgenus Dereconcha. Both lineages show similar evolutionary change in form. Alatoconchids are limited in occurrence to the Old World Permian Tethyan province, having a distribution similar to that of the verbeekinid fusulinaceans. Rapid evolution of species, and their distinctive form, make them stratigraphically useful in the Tethyan province.Alatoconchids are the most unusual of late Paleozoic bivalves. They are strongly flattened in a dorsal-ventral direction, producing wide wing-like flanges on each valve. Some species had very large and heavy shells, as much as 1 m in length and 10 kg or more in weight, making them nearly the largest known bivalves. Small shells (mostly juveniles) were byssally attached, but large ones were free-living on a soft sediment surface, and are one of the few groups of recliners to have a vertically aligned plane of commissure.