Probable ancestral type of actinodont hinge in the Ordovician bivalve Pseudocyrtodonta Pfab, 1934

The type species of Pseudocyrtodonta, P. ala and other two species P. incola, P. obtusa are known from the Middle and Upper Ordovician of the Prague Basin, Bohemia. Because of the actinodont type of the hinge, Pseudocyrtodonta is excluded from the subclass Protobranchia and is transferred to the Autobranchia, family Cycloconchidae. The family Pseudocyrtodontidae is considered invalid. Early and Middle Ordovician Cycloconchidae were highly diversified, containing 24 genera. Their diversity suddenly decreased during the Upper Ordovician to three genera only. The hinge of Pseudocyrtodonta could be considered morphologically close to the ancestral type of the hinge of the actinodonts. A complete species list of Ordovician actinodonts, including schematic figures of their hinge for the most important genera is presented. Pseudocyrtodonta was most probably an active burrower.