Epidiceratidae Rengarten 1950

FamilyEpidiceratidae Rengarten, 1950 [= Plesiodiceratidae Pchelintsev, 1959 = Heterodiceratidae Pchelintsev, 1959] Remarks. The distinction between Epidiceratidae and Requieniidae Kutassy, 1934, the two families included in Requienioidea, is primarily based on the shift of the commissural plane relative to the coiling axis of the left valve (Gourrat et al. 2003; Scott et al. 2010; Fig. 3). In Epidiceratidae, the angle between the commissural plane and the coiling axis is relatively large (usually 45 degrees or more), resulting in a comparatively erect growth habit (Fig. 3 A). In Requieniidae, this angle is distinctly smaller. As a result, the commissural plane intersects the spiral portion of the shell (Fig. 3 B), while this is generally not the case in Epidiceratidae. This change in whorl geometry brings the usually flattened anterior face of the requieniid left valve into close contact with the substrate. The resulting shell forms were termed frictional or attached clingers by Gili et al. (1995), and thus also differ ecologically from epidiceratids. In Epidiceras, Plesiodiceras, Heterodiceras and Saalidiceras gen. nov., the plesiomorphic, epidiceratid condition of this character is clearly met. In Hypelasma fritzlangi sp. nov., the commissural plane intersects the spiral shell in most specimens, but the growth habit remains relatively erect; this species thus might link Epidiceratidae and Requieniidae phylogenetically, but is assigned to the latter herein.