Hypelasma salevensis (FAVRE, 1913) from the Upper Kimmeridgian of the French Jura, and the Origin of the Rudist Family Requieniidae

Abstract

The requieniid rudist species ‘Matheronia’ salevensis FAVRE, first described from the Tithonian of Mont Salève, eastern France, is transferredto the genus Hypelasma PAQUIER, which is distinguished from Matheronia by possession of a posterior myophoral ledge in the left (attached) valve. Diminutive specimens from the Upper Kimmeridgianof the southern Jura are described and placed in this species.Hence, Hypelasma salevensis (FAVRE) is the stratigraphically oldest known member of the Family Requieniidae. It may also provideanother example of phyletic size increase among rudists. Revised diagnoses are given for the family, genus and species.The main distinction between the requieniids and the diceratids, from among which they arose, concerns the angle between the coiling axis of the left valve and the commissural plane. In diceratids, this angle is large, such that the often sub-equal umbones tend to twist outwards from the commissural plane, so avoiding mutual interference.In requieniids, by contrast, this angle is small, such that the prominent umbo of the left valve tends to coil across the commissuralplane in trochospiral to helicospiral fashion, while that of the right valve is suppressed in compensation, producing an exogyriform morphology. The requieniid modification of growth geometry, already present in H. salevensis, generated an extended basal surface on the flattened anterior wall of the left valve, implying specialized adaptationof these rudists as frictional or attached clingers.Requieniid ancestry should be sought among species of the pre-existing diceratid genera Epidiceras or Plesiodiceras, which also attached by the left valve. Although Plesiodiceras is favoured by its already more or less operculiform right valve and relatively small size, the derived condition of its posterior myophoral organisation is problematical. However, its juvenile shell shows some similarity of external form to H. salevensis, suggesting the possibility of paedomorphicevolution.