NEW SPECIES OF THE GENUS HORIOPLEURA (BIVALVIA, HIPPURITIDA, POLYCONITIDAE) FROM THE BARREMIAN–LOWER APTIAN DEPOSITS OF THE MEDITERRANEAN REGION: EVOLUTIONARY IMPLICATIONS

Two new species of the genus Horiopleura (Family Polyconitidae) are described from the Lower Cretaceous deposits of the Mediterranean Tethys margins. Horiopleura arabica comes from the lower Aptian Qishn Formation of southern Oman and Horiopleura helviorum, which occurs in the upper Barremian, Urgonian, Rustrel fauna but was primarily recorded in the lower Aptian deposits of Ardèche (SE France). In Horiopleura arabica the left valve projects dorsally off the commissural plane and the anterior myophore is a small crest, the right valve posterior myophore is a ledge; radial bands are weak but present. Horiopleura helviorum, the smallest species of the genus is characterized by a parietal lamellar anterior myophore on right valve. The two new species, are small and possess a domal left valve. They have a significant biostratigraphic and palaeobiogeographic potential. The new species illustrate a particular mode of arrangement of the anterior myophore on the right valve of Horiopleura, leading to a distinctive species grouping during the late Barremian-early Aptian. Our discoveries document a remarkable contemporaneous radiation of the Polyconitidae.