Upper Barremian-lower Aptian rudist faunas of Urgonian-type platform formations from Ardèche (southeastern France)

Abstract

Upper Barremian-lower Aptian Urgonian-type platform carbonates from Ardèche yield a rich rudist fauna belonging to the families Requieniidae, Monopleuridae, Caprinidae, and Polyconitidae. The upper Barremian assemblage from the Ardèche river gorges consists of eighteen species, it conforms to the Rustrel fauna of the Monts-de-Vaucluse but contains a new genus Valclusella, and a new species of Bicornucopina, a genus unknown so far in Western Europe, whereas Mathesia darderi is identified for the first time in SE France. We define the "Saint-Montan limestones", of early Aptian age, as the uppermost member of the Urgonian Formation in this region. The corresponding Saint-Montan fauna is described for the first time: it contains twenty-five rudist species, of which eight are new and seventeen are inherited from the antecedent late Barremian Orgon and Rustrel fossil type localities. Of the eight new species, two are ascribed to earlier genera, i.e., Homopleura parva and Bicornucopina acuminata, three are assigned to a new genus, Myostyla ardescensis, while Valclusella valclusensis and Valclusella biconvexa preexisted in the late Barremian. Aside from inherited taxa, e.g., Pachytraga paradoxa, Praecaprina varians-gaudryi, Caprina douvillei, important novelties regard the Caprinidae, characterized by the appearance of probable new species of Pachytraga, P. aff. paradoxa, Offneria, O. aff. rhodanica, and three new species of Praecaprina: Pr. tubifera n. sp., Pr. paquieri n. sp., and Pr. vacua n. sp. The early Aptian p.p., therefore, coincides with the acme of the genus Praecaprina, SE France being unique in this respect. In the Saint-Montan fauna, up to about nine species from the Rustrel fauna are missing. The Saint-Montan rudist assemblage is unique, thus far, in SE France, even as the study of its composition is still in progress. It has a West European character, similar to that of its antecedent late Barremian one, and the succeeding late early Aptian Villaroya de los Pinares fauna from Spain, but the three have distinctive taxonomic compositions. The Saint-Montan stratigraphic succession is framed by two drowning boundary events. The taxonomic composition of its rudist assemblage fills a gap between the late Barremian and the late early Aptian faunas, and represents a distinctive episode in the Barremian-lower Aptian Urgonian-type rudist sequence of southern France and Spain, and possesses a significant biostratigraphic potential.