Pachytraga gracilis sp. nov. (Hippuritida, Caprinidae) from the Barremian of the Sub-Alpine region (SE France). Evolutionary and biogeographic implications for the genus Pachytraga Paquier

Abstract

A new species of Pachytraga (family Caprinidae) is described from the Forêt de Coulmes (Northern Vercors, Subalpine region of SE France) and dated from the Taveraidiscus hugii- Nicklesia nicklesi ammonite Zones (lowermost Barremian). Pachytraga gracilis sp. nov. has an undivided posterior myocardinal cavity on the left valve and an anterior myophore supported by a single row of rectangular canals. The right valve bears a row of anterior canals, a partitioned posterior perimyophoral cavity and a network of minute pallial canals on the dorsal side. The new species represents an evolutionary link in between the Hauterivian Pachytraga tubiconcha and the Late Barremian–early Aptian Pachytraga paradoxa, so shortening the West European Barremian Lazarus phase of the genus Pachytraga. The possibility of a Barremian p.p. ecological rather than geographical refuge is proposed. We consider the new species as a potential biostratigraphic marker of the West European lowermost Barremian. It is the oldest member of canaliculated Caprinidae. Number of traits of Pachytraga gracilis sp. nov. (shape, mode of canaliculation), are absent in its assumed descendant P. paradoxa, but present in other advanced taxa, Praecaprina, Caprina, hence the new taxon was a precursor in more ways than one.