New Pennsylvanian Bivalvia (Mollusca), the Early Permian glaciation and the Carboniferous-Permian boundary in western Argentina

This paper deals with new finds from the Upper Paleozoic Santa Elena Formation that crops out to the east of Uspallata, western Argentina. Current studies reveal the existence in these rocks of a faunal assemblage characterized by the bivalve Myonia aparicioi sp. nov, fossil guide of a new biozone. This fauna is regarded as the youngest of the Pennsylvanian, age confirmed by its relationships with other Late Carboniferous faunas and flora. During the past several years, the relationships between the Late Carboniferous and early Permian faunas contained in this formation had been obscured by strong faulting. However, this uncertainty was answered by the discovery of a structurally uncomplicated section in which the Myonia aparicioi and the early Permian Costatumulus amosi faunas are separated by some tens of meters. These finds reveal the actual relationships between these important key fossils and allow a more precise definition of the Carboniferous-Permian boundary. This locality is recommended for establishing the stratotype and section of this systemic boundary in western Argentina. The existence of an earliest Permian glacial phase is confirmed by the association of these rocks with the brachiopod Costatumulus amosi. The new faunal assemblage occurs in various localities to the east of Uspallata city; the material is very poorly preserved, and the identification of the specimens is based mainly on shape. However, because of its biostratigraphic interest, three new bivalve species are described: Promytilus rochacamposi sp. nov., Permophorus? yrigoyeni sp. nov, and Myonia aparicioi sp. nov. Six further species are also included with nomina aperta because of insufficient material or doubtful generic assignment; these are: Aphanaia? sp., Leptodesma (Leiopteria) sp., Schizodus sp., Vacunella? sp., Myonia? sp., and Streblopteria sp. Specimens previously thought to be "eurydesmatids" probably belong to a new genus.