Taxonomic revision and palaeobiogeographic affinities of Berriasian–Valanginian oysters from the Vaca Muerta and Mulichinco formations, southern Mendoza, Neuquén Basin, Argentina

Abstract

Oysters are very abundant in marine Jurassic–Cretaceous deposits across the world, which has led to multiple taxonomic studies since the nineteenth century. In contrast, South American Jurassic–Cretaceous oysters have been largely neglected in the literature. Here, focus is on Berriasian–Valanginian oysters from previously undocumented benthic faunas of southern Mendoza (Neuquén Basin, Argentina). Four species belonging to three genera of the Family Gryphaeidae, Subfamily Exogirinae are described, including their shell microstructure characterization, and their taxonomic status is discussed. Of the studied genera, Aetostreon and Ceratostreon have been widely recorded in the Neuquén Basin previously, whereas the genus Nanogyra is recorded for the first time in Argentina. Aetostreon is represented by A. subsinuatum and A. latissimum, and Ceratostreon and Nanogyra by C. hilli and N. (N.) brevisulcata n. sp. The oldest worldwide records of A. subsinuatum (Berriasian) and C. hilli (Valanginian) are documented here. The paleobiogeographic distribution of these species suggests a Tethyan distribution for A. subsinuatum and A. latissimum and a Trans-Temperate-Pacific distribution for C. hilli, while N. (N.) brevisulcata is endemic to the basin. The three genera appear as isolated records and different types of oyster mass occurrences (OMOs), classified according to its lateral extension, geometry, maximum thickness, and stratigraphic relevance. The different hierarchy of the OMOs could possibly represent different scales of palaeoenvironmental and/or stratigraphic changes, from basin-wide to local level, and could provide tools to identify variations in palaeoenvironmental parameters such as sedimentation rate, siliciclastic and nutrient input and salinity variations.