Revision of the lower Permian bivalves from the Río Bonito Formation, Paraná Basin, Río Grande do Sul, Brazil

Taxonomy of mollusks in lower Permian rocks of southern Brazil is relatively well established for the state of Santa Catarina, where they carry a diversified community of bivalves dominated by Pectinidae (Heteropecten catharinae Reed) accompanied by Trigoniidae (Schizodus occidentalis Reed), Pholadomyidae, Myoconchidae (Stutchburia brasiliensis Reed), Sanguinolitidae?, Crassatelidae (Oriocrassatela itajaiensis Rocha Campos), gastropods, brachiopods and echinoderms. However, in Río Grande do Sul only two forms of this group were previously known, Aviculopecten cambahyensis Martins and Stutchburia sp. New field collections were made in order to attempt testing diversity and taphonomic constrains in these more southern areas. Additionally, samples stored at UNISINOS Museum and collected also at Río Grande do Sul were studied. A total of 313 samples containing right and left valves were tabulated, measured and described. Analysis revealed the dominance -as in Santa Catarina- of disarticulated valves of pectinids (674 forms), represented by dominant A. cambahyensis, Stutchburia sp. (28 valves), and species of two genera not previously described for this region, i.e., Schizodus sp. (7 valves) and Oriocrassatella sp. (one form). The pectinids are dispersed throughout all the eleven meters thick pelitic succession with wavy lamination, but all other elements appear only in the uppermost three meters, accompanied by rare fragments of plants and suggesting an upward shallowing sequence. The basal and upper beds including broad cross-stratified sandstones suggest a deltaic context in a shallow nearshore environment. The lower diversity observed in the southern areas of the basin, where the absence of more typical marine forms is noteworthy, suggests the existence of more stressing conditions possibly linked to colder waters and a consequent low salinity. Sesiones libres Facultad de Ciencias Naturales y Museo