The last chapter of 30 million years of molluscan evolution: Permian non-marine bivalves of the Rio do Rasto Formation, Parana Basin, Brazil

Abstract

The last chapter of the evolutionary history of the Permian (Artinskian-Wuchiapingian) endemic bivalves of the Passa Dois Group, Brazil, is depicted. They evolved in a vast, isolated epeiric sea under progressive continentalization. Previously, bivalves that thrived during times of marked non-marine (limnic) conditions have been poorly investigated. Hence, a systematic survey of the bivalves between theLeinzia similisassemblage (Serrinha Member) and thePalaeomutela platinensisassemblage (Morro Pelado Member), Rio do Rasto Formation, is presented. Over two hundred specimens were examined, and three bivalve assemblages were identified: (a) theTerraia decarinataassemblage in the uppermost part of the Serrinha Member, includingRelogiincola delicatagen. et sp. nov.,Palaeomutela australissp. nov., andPalaeomutela platinensis(Reed,1935); (b) theTerraiacf.decarinataassemblage in the transition between the Serrinha and Morro Pelado members, includingT. decarinata, andP. platinensis, and (c) thePalaeomutela australisassemblage in the lowermost part of the Morro Pelado Member, also includingR. delicataandP. platinensis. The investigated bivalves were recorded above the last occurrences ofTerraia altissima(Holdhaus,1918) andCowperesia emeritaMendes,1952in the Serrinha Member. In the Gai-As Formation, Huab Basin, Namibia, these species are found in well-constrained Wordian-Capitanian strata, suggesting a Capitanian-Wuchiapingian age for the studied assemblages. The Permian cosmopolitan freshwater genusPalaeomutelaoccurs in all three assemblages and is also found in the lower Beaufort Group (South and Central Africa). Therefore, our record may represent the maximum paleobiogeographic expansion ofPalaeomutelain Western Gondwana.