The last chapter of 30 million years of molluscan evolution: Permian non-marine bivalves of the Rio do Rasto Formation, Paraná 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 the Leinzia similis assemblage (Serrinha Member) and the Palaeomutela platinensis assemblage (Morro Pelado Member), Rio do Rasto Formation, is presented. Over two hundred specimens were examined, and three bivalve assemblages were identified: (a) the Terraia decarinata assemblage in the uppermost part of the Serrinha Member, including Relogiincola delicata gen. et sp. nov., Palaeomutela australis sp. nov., and Palaeomutela platinensis (Reed, 1935); (b) the Terraia cf. decarinata assemblage in the transition between the Serrinha and Morro Pelado members, including T. decarinata, and P. platinensis, and (c) the Palaeomutela australis assemblage in the lowermost part of the Morro Pelado Member, also including R. delicata and P. platinensis. The investigated bivalves were recorded above the last occurrences of Terraia altissima (Holdhaus, 1918) and Cowperesia emerita Mendes, 1952 in 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 genus Palaeomutela occurs 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 of Palaeomutela in Western Gondwana.