Two new species are described based on material from the Late Miocene Paraná Formation, Entre Ríos, Argentina: Polymesoda muravchiki, nov. sp., and Erodona doellojuradoi, nov. sp. The bearing rocks were deposited during the Entrerriense ingression that covered part of central-northern Argentina, reaching as far north as Bolivia, Paraguay, and southern Brazil. The exposure from where the material was collected represents a rich parautochtonous assemblage — also including marine taxa — that has been assigned to the Late Miocene. Extant representatives of Polymesoda and Erodona are known to inhabit mostly brackish environments in the Caribbean region, the eastern Pacific coast of America, and Southeast Asia (in the case of Polymesoda), and the Atlantic coast of southern South America (Erodona). The presence of these bivalves in the Paraná Formation suggests that at least a marginal connection may have existed between a southern arm of the Amazonian Sea and the Paraná Sea during the Miocene. It is highly unlikely that these taxa could have migrated along the Atlantic coast of South America — contrarily to the case of the fully marine taxa — in view of their peculiar ecological requirements.
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