Abstract
The Solimões Formation (late early Miocene to Pliocene, Brazil) yields an important fossiliferous content, but the mollusks have been scarcely studied. We studied 58 bivalve specimens from Aquidabã and Morada Nova, Eirunepé Municipality, southwestern Amazonas. They belong to Castalia ambigua (Hyriidae) and Haasica cf. balzani (Mycetopodidae). The two genera are restricted to South America. Castalia has its oldest occurrence in the late Cretaceous at Itapecuru Formation, Parnaíba Basin (Maranhão State, Brazil), and it presently has a wide distribution in South America. Haasica is currently restricted to the Paraná River Basin, and its presence in the Solimões Formation is the first in the fossil record. These strictly freshwater bivalves suggest a lacustrine environment to the upper part of the Solimões Formation.
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