New freshwater mussels (Bivalvia, Unionida) with potential trigonioidid and hyriid affinities from the Early Cretaceous of Brazil.

Abstract

Two new taxa of freshwater mussels (Bivalvia: Unionida) from the Aptian Crato Formation of the Araripe Basin, NE Brazil, are described. The fossil bivalves are confined to 30- to 130-cm-thick bioturbated

mudstones overlying the fossil-rich laminated limestones of the Crato Formation Konservat- Lagersta ̈tte. Individuals are often preserved with closed or splayed articulated valves, some of them

potentially in life position, forming an autochthonous to parautochthonous assemblage. Monginellopsis bellaradiata nov. gen., nov. sp. shares key characters with the Trigonioidoidea: (i) the anterior pedal

retractor muscle scar is clearly separated from the anterior adductor muscle scar; (ii) the shell has fold- like radial ribs on the posterior half; (iii) a right valve anterior tooth has a striated facet. Araripenaia

elliptica nov. gen., nov. sp. is the most abundant and widely distributed unionid of the Crato Formation. Its ornament of anterior inverted V-shaped riblets, and central and posterior radial and sub-radial riblets resembles modern and fossil Hyriidae from the Americas, but also Trigonioidoidea from Eurasia. Its dentition of two smooth anterior pseudocardinals and two smooth posterior laterals in each valve provides no further clues for systematic assignment; muscle scars are not preserved. Assignment to the Hyriidae would make Araripenaia the oldest member of this family known from South America. Moreover, this bivalve assemblage of trigonioidoidids, hyriids, and previously reported silesunionoids suggests palaeobiogeographic links to other areas in both Gondwana and Laurasia.