Duplexium jatobensis & Anhapoa munizi: Freshwater Mussels from the Early Cretaceous of northeastern Brazil.

Freshwater Mussels, Unionida, are the only order of Bivalves found exclusively in freshwater. There are about 900 species today, found on every continent, with some species considered economically significant, due to their use as food, their ability to produce pearls, their ability to modify environments, or their impact upon Fish stocks. Unionids have a unique life cycle among Bivalves, with a larval stage which lives commensually or parasitically on the skin, fins, or gills of a Fish host.

In a paper published in the Journal of South American Earth Sciences on 20 May 2025, Débora Eliza Baumann of the Laboratório de Paleontologia de Macroinvertebrados at São Paulo State University, Luiz Ricardo Simone of the Laboratório de Malacologia at the Museu de Zoologia of the Universidade de São Paulo, Rafael Costa da Silva of the Museu de Ciéncias da Terra of the Serviço Geológico do Brasil, and Renato Pirani Ghilardi, also of the Laboratório de Paleontologia de Macroinvertebrados at São Paulo State University, describe two new species of Unionid Freshwater Mussels from the Early Cretaceous Salvador Formation of Pernambuco State, Brazil.

The Salvador Formation outcrops in the Jatobá Basin of Pernambuco State, and records a fan delta laid down within a palaeolake in the Late Berriasian age (roughly between 140 and 137 million years ago). Both new species are described from specimens collected at a site designated BL-42 within the city of Floresta, and held within the collection of the Museu de Ciéncias da Terra of the Serviço Geológico do Brasil.

Location of Jatoba Basin (08◦30′ to 9◦06′ S; 37◦06′ to 38◦30′ W) in Northeast Brazil, location where the fossils are collected (8◦41′48.60″S 38◦16′34.30″W) and stratigraphy of the Jatoba Basin. Baumann et al. (2025).

The first new species is named Duplexium jatobensis, where 'Duplexium' refers to the fact that it has two types of teeth on its hinge, and 'jatobensis' means 'from Jatobá' in reference to the Jatobá Basin where it was discovered. The species is described from four specimens, which are between 47 and 50 mm in length, 18-19 mm in width, and 6-8 mm in thickness, with three teeth on the hinge, the central one of which is distinctly larger than the other two.

Holotype, MCT.I.7202. Duplexium jatobensis; Salvador Formation (Berriasian), general morphology and muscle scars. Abbreviations: aa, anterior adductor; pa, posterior adductor; pl, pallial line; ht, heterodont teeth; tt, taxodont teeth; pp, pedal protractor; apr, anterior pedal retractor; pe, pedal elevator; S1, posterior pedal retractor? S2, posterior adductor? or S1 + S2, slow and fast components of the posterior adductor? Scale bar is 10 mm. Baumann et al. (2025).

The second new species is named Anhapoa munizzi, where 'Anhapoa' derives from 'Anhapoā', which means 'canine tooth' in the Tupi-Guarani indigenous language of Brazil, in reference to a well-pronounced cardinal tooth on the hinge of the right valve of the shell. Anhapoa munizzi is described from five specimens, which range from 43 to 52 mm in length, 23 to 27 mm in width, and from 8 to 14 mm in thickness. The hinge of the right valve has a large cardinal tooth between two pits, and small lateral teeth.

Holotype, MCT.I.7147, Anhapoa munizi, Salvador Formation (Berriasian), general morphology and muscle scars. Abbreviations: aa, anterior adductor; pa, posterior adductor; pl, pallial line; ht, heterodont teeth; tt, taxodont teeth; pp, pedal protractor; apr, anterior pedal retractor; pe, pedal elevator, S1, posterior pedal retractor? S2, posterior adductor? or S1 + S2, slow and fast components of the posterior adductor? Scale bar is 10 mm. Baumann et al. (2025).

Both Duplexium jatobensis and Anhapoa munizi lack any form of ornamentation, have smooth beak, lack a prodissoconch, have a heterodont hinge and an additional taxodont hinge, have an elevator muscle scar, and lack a pallial sinus, all traits consistent with assignment to the Family Iridinidae within the Superfamily Etherioidea, one of the two major subdivisions of the Unionida.

The oldest members of the Family Iridinidae described to date come from the Middle Cretaceous of Africa, with some possible Iridinid fossils from the End Cretaceous of South America. Modern Iridinids are restricted to Africa, leading malacologists toconclude that the genus originated there, after the continents of Africa and South America split apart, between 140 and 120 million years ago. The Salvador Formation of the Jatobá Basin, however, was laid down during the earliest part of this rifting process, when Africa and South America were still joined, raising the possibility that the Iridinidae originated in South America, but died out there during the End Cretaceous Extinction.

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