After a major reduction due to the end-Triassic extinction event (only three surviving genera according to Ros-Franch et al., 2014), the order Trigoniida (Mollusca, Bivalvia) experienced a strong diversification during the Early Jurassic. In southern basins from South America at least eleven genera were recognized between the Hettangian and the Toarcian (Leanza 1993, Pérez et al. 2008). Three of them (Trigonia, Prosogyrotrigonia and Frenguelliella) were Triassic survivors, while the other eight genera evolved during the Jurassic, with the respective oldest records for many of them occurring within these basins. The genera Groeberella, Neuquenitrigonia and Quadratojaworskiella are exclusive from South America. The possibility that Groeberella evolved from myophorid ancestors as suggested by Pérez et al. (1995) implies a fourth surviving lineage from the Triassic. Neuquenitrigonia is clearly related to the Trigonia lineage (Pérez et al. 2008). The genus Quadratojaworskiella contains only two species; certain differences between them, nevertheless, suggest relationships to different genera for each. The genus Jaworskiella includes a few species from South and North America (Poulton 1979, Leanza 1993, Pérez et al. 2008), though they might represent different offshoots from Frenguelliella, perhaps providing an example of parallel evolution. Myophorella, a widespread and conspicuous Jurassic genus, most likely evolved from some species of Jaworskiella within southern South America during the Pliensbachian. Scaphorella seems to be closely related to Myophorella. Psilotrigonia is a genus of uncertain affinities; its oldest record (the Late Sinemurian or Early Pliensbachian species P. vegaensis) is from northern Chile (Pérez et al. 2008) being found later on in North America (Poulton 1979). The list of genera is completed with Vaugonia, most likely originating in Japan during the Hettangian and subsequently arriving to South America in the late Sinemurian via North America (Poulton 1979), though a wide variety of species have been included within the genus, which deserves detailed phylogenetic revision.Facultad de Ciencias Naturales y Muse
