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Abstract:The 'perleidiform' Mendocinichthys and Pseudobeaconia from the Potrerillos and Santa Clara Abajo formations (Upper Triassic; Argentina) are reviewed. Mendocinichthys has been known from a review of this species that is not based on the type material, but on referred specimens in the American Museum of Natural History. However, those specimens are found here to represent a new species of Pseudobeaconia, P. celestae sp. nov. Consequently, Mendocinichthys is restricted here to the type material and, within it, to the only specimen that clearly represents a distinct taxon and is thus designated lectotype. We further performed a cladistic analysis and propose a new family, Pseudobeaconiidae for these two South American Triassic genera, which is mainly characterized by the presence of an incomplete dorsal ridge of spine-like scales between the skull and the dorsal fin, and scales with straight posterior border, an elevated central region and marginal concentric ridges of ganoine. The cladistic analysis further indicates the existence of some lineages endemic to certain areas of Gondwana and Europe. Pseudobeaconia celestae sp. nov. represents the first record of Pseudobeaconia in the Cacheuta sub-basin of the Cuyana Basin. The genus was previously known from the Santa Clara sub-basin of the Cuyana Basin only, and the new record confirms the previous hypothesis of correlation between the sedimentary infilling of these sub-basins. The Cuyana Basin, with c. 60,000 km 2 of fluvio-lacustrine Triassic sediments and a rich fossil flora and fauna, constitutes one of the most important records of the continental life on Earth during that period of biological revolution. Within the Cuyana Basin, mainly the extensive outcrops of the Uspallata Group have yielded abundant vertebrate remains. With the exception of the Río Blanco Formation, fish remains are known from all of the units of the Uspallata Group, with a total of 26 fish species having been named (Ló pez-Arbarello 2004). However, the taxonomy of many of these taxa is problematic. Most of them are based on poorly preserved or even undiagnostic material. Therefore, a taxonomic revision of these faunas [Palaeontology, Vol. 51, Part 5, 2008, pp. 1025-1052 is necessary and essential towards a better understanding of the evolution of basal actinopterygians in Gondwana. Only three species from the Triassic of South America have received some attention in the literature on basal actinopterygian systematic and phylogeny (Hutchinson 1973a, b; Gardiner and Schaeffer 1989): Mendocinichthys brevis (Bordas, 1944), Pseudobeaconia bracaccinii Bordas, 1944, and P. elegans Bordas, 1944. Bordas (1944 created the taxon Mendocinia brevis for two specimens from lacustrine sediments of the Potrerillos Formation (Upper Triassic) of the Cuyana Basin, Argentina. Whitley (1953) recognized that the genus name Mendocinia was preoccupied by a hemipteran and replaced it with Mendocinichthys. Schaeffer (1955) studied several specimens in the American Museum of Natural History, New York, fro...

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