Tournasian (Lower Carboniferous) Actinopterygian and Sarcopterygian fish remains from Agua de Lucho Formation, La Rioja, Argentina

SFERCO E.; RUSTAN, JUAN; VACCARI N.E.; STERREN A.; BALSEIRO, D.; EZPELETA M.; PRESTIANNI, C.; ASTINI, R.A.

Congreso; RCAPA 2022; 2022

ASOCIACIÓN PALEONTOLÓGICA ARGENTINA

Upper Palaeozoic osteichtyan fossil record is rather abundant in South America (especially in Uruguay, Bolivia and Chile); however, it is scarce and only represented by few late Carboniferous and early Permian isolated scales and bone remains in Argentina. Here we present well-preserved osteichthyan remains from the Agua de Lucho Formation in Sierra de las Minitas, Río Blanco Basin, La Rioja province, Argentina. This is a marine siliciclastic succession (1250 m thick), recording glacigenic and deltaic environments of middle to late Tournaisian age, based on palynological data and biostratigraphic correlations. Fishes were collected from four stratigraphic intervals. The lowermost, interval 1, bears actinopterygian remains in nodules within massive mudstones underlying diamictitic beds, including isolated bones and a remarkable articulated specimen, preserving the right opercular series, cleithrum, a branchiostegal plate, pectoral fin and the anterior portion of the trunk showing articulated scales mainly in medial view. The scales are typically ganoid, with a peg and socket articulation, rectangular (deeper than long), strongly serrated posteriorly, exhibiting a high peg and anterior process. Interval 2 includes a tetrapodomorph sarcopterygian skull roof bone and an actinopterygian branchiostegal plate preserved as molds in fine-grained sandstones, around the marker palynomorph level of Waltzispora lanzonii. Both intervals also record abundant and diverse invertebrate fauna and plants. Interval 3, located slightly above levels with the plants Pseudosporogonites cf. hallei and Porongodendron minitensis, bears isolated actinopterygian bones (i.e., maxilla, nasal) and smooth longer than deep, posteriorly serrated scales, exhibiting a short peg and anterior process. Fishes occur in fine-grained sandstones,only accompanied by abundant plants. Finally, an isolated actinopterygian subopercle occurs within the mudstones of interval 4, associated only with abundant invertebrate fauna. All studied actinopterygian bones are ornamented with densely packed ganoin tubercles and short, confluent ridges, whereas posteriorly serrated scales are only smoothly ornamented with low ridges. Similar bone ornamentation is found in Devonian early actinopterygians (i.e, Mimipiscis), but similar scale configuration and ornamentation also occur in many Carboniferous actinopterygian groups (like Radinichthyidae); both with a broad Gondwanan distribution. In South America, Devonian early actinopterygians are mostly found in marine deposits of Bolivia, whereas Carboniferous forms have been mainly recovered from upper Carboniferous marine deposits of Uruguay. The detailed anatomical study of these remains will not only improve our knowledge on early actinopterygian evolution and diversification, but will also be important to understand its paleobiogeographic implications, within South America and the rest of Gondwana