First Fish Remains from the Earliest Late Triassic of the Chañares Formation (La Rioja, Argentina) and Their Paleobiogeographic Implications

Abstract

We hereby describe fish material collected from the lowermost Upper Triassic continental levels of the Chañares Formation (Campo de Córdoba and El Torcido localities). The specimens consist of a fragmentary dermal skull roof bone assigned to Mawsoniidae based on its ornamentation marked by coarse ridges and wide shallow grooves, plus patches of ganoid scales referred to cf. Pseudobeaconiidae based on the presence of an elevated central region in most of the scales. However, unlike that which is observed in pseudobeaconiids, these scales are characterized by an ornamentation pattern consisting of concentric and/or parallel ganoine ridges covering their surface. These scale patches, for which a brief taphonomic approach is hereby provided, are the first vertebrate fossil remains to be collected from the upper member of the Chañares Formation. The Mawsoniidae described herein represents the southernmost record of the family, being the first coelacanth from Argentina and the first Triassic mawsoniid from South America. Freshwater mawsoniid coelacanths established in Laurasia during the Late Triassic and the new Gondwanan record suggests that similar weather conditions (hot and arid but with seasonal precipitations) may have affected north and south Pangea during this period.