The discovery of a rich assemblage of fossil vertebrates in the lower section of the Loncoche Formation in Ranquil-Co, southern Mendoza province, Argentina, is reported herein. This assemblage comprises fishes, amphibians and reptiles. The fishes include Rajiformes indet., cf. Atractosteus sp. (Lepisosteidae, Semionotiformes), Percodei indet. (Perciformes) and Ceratodus sp. (Ceratodontidae, Dipnoi). The amphibians include ?Leptodactylidae indet. (Anura). The reptiles are represented by Chelidae indet. (Chelonii), Boidae indet. (Serpentes), Theropoda indet. and Titanosauria indet. (Saurischia), and Plesiosauria indet. The remains were found in a tidal paleoenvironment; this accounts for transportation, mixing and deposition of continental animal remains mixed with those from near coastal environments. This agrees with our sedimentological interpretation of the Loncoche Formation. Similar stratigraphic units from northern Patagonia (e.g. Alien and Los Alamitos Formations) record a progressive marine transgression up to the end of the Cretaceous and show similar paleovertebrate assemblages. In particular, the Loncoche fauna is similar to that described from the Los Alamitos Formation (Rio Negro Province); this suggests a tentative late Campanian to early Maastrichtian age. This age is in agreement with that indicated by micropaleontological data in both units.
