The Late Jurassic fish assemblage from the Cañadón Calcáreo Formation of Patagonia

SFERCO, E., ; RAUHUT, O.W. M.; A. LÓPEZ.ARBARELLO

Bristol, UK

Congreso; 69th Annual Meeting of the Society of Vertebrate Paleontology and the 57th Symposium of Vertebrate Palaeontology and Comparative Anatomy.; 2009

Society of Vertebrate Paleontology

The Cañadón Calcáreo Formation, Argentina, bears one the few Late Jurassic freshwater fish faunas known so far. Only two other important freshwater fish assemblages of Late Jurassic age are known, the Talbragar beds in Australia and the Morrison Formation in USA. The main fish-bearing stratum of the Cañadón Calcáreo Formation is found in at least six widely spread localities, up to 30 km apart from each other, all of which have a remarkably uniform fish composition. This suggests that a single large lake, or a series of smaller interconnected lakes were present. The absence of fossil plants and tetrapods support the hypothesis of a single large lake. Although a total of up to 60 m of lacustrine sediments are present at the base of the formation, abundant fishes only occur in a single layer of some 80 cm near the top of the sequence in the majority of known localities. Only in the most distant locality of Sierra de la Manea, several fish layers and desiccation cracks can be recognized, indicating closer proximity to the coast of the lake. This stratigraphically isolated occurrence of fish remains in the basal lacustrine part of the formation indicates that these layers represent mass mortality events, possibly associated with volcanic activity. Every studied locality shows the same association of teleosts with basal chondrosteans, with a marked dominance of teleosts. Together with the occurrence of chondrostean specimens with teleost remains in their digestive system, this suggests a predator-prey relationship among these taxa. Interestingly, recent fieldwork resulted in the discovery of a second fish-bearing level within the 50 m above the layer in the basal part. This second layer is less than 50 cm thick and intercalated between fluvial sandstones, but also seems to be dominated by teleosts. The fish association of the Cañadón Calcáreo Formation thus seems to be remarkably similar to that of the roughly contemporaneous Talbragar Beds. The latter also contain abundant teleosts and rarer chondrosteans(together with a few other taxa) that occur in a mass-mortality layer due to volcanic activity within a fresh water environment.