Augusto N. Varela

The Oldest Lower Upper Cretaceous Plesiosaurs (Reptilia, Sauropterygia) from Southern Patagonia, Argentina

Abstract. Plesiosaurs are recorded for the first time from the lower section of Mata Amarilla Formation, Santa Cruz Province, Patagonia, Argentina. The stratigraphic succession consists of mudstones and siltstones interbedded with medium to fine-grained sandstone, deposited in a littoral environment during the Cenomanian-Santonian; therefore the material is the oldest record of plesiosaurs from the lower Late Cretaceous rocks of Argentina. The remains include teeth, some vertebrae, and one propodium assigned to Elasmosauridae indet. and Plesiosauria indet. The status of Polyptychodon patagonicus Ameghino, 1893, as well as its stratigraphic position are discussed, leading to the conclusion that the material described by Ameghino is probably from the Mata Amarilla Formation and can only be referred to Plesiosauria indet. Analysis of sedimentologic features suggests that the material described here was deposited in an estuarine environment, strongly influenced by tides. The characters of the inferred environment are consistent with the type of preservation of the materials.

ICNOLOGÍA DE LA FORMACIÓN RÍO MAYER, CRETÁCICO INFERIOR, SUDOESTE DE GONDWANA, PATAGONIA, ARGENTINA

Abstract ICHNOLOGY OF THE RÍO MAYER FORMATION, LOWER CRETACEOUS, SOUTHWESTERN GONDWANA, PATAGONIA, ARGENTINA. The Río Mayer Formation was deposited during the Early Cretaceous in the Austral Basin (Santa Cruz Province, Argentina) and includes black shales with intercalated sandstones and marls. Abundant information is available on its palaeontological content, especially on ammonoids, but detailed ichnological studies are lacking. Sedimentological and ichnological data were collected in the Río Guanaco area (49°57′11″S—72°04′56″W). Three sections were identified and described in detail. The lower Section was deposited during the Berriasian—Valanginian and is composed of black shales. The middle section including black marls with intercalated black shales was deposited during the Valanginian—Hauterivian. The upper section is composed of black shales with frequent thin intercalations of finegrained sandstones (Barremian—Albian). Six ichnogenera were identified: Zoophycos, Chondrites, Bergaueria, Ophiomorpha, Palaeophycus and Teredolites. The most abundant and constant is Zoophycos, arranged in three morphological groups. All these ichnogenera are grouped —in the Río Mayer Formation— into two ichnoassociations. The first is found in the middle section, and is a typical Zoophycos ichnofacies with associated Bergaueria. The second ichnoassociation appears at the top of the upper section, and contains a Zoophycos ichnofacies with associated Ophiomorpha and Teredolites. The unusual presence of Bergaueria in low oxygen offshore environments, exceeds the bathymetric range typical of this ichnogenus. The unusual presence of O. cf. rudis in outer shelf environments reveals the existence of doomed pioneers in the analyzed unit.