First record of Dromaeosauridae (Dinosauria: Theropoda) in the Early Late Cretaceous Bajo Barreal Formation of Chubut Province, Argentina

CASAL, GABRIEL A.; MARTÍNEZ, RUBÉN D.; CANDEIRO, CARLOS ROBERTO; LAMANNA, MATTHEW C.; IBIRICU, LUCIO M.

Austin; Texas

Congreso; SVP, Reunión anual de Paleontología; 2007

SVP (Society of Vertebrate Paleontology)

We report three teeth of dromaeosaurid theropods that constitute the first record of these dinosaurs in the Upper Cretaceous Bajo Barreal Formation of central Patagonia, Argentina. The teeth were recovered from the highest levels of the early Late Cretaceous (Cenomanian-Coniacian) Lower Member of the Bajo Barreal at the Estancia ?Ocho Hermanos? in southern Chubut Province. The teeth are labiolingually compressed with curved mesial and distal carinae. In lingual view, the distal carina is rotated near its apex.  Denticles are present but very fine on the distal carina, averaging 19 per 5 mm near its basoapical midpoint, and are absent on the mesial carina. The distal denticles are oriented perpendicular to the distal edge of the crown, and, as seen in labial view, are quadrangular in shape, measuring an average of 0.35 mm long by 0.40 mm wide. The denticles are even smaller toward the base of the crown, and are projected toward its apex. A series of wrinkles or concave undulations crosses the entire surface of the base of each crown. These structures occur on both the labial and lingual faces of each tooth, although they are better developed on the lingual face. The labiolingual section at the base of the crowns is characterized by its subrectangular form. The general morphology of the teeth and denticles is very similarto that described in the dromaeosaurid Dromaeosaurus albertensis from the Late Cretaceous (Campanian) of western North America. These teeth demonstrate the presence of Dromaeosauridae in the Bajo Barreal Formation, and supplement the record of these theropods in Argentina Patagonia during the Late Cretaceous. Moreover, the presence of Dromaeosauridae in the Bajo Barreal broadens the paleogeographic distribution of the clade, as it represents the most southerly occurrence of these theropods in South America. The nonavian theropod fauna of the Bajo Barreal Formation is now one of the most diverse known from the Southern Hemisphere, comprising abelisauroids such as Xenotarsosaurus bonapartei alongside a Megaraptor-like primitive tetanuran, the coelurosaur Aniksosaurus darwini, and the highly derived Dromaeosauridae.