The occurrence and geological implications of the first Cretaceous fauna from La Rioja, Argentina

LUCAS FIORELLI; GERALD GRELLE-TINNER; ELOISA ARGAÑARAZ; LAURA CHORNOGUBSKY; MARTÍN HECHENLEITNER

San Juan

Congreso; IV Congreso Latinoamericano Paleontologia de Vertebrados; 2011

The Cretaceous terrestrial ecosystems in Argentina are well known especially from Patagonia, namely the Neuquén, Colorado, Somuncurá-Cañadón Asfalto, San Jorge, and Austral basins. Conversely, the northwestern continental Cretaceous basins are virtually unknown besides the rift basin of the Salta Group. Bodenbender (1911) named the ?Estratos de Los Llanos? a suite of sedimentary units scattered throughout the La Rioja Province with an Upper Cretaceous age. However subsequent authors correlated these strata with Cenozoic outcrops, a dating that prevailed (see Ezpeleta et al., 2006) until fragmentary dinosaur eggshells were discovered in the Sanagasta (Tauber, 2007). Following this discovery, the age of this locality was reverted to the previous Bodenbender?s Cretaceous dating (Grellet-Tinner and Fiorelli, 2010). The 2007-2010 Sanagasta-Tama Project was spearheaded to carry out geo-paleontologic field works in the poorly known Los Llanos Formation, La Rioja province, NW Argentina. The main goal of the project was to explore and thoroughly investigate the Sanagasta neosauropod dinosaurnesting site (Grellet-Tinner and Fiorelli, 2010) and correlate it with other outcrops to better understand the paleontology and paleoenvironments of the Formation. Recent field work led to the identification of numerous fossil localities, with specimens ranging from fragmentary bones to semi-articulated skeletons. The aim of this communication is to report the discovery of a new Cretaceous fossiliferous deposit at the type locality for Los Llanos Formation. The locality offers a diverse Cretaceous terrestrial fauna that consists of turtles, crocodyliforms, and a variety of dinosaurs. A Notosuchia crocodyliform is represented by diagnostic rostral and teeth characters shared with Notosuchus terrestris Woodward, 1896 and Sphagesaurus huenei Price, 1950, typical Cretaceous taxa from Gondwana (Fiorelli and Calvo, 2008). Dinosaurs are represented by titanosaurs and several fragmentary theropod taxa. The paleo-fauna assemblage is very similar to others reported in the Cretaceous Neuquén and Baurú Basins. The Los Llanos Formation exposuresin the area are characterizedby a succession of paleosols, composed by quartz sandstone with sparry cement. Paleosols display typical structures (e.g. calcareous nodules, laminar gypsum, rhizocretions and micro-silica sinter) typical of aridisols with calcitic horizons formed in hot and semi-arid climates. Bioturbations, burrows, and pupal chambers (in ephemeral lake facies) are also ubiquitous. The presence of isolated sandy river channels (main fossil-bearing) indicates the occurrence of ephemeral rivers. In conclusion, this study with the research at the Sanagasta site resolve the longstanding geological debate about the relative age of the sedimentary basins from the Sierras Pampeanas by offering definitive geological and paleontological evidence of a Cretaceous age for the Los Llanos Formation.