Record of the first Cretaceous continental fauna from La Rioja province, northwestern Argentina: geo-paleontological implications.

FIORELLI, LUCAS; GRELLET-TINNER, GERALD; ARGAÑARAZ, ELOISA; LARROVERE, MARIANO; CHORNOGUBSKY, LAURA; TORRENS, JAVIER; HECHENLEITNER, MARTÍN

Heidelberg

Conferencia; 22° International Lateinamerika-Kolloquium; 2011

The Cretaceous terrestrial ecosystems in Argentina are relatively well known especially from Patagonia, namely Neuquén, Colorado, San Jorge and, Austral basins. Conversely, the northwestern continental Cretaceous sedimentary basins are virtually unknown besides the rift basin of the Salta Group in the Salta Province. Bodenbender (1911) named the “Estratos de Los Llanos” a suite of sedimentary units scattered throughout the La Rioja Province and dated them to the Upper Cretaceous. However subsequent authors correlated these strata with Cenozoic outcrops from neighboring provinces, a dating that prevailed until (see Ezpeleta et al., 2006) fragmentary dinosaur eggshells were discovered in the Sanagasta Valley (Tauber, 2007). Following this discovery, the age of this locality was reverted to the previous Bodenbender’s Cretaceous dating (Grellet-Tinner & Fiorelli, 2010). In 2010, preliminary explorations of the 2007-2010 Sanagasta-Tama Project started with a geo-paleontologic field work in the Los Llanos Formation in La Rioja province, Northwestern Argentina. The main goal of the project was to explore and thoroughly investigate what is now the Sanagasta neosauropod dinosaur nesting site (Grellet-Tinner & Fiorelli, 2010). Subsequent work focused on, correlating this site with other outcrops to better understand the paleontology and paleoenvironments of the Los Llanos Formation. Recent field work led to the identification of numerous fossil-bearing localities, with specimens ranging from fragmentary bones to semi-articulated vertebrates. The main objective of this communication is to report the discovery of a new Cretaceous fossiliferous deposit at Tama –the type locality for Los Llanos Formation–, La Rioja. The locality offers a diverse Cretaceous terrestrial fauna composed of several major vertebrate clades, namely turtles, crocodyliforms, and several sauropod and theropod fossils. Crocodyliform is represented by very diagnostic notosuchian cranial remains. The rostral and teeth share characters with Notosuchus terrestris and Sphagesaurus huenei that are typical Cretaceous taxa from the Gondwana (Fiorelli & Calvo, 2008). Likewise, dinosaurs are represented by indeterminate neosauropod and several theropod taxa possibly affiliated with Titanosauria, Abelisauria, Carcharodontosauridae and Coelurosauria, respectively. The paleofaunal assemblage is very similar to those from the Cretaceous Neuquén and Baurú basins as well as African Cretaceous basins. The Los Llanos Formation exposures in Tama are characterized by a succession of well developed paleosols, composed by quartz sandstone cemented by sparry calcite. Paleosols display pedogenetic structures (e.g. calcareous nodules, laminar gypsum, and abundant rhizocretions) suggesting a typical aridisol calcitic horizon intersperced by ephemeral rivers, represented by the presence of isolated and fossil-bearing sandy river channels. Bioturbations, burrows, and pupal chambers (in ephemeral lake facies), and indeterminate trace fossils are also ubiquitous. Overall, the results of the field work at Tama coupled with the research at Sanagasta resolve the longstanding geological debate about the relative age of the sedimentary basins from the Sierras Pampeanas Orientales (central-west area of Argentina) by offering definitive geological (Gondwanic Cycle) and paleontological evidence of a Cretaceous age for the Los Llanos Formation.