Sanagasta: record of a Neosauropod nesting site in a Lower Cretaceous paleohydrothermal landscape.

FIORELLI, LUCAS; GRELLET-TINNER, GERALD; ARGAÑARAZ, ELOISA; ALASINO, PABLO; LARROVERE, MARIANO

Heidelberg

Conferencia; 22° International Lateinamerika-Kolloquium; 2011

Cretaceous dinosaur nesting sites have been discovered on every continent, except Antarctica. Despite these discoveries, the factors that influenced the choice of selected colonial nesting sites by Cretaceous dinosaur still remained enigmatic. A recent study of the Sanagasta nesting site demonstrated the relation between dinosaurs nesting behaviors and a specific paleoenvironment: the Sanagasta neosauropod dinosaurs nested in an Early Cretaceous geothermal paleosetting (Grellet-Tinner & Fiorelli, 2010). Although the Los Llanos Formation outcrops throughout the La Rioja Province, the new nesting site is geographically restricted to a relatively small area (~300,000 m2) mostly located within the Sanagasta Geologic Park boundaries, in the central region of the Sierra de Velasco, La Rioja province, NW Argentina. There, the neosauropods ovideposited clutches of 21cm sub-spherical eggs in dug out holes systematically grouped nearby paleohydrothermal conduits, discharge channels, fountain geysers, domal mounds, paleo-hot spring terraces, ponds, travertine dams and mini-dams (Grellet-Tinner & Fiorelli, 2010). After detailed microscopic characterizations the original ~7.5 mm thick eggshells are hypothesized as an adaptation to this specialized geological paleoenvironment by compensating for the acidity of the hydrothermal solutions during the necessary incubation period. The number and position of the clutches combined with geological dating support colonial and nesting fidelity behaviors, migrations for reproductive purposes, and adaptation to soil moisture and thermoradiance for incubation, which for the latter had not been suspected previously for extinct dinosaurs but still observed in modern saurians. Indeed, several species, i.e. like the megapodes birds (Megapodius) and the Galapagos land iguanas (Conolophos) (Werner, 1983; Göth & Vogel, 1997) still rely on similar geological conditions to incubate their eggs. Although most of the 80 recorded clutches in the Sanagasta Geologic Park contain an average of 10 eggs, several displays up to 35 eggs with a maximum axis of 220 cm. Thorough field and taphonomic observations and ensuing statistical and geochemical analyses, coupled with microscopic characterizations indicate that the reproductive behaviors of the Sanagasta neosauropods was symbiotic and concomitant with the Early Cretaceous geothermal activities dated by the Gondwanic geothermal cycle. This investigation resolves longstanding geological issues related to the dating of the sedimentary basins in the Sierras Pampeanas Orientales (central-west Argentina) and the Los Llanos Formation by assigning them an Early Cretaceous date (Hauterivian to Aptian, ~134 and ~117 million years; Mutti et al., 2005; Grellet-Tinner & Fiorelli, 2010). Furthermore, this research provides the first definitive answer to the question to why neosauropod nesting sites were confined to a few selected geographical localities during the Cretaceous times. As such, the Cretaceous Sanagasta nesting site represents to date one of the most important dinosaur nesting sites in the world, as, for the first time, it shed lights on a particular reproductive behavior of neosauropods.