The geology and palaeoecology of the newly discovered Cretaceous neosauropod hydrothermal nesting site in Sanagasta (Los Llanos Formation), La Rioja, northwest Argentina

The recent discovery of the Cretaceous Sanagasta geothermal nesting site in the Los Llanos Formation, La Rioja Province, northwestern Argentina, has shed light on new and unexpected neosauropod reproductive behaviours. Here we recapitulate the palaeontological discovery at Sanagasta and the oological characterization of the nesting site (reported in 2010). In addition, we describe in detail the geology, sedimentology, petrology, and geochemistry of the nesting site with the goals of assessing the palaeoenvironment of the Los Llanos Formation at this site. The Sanagasta Geologic Park, where the nesting site is located, represents a local expression of a geothermal process that occured in the Sierras Pampeanas during the Cretaceous Period. This geothermal cycle allows for the first time an Early Cretaceous dating (Hauterivian–Aptian) for the Los Llanos Formation, which is further supported by the occurrence of notosuchians, and ornithopod, sauropod and theropod dinosaurs in other exposures of the same formation in La Rioja Province. As such, in addition to shedding light on neosauropod palaeobiology and their nesting environment, this investigation clarifies longstanding geological issues pertaining to the sedimentary basins in the Sierras Pampeanas Orientales (central-west Argentina).