Present knowledge of Late Triassic tetrapod evolution, including the rise of dinosaurs, relies heavily on the fossil-rich continental deposits of South America, their precise depositional histories and correlations. We report on an extended succession of the Ischigualasto Formation exposed in the Hoyada del Cerro Las Lajas (La Rioja, Argentina), where more than 100 tetrapod fossils were newly collected, augmented by historical finds such as the ornithosuchid Venaticosuchus rusconii and the putative ornithischian Pisanosaurus mertii. Detailed lithostratigraphy combined with high-precision U–Pb geochronology from three intercalated tuffs are used to construct a robust Bayesian age model for the formation, constraining its deposition between 230.2 ± 1.9 Ma and 221.4 ± 1.2 Ma, and its fossil-bearing interval to 229.20 + 0.11/− 0.15–226.85 + 1.45/− 2.01 Ma. The latter is divided into a lower Hyperodapedon and an upper Teyumbaita biozones, based on the ranges of the eponymous rhynchosaurs, allowing biostratigraphic correlations to elsewhere in the Ischigualasto-Villa Unión Basin, as well as to the Paraná Basin in Brazil. The temporally calibrated Ischigualasto biostratigraphy suggests the persistence of rhynchosaur-dominated faunas into the earliest Norian. Our ca. 229 Ma age assignment to Pi. mertii partially fills the ghost lineage between younger ornithischian records and the oldest known saurischians at ca. 233 Ma.

With one of the richest land biotas recorded worldwide, the Ischigualasto Formation of north-western Argentina represents a unique “window” into Late Triassic biodiversity and evolution. This stratigraphic unit is well known from the Ischigualasto Provincial Park (IPP), San Juan Province, with a fossil record composed of plants, fishes, and most of the known tetrapod groups of the time, i.e., temnospondyls, rhynchosaurs, archosauriforms (including dinosaurs), dicynodonts, and cynodonts^1,2,3. Radioisotopic dates of various vintages have given the Ischigualasto fauna a temporal context, elevating its global significance in understanding the Triassic land ecosystems, as well as the early evolution of dinosaurs³. Nevertheless, exposures of the Ischigualasto Formation outside the IPP have only been briefly explored, delivering only subordinate fossil records³. One exception is the site known as Hoyada del Cerro Las Lajas^4,5,6 in La Rioja Province, where the northernmost known outcrops of the formation are exposed (Fig. 1; see also fig. 1 in Baczko et al.⁷). Explored by several expeditions starting in the early sixties (see Historical background and motivation in the Supplementary Information), the fossil record of the area appears meagre compared to that of the IPP and it has been described as “a poorly fossiliferous outcrop” (p. 20 in Martínez et al.³), but includes key specimens, such as the holotypes of the ornithosuchid Venaticosuchus rusconii and the probable ornithischian Pisanosaurus mertii.

Journal Reference: Julia B. Desojo, Lucas E. Fiorelli, Martín D. Ezcurra, Agustín G. Martinelli, Jahandar Ramezani, Átila. A. S. Da Rosa, M. Belén von Baczko, M. Jimena Trotteyn, Felipe C. Montefeltro, Miguel Ezpeleta, Max C. Langer. The Late Triassic Ischigualasto Formation at Cerro Las Lajas (La Rioja, Argentina): fossil tetrapods, high-resolution chronostratigraphy, and faunal correlations. Scientific Reports, 2020; 10 (1) DOI: 10.1038/s41598-020-67854-1

@WFS,World Fossil Society,Riffin T Sajeev,Russel T Sajeev