Abstract
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.
Highlights
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, dicynodonts, and c ynodonts[1,2,3]
Explored by several expeditions starting in the early sixties, 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
Our Bayesian age-stratigraphic model based on three new high-precision U–Pb CA-ID-TIMS tuff dates from the Hoyada del Cerro Las Lajas can be extended to the Ischigualasto Formation and its rich fossil record at IPP based on the following considerations: (a) the dated ‘Toba-2’ marker tuff bed, 107 mab at Hoyada del Cerro Las Lajas, is a direct correlative of the Herr Toba bentonite, ~ 20 m above base of the formation at IPP and, (b) the relatively sharp, but conformable contact of the Ischigualasto Formation with the overlying Los Colorados Formation is expected to be the same age at both location
Summary
Rhynchosaurs still abound and proterochampsids still occur in such strata, differing from younger beds that lack such taxa[93] It seems that the major Late Triassic turnover seen in the terrestrial tetrapod biotas of western Gondwana post-dates the assemblages currently known for the Hoyada del Cerro Las Lajas (and most probably the Carnian-Norian boundary), which is followed by Norian non-fossiliferous deposits that evidence an increase in humidity (see above). We agree with Agnolín and Rozadilla[107] that the posterolateral margin of the tibia of Pi. mertii is not concave as in early ornithischians (L. diagnosticus, NHMUK PV RU B17; Sc. lawleri, MNA V1752), but we disagree that this feature supports its silesaurid affinity, as a similar plesiomorphic condition is seen in most early dinosauromorphs, including d inosaurs[142]. At the same time, it shows how deficient is our current knowledge of the first million years of evolution of this main dinosaur lineage
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