A new rhynchosaur maxillary tooth plate morphotype expands the disparity of the group in the Ischigualasto Formation (Late Triassic) of Northwestern Argentina

et al. 2020

Sci Rep

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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.

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 87%

Section: Introductionmentioning

confidence: 90%

Section: Introductionmentioning

confidence: 99%

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The Late Triassic Ischigualasto Formation at Cerro Las Lajas (La Rioja, Argentina): fossil tetrapods, high-resolution chronostratigraphy, and faunal correlations

Desojo

Fiorelli

et al. 2020

Sci Rep

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Skull osteology of the holotype of the rhynchosaur Hyperodapedon sanjuanensis (Sill, 1970) from the Upper Triassic Ischigualasto Formation of Argentina

Gentil

2021

The Anatomical Record

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Rhynchosaurs are bulky quadrupedal herbivores that achieved a cosmopolitan distribution during the Middle and Late Triassic. Rhynchosaurids are characterized by a pair of premaxillae modified into an edentulous beak that had a bone‐to‐bone occlusion with the tips of the dentaries, and a specialized masticatory apparatus composed of groove(s) on the maxilla and ridges(s) on the dentary. The Argentinian fossil record of rhynchosaurs is abundant, but only two nominal species have been named so far. One of them, the hyperodapedontine Hyperodapedon sanjuanensis, is the most abundant tetrapod of the Ischigualasto Formation of northwestern Argentina. However, the anatomy of H. sanjuanensis remains poorly studied. The repreparation and computed tomographic (CT) scanning of its holotypic skull (MACN‐Pv 18185) allowed a detailed description of its anatomy. Among the novel anatomical data presented here, we provide the first tridimensional complete reconstruction of a rhynchosaurid palate. This structure is extremely complex in comparison to the plesiomorphic archosauromorph condition, and more data on other rhynchosaurid palates are needed to shed light on the sequence of character states acquisition that resulted in this peculiar anatomy. The presence of a pair of septomaxillae in H. sanjuanensis is the first report of these bones in a rhynchosaurid, and this new information allowed to identify septomaxillae also in the hyperodapedontines Teyumbaita sulcognathus and Hyperodapedon mariensis. The better understanding of the skull anatomy of the holotype of H. sanjuanensis is a first step towards an improved knowledge of the morphology and taxonomy of the South American rhynchosaurs.

The stem-archosaur evolutionary radiation in South America

Journal of South American Earth Sciences

Montefeltro

Pinheiro

et al. 2021

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The oldest archosauromorphs (dinosaurs, birds, crocodiles, and their stem-taxa) are recorded in middle− upper Permian rocks, but it was not after the Permo− Triassic mass extinction that the group shows a substantially high taxonomic richness and ecomorphological disparity. The early evolutionary history of the Archosauromorpha during the Early and Middle Triassic is mainly based on fossils recovered from rocks in southern Africa, Europe and Asia, whereas South America possesses a more complete fossil record of the group only in the Late Triassic.Here we revisit, discuss, and reanalyse the non-archosaurian archosauromorph fossil record of the current-day South America. The Early Triassic archosauromorph record in this continent is still scarce, but it documents the early evolution of the group in western Pangaea and is crucial to understand more globally the biotic recovery after the Permo− Triassic mass extinction. The Middle Triassic record is extremely scarce, but the Late Triassic archosauromorph assemblage of South America is among the most diverse and abundant worldwide. The last decade has witnessed a considerable improvement in our knowledge of the record, taxonomy, phylogeny, and macroevolution of the group with the input from the South American fossils. Nevertheless, a considerable amount of research is needed and ideally should be focused on some particular aspects of the Triassic evolutionary radiation of Archosauromorpha. Among them, the Early Triassic record should be expanded, more numerous and more complete Middle Triassic archosauromorph specimens are crucial to have a more complete picture of the evolution of the group, and the taxonomy of groups like proterochampsids and hyperodapedontine rhynchosaurs should be clarified through detailed anatomical work.