Ricardo N. Martinez

The early diversification of dinosaurs produced a major ecological change in the terrestrial ecosystems, culminating with tetrapod assemblages dominated in abundance by dinosaurs by the Triassic/Jurassic boundary (~201 million years ago (Ma)). Therefore, studying the initial diversification of dinosaurs is crucial to understand the establishment of Mesozoic assemblages. However, the lack of stratigraphically continuous fossil data in the few geological units that preserve the oldest known dinosaurs (~233–227 Ma, Carnian age) obscures our understanding of this initial diversification. The Ischigualasto Formation in northwestern Argentina (231.4–225.9 Ma) yields a rich vertebrate assemblage and new studies resulted in an abundant and stratigraphically near-continuous fossil record, which offers new insights into the early diversification of dinosaurs. Among the discoveries, we report Anteavis crurilongus gen. et sp. nov., an early-diverging theropod, which supports the notable diversity of small- to medium-sized dinosaurs during the late Carnian. Anteavis is recovered outside Neotheropoda, but it has features previously thought to be exclusive to that group. We show that dinosaur diversity and abundance in the Ischigualasto Formation were higher than previously recognized, particularly among small herbivores (<30 kg) and medium-sized (30–200 kg) predators. This diversification occurred in Ischigualasto during a climatic shift to semi-arid conditions, but the return of more humid conditions resulted in a gap in the dinosaur record that started at 228.91 ± 0.14 Ma. Only 15 million years (Myr) later, in the middle Norian age, the dinosaur record recovered its abundance and diversity in the basin, but now it was characterized by larger-bodied species. Our findings demonstrate an early dinosaur diversification probably punctuated by a climate-driven faunal turnover in, at least, southwestern Pangaea.

Herrerasauridae were among the first large carnivorous dinosaurs. Although their anatomy and phylogenetic relationships have been previously studied, little is known about their paleobiology. Bone pathologies provide clues about the behaviour of ancient organisms, and this study evaluates presumed craniofacial pathologies in herrerasaurid specimens to infer their etiology and implications. Our analysis reveals that the craniofacial marks analysed are most likely pathological in nature and possibly result from intraspecific agonistic interactions, rather than post-mortem alterations. The relatively high frequency of lesions identified supports this claim. In the sample of herrerasaurids with preserved skulls analysed, nearly half presented craniofacial lesions. The bone traumas identified here are the oldest evidence of pathology in dinosaurs. Identification and analysis of pathologies in early dinosaurs can provide insights into their paleobiology and evolution of behavioural characteristics.

Pterosaurs were the first vertebrates to evolve powered flight. The timing of their origin is still debated, and hypotheses range from the end of the Permian Period, to the lower Mesozoic Era, and through to the Middle–Late Triassic epochs. Regardless of when they originated, the oldest records are restricted to the Upper Triassic Norian Stage in the northern hemisphere (Europe, USA and Greenland). We report two new raeticodactylid pterosaurs, Yelaphomte praderioi gen. et sp. nov. and Pachagnathus benitoi gen. et sp. nov. from the upper Norian to Rhaetian Quebrada del Barro Formation in north‐western Argentina. The new specimens (an isolated dentary symphysis, partial rostrum, and distal half of ulna) are the first unequivocal Triassic records of pterosaurs in the southern hemisphere, confirming that the absence of pterosaurs outside north‐western Pangaea during the Late Triassic was the result of poor sampling rather than true absence. These new discoveries provide evidence of a greater diversity of pterosaurs living in terrestrial habitats and a wider global distribution of pterosaurs from the beginning of their evolution on Earth.

The early evolution of diapsid reptiles is marked by a deep contrast between our knowledge of the origin and early evolution of archosauromorphs (crocodiles, avian and non-avian dinosaurs) to that of lepidosauromorphs (squamates (lizards, snakes) and sphenodontians (tuataras)). Whereas the former include hundreds of fossil species across various lineages during the Triassic period¹, the latter are represented by an extremely patchy early fossil record comprising only a handful of fragmentary fossils, most of which have uncertain phylogenetic affinities and are confined to Europe^1,2,3. Here we report the discovery of a three-dimensionally preserved reptile skull, assigned as Taytalura alcoberi gen. et sp. nov., from the Late Triassic epoch of Argentina that is robustly inferred phylogenetically as the earliest evolving lepidosauromorph, using various data types and optimality criteria. Micro-computed tomography scans of this skull reveal details about the origin of the lepidosaurian skull from early diapsids, suggesting that several traits traditionally associated with sphenodontians in fact originated much earlier in lepidosauromorph evolution. Taytalura suggests that the strongly evolutionarily conserved skull architecture of sphenodontians represents the plesiomorphic condition for all lepidosaurs, that stem and crown lepidosaurs were contemporaries for at least ten million years during the Triassic, and that early lepidosauromorphs had a much broader geographical distribution than has previously been thought.

The origin and homology of the turtle shell is one of the most captivating topics in amniote evolution. In this contribution, we present a new species of turtle from the Late Triassic of Argentina whose peripheral plates raise questions about the homology of these bones in turtles. The external morphology of the peripheral plates of Waluchelys cavitesta gen. et sp. nov. (Testudinata, Australochelyidae) is as in any other turtle, however, appearances can be deceptive. Internally, these plates have an unexpected internal cavity. The absence of structural similarities and of ontogenetic or phylogenetic transitional forms between the peripheral plates of W. cavitesta and other testudinatans might suggest that the periphery of turtles represents a case of deep homology. Furthermore, the present and recent findings suggest that the structure and ossification patterns of the periphery of the turtle shell were more plastic and subject to variation than other elements of the shell, at least in the earliest stages of turtle evolution. These findings also suggest that the typical mesochelydian turtle shell could have been acquired in a two‐stage process.

Protosuchids are known from the Late Triassic to the Early Cretaceous and form a basal clade of Crocodyliformes. We report here a new protosuchid crocodyliform, Coloradisuchus abelini, gen. et sp. nov., from the middle Norian Los Colorados Formation, La Rioja, northwestern Argentina. Our phylogenetic analysis recovers Coloradisuchus abelini within Protosuchidae, as the sister group of the clade formed by Hemiprotosuchus and two species of Protosuchus (P. richardsoni and P. haughtoni). The new protosuchid C. abelini increases the diversity of crocodyliforms in the Late Triassic and, together with H. leali from the same stratigraphic levels of the Los Colorados Formation, shows that the diversification of basal crocodyliforms was probably faster and/or older than thought previously. http://.zoobank.org/urn:lsid:zoobank.org:pub:66629B34-42E4-4F6D-A56C-B082D749AB4B SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP Citation for this article: Martínez, R. N., O. A. Alcober, and D. Pol. 2019. A new protosuchid crocodyliform (Pseudosuchia, Crocodylomorpha) from the Norian Los Colorados Formation, northwestern Argentina. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2018.1491047.

Sphenodontians were a successful group of rhynchocephalian reptiles that dominated the fossil record of Lepidosauria during the Triassic and Jurassic. Although evidence of extinction is seen at the end of the Laurasian Early Cretaceous, they appeared to remain numerically abundant in South America until the end of the period. Most of the known Late Cretaceous record in South America is composed of opisthodontians, the herbivorous branch of Sphenodontia, whose oldest members were until recently reported to be from the Kimmeridgian–Tithonian (Late Jurassic). Here, we report a new sphenodontian,Sphenotitan leyesigen. et sp. nov., collected from the Upper Triassic Quebrada del Barro Formation of northwestern Argentina. Phylogenetic analysis identifiesSphenotitanas a basal member of Opisthodontia, extending the known record of opisthodontians and the origin of herbivory in this group by 50 Myr.