Cecilia Apaldetti

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.

Sauropodomorph dinosaurs were the dominant medium to large-sized herbivores of most Mesozoic continental ecosystems, being characterized by their long necks and reaching a size unparalleled by other terrestrial animals (> 60 tonnes). Our study of morphological disparity across the entire skeleton shows that during the Late Triassic the oldest known sauropodomorphs occupied a small region of morphospace, subsequently diversifying both taxonomically and ecologically, and shifting to a different and broader region of the morphospace. After the Triassic–Jurassic boundary event, there are no substancial changes in sauropodomorph morphospace occupation. Almost all Jurassic sauropodomorph clades stem from ghost lineages that cross the Triassic–Jurassic boundary, indicating that variations after the extinction were more related to changes of pre-existing lineages (massospondylids, non-gravisaurian sauropodiforms) rather than the emergence of distinct clades or body plans. Modifications in the locomotion (bipedal to quadrupedal) and the successive increase in body mass seem to be the main attributes driving sauropodomorph morphospace distribution during the Late Triassic and earliest Jurassic. The extinction of all non-sauropod sauropodomorphs by the Toarcian and the subsequent diversification of gravisaurian sauropods represent a second expansion of the sauropodomorph morphospace, representing the onset of the flourishing of these megaherbivores that subsequently dominated in Middle and Late Jurassic terrestrial assemblages.

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.

Dinosaurs dominated the terrestrial ecosystems for more than 140 Myr during the Mesozoic era, and among them were sauropodomorphs, the largest land animals recorded in the history of life. Early sauropodomorphs were small bipeds, and it was long believed that acquisition of giant body size in this clade (over 10 tonnes) occurred during the Jurassic and was linked to numerous skeletal modifications present in Eusauropoda. Although the origin of gigantism in sauropodomorphs was a pivotal stage in the history of dinosaurs, an incomplete fossil record obscures details of this crucial evolutionary change. Here, we describe a new sauropodomorph from the Late Triassic of Argentina nested within a clade of other non-eusauropods from southwest Pangaea. Members of this clade attained large body size while maintaining a plesiomorphic cyclical growth pattern, displaying many features of the body plan of basal sauropodomorphs and lacking most anatomical traits previously regarded as adaptations to gigantism. This novel strategy highlights a highly accelerated growth rate, an improved avian-style respiratory system, and modifications of the vertebral epaxial musculature and hindlimbs as critical to the evolution of gigantism. This reveals that the first pulse towards gigantism in dinosaurs occurred over 30 Myr before the appearance of the first eusauropods.