The comparative description improves our understanding of the early dinosaur braincase, which is poorly known relative to that of later representatives of the group. The braincase is characterized by a semilunar depression on the lateral surface of the basisphenoid, an occipital condyle whose ventral margin lies dorsal to the ventral margin of the cultriform process of the parabasisphenoid, a poorly developed preotic pendant, and anteriorly oriented basipterygoid processes. Whereas the oldest dinosaurs were geographically restricted to south Pangea, including rare ornithischians and more abundant basal members of the saurischian lineage, the group achieved a nearly global distribution by the latest Triassic, especially with the radiation of saurischian groups such as ‘‘prosauropods’’ and coelophysoids.The braincase anatomy of the sauropodomorph dinosaur Saturnalia tupiniquim from the Upper Triassic (Carnian) Santa Maria Formation of Brazil is described for the first time using computed tomography (CT). Dinosaurs arose from fully bipedal ancestors, the diet of which may have been carnivorous or omnivorous. These are mainly characterized by early diversification in Carnian times, a Norian increase in diversity and (especially) abundance, and the occupation of new niches from the Early Jurassic onwards. In fact, the radiation of the group comprises at least three landmark moments, separated by controversial (Carnian-Norian, Triassic-Jurassic) extinction events. Yet, contrary to the classical ‘‘competitive’’ models, dinosaurs did not gradually replace other terrestrial tetrapods over the Late Triassic. Some of these are connected to the acquisition of a fully erect bipedal gait, which has been traditionally suggested to represent a key adaptation that allowed, or even promoted, dinosaur radiation during Late Triassic times. Nonetheless, Dinosauria can be diagnosed by a suite of derived traits, most of which are related to the anatomy of the pelvic girdle and limb. The identification of dinosaur apomorphies is jeopardized by the incompleteness of skeletal remains attributed to most basal dinosauromorphs, the skulls and forelimbs of which are particularly poorly known.
candelariensis as basal theropods or basal saurischians, and the affinity and/or validity of more fragmentary taxa such as Agnosphitys cromhallensis, Alwalkeria maleriensis, Chindesaurus bryansmalli, Saltopus elginensis, and Spondylosoma absconditum.
On the contrary, several aspects of basal dinosaur phylogeny remain controversial, including the position of herrerasaurids, E. Recent cladistic analyses of early dinosaurs agree that Pisanosaurus mertii is a basal ornithischian that Herrerasaurus ischigualastensis and Staurikosaurus pricei belong in a monophyletic Herrerasauridae that herrerasaurids, Eoraptor lunensis, and Guaibasaurus candelariensis are saurischians that Saurischia includes two main groups, Sauropodomorpha and Theropoda and that Saturnalia tupiniquim is a basal member of the sauropodomorph lineage. The first phylogenetic definition to fit the current understanding of Dinosauria as a node-based taxon solely composed of mutually exclusive Saurischia and Ornithischia was given as ‘‘all descendants of the most recent common ancestor of birds and Triceratops’’. These include the typical Marasuchus lilloensis, more basal forms such as Lagerpeton and Dromomeron, as well as silesaurids: a possibly monophyletic group composed of Mid-Late Triassic forms that may represent immediate sister taxa to dinosaurs. No uncontroversial dinosaur body fossils are known from older strata, but the Middle Triassic origin of the lineage may be inferred from both the footprint record and its sister-group relation to Ladinian basal dinosauromorphs. The better known of these are Herrerasaurus ischigualastensis, Pisanosaurus mertii, Eoraptor lunensis, and Panphagia protos from the Ischigualasto Formation, Argentina, and Staurikosaurus pricei and Saturnalia tupiniquim from the Santa Maria Formation, Brazil. The oldest unequivocal records of Dinosauria were unearthed from Late Triassic rocks (approximately 230 Ma) accumulated over extensional rift basins in southwestern Pangea.
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