The origin and early evolution of dinosaurs
Authors: Langer Max C. , Ezcurra Martin D. , Bittencourt Jonathas S. , Novas Fernando E. 
Published: November 2009

The oldest unequivocal records of Dinosauria were unearthed from Late Triassic rocks (approximately 230 Ma) accumulated over extensional rift basins in southwestern Pangea. The better known of these areHerrerasaurus ischigualastensis,Pisanosaurus mertii,Eoraptor lunensis, andPanphagia protosfrom the Ischigualasto Formation, Argentina, andStaurikosaurus priceiandSaturnalia tupiniquimfrom the Santa Maria Formation, Brazil. 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. These include the typicalMarasuchus lilloensis, more basal forms such asLagerpetonandDromomeron, as well as silesaurids: a possibly monophyletic group composed of Mid‐Late Triassic forms that may represent immediate sister taxa to dinosaurs. 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 andTriceratops”. Recent cladistic analyses of early dinosaurs agree thatPisanosaurus mertiiis a basal ornithischian; thatHerrerasaurus ischigualastensisandStaurikosaurus priceibelong in a monophyletic Herrerasauridae; that herrerasaurids,Eoraptor lunensis, andGuaibasaurus candelariensisare saurischians; that Saurischia includes two main groups, Sauropodomorpha and Theropoda; and thatSaturnalia tupiniquimis a basal member of the sauropodomorph lineage. On the contrary, several aspects of basal dinosaur phylogeny remain controversial, including the position of herrerasaurids,E. lunensis, andG. candelariensisas basal theropods or basal saurischians, and the affinity and/or validity of more fragmentary taxa such asAgnosphitys cromhallensis,Alwalkeria maleriensis,Chindesaurus bryansmalli,Saltopus elginensis, andSpondylosoma absconditum. 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. 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. 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. Yet, contrary to the classical “competitive” models, dinosaurs did not gradually replace other terrestrial tetrapods over the Late Triassic. In fact, the radiation of the group comprises at least three landmark moments, separated by controversial (Carnian‐Norian, Triassic‐Jurassic) extinction events. 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. Dinosaurs arose from fully bipedal ancestors, the diet of which may have been carnivorous or omnivorous. 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.