The name "Dinosauromorpha" was briefly coined by Michael J. Benton in 1985. It was considered an alternative name for the group "Ornithosuchia", which was named by Jacques Gauthier to correspond to archosaurs closer to dinosaurs than to crocodilians. Although "Ornithosuchia" was later recognized as a misnomer, it was still a more popular term than Dinosauromorpha in the 1980s. The group encompassed by Gauthier's "Ornithosuchia" and Benton's "Dinosauromorpha" is now given the name Avemetatarsalia. In 1991, Paul Sereno popularized the group, and defined it as a node-based clade, defined by a last common ancestor and its descendants. In his definition, Dinosauromorpha included the last common ancestor of Lagerpeton, MarasuchusPseudolagosuchus'', Dinosauria, and all its descendants. This definition was intended to correspond to a clade including dinosauriforms and lagerpetids, but not pterosaurs or other archosaurs. In 2011, Dinosauromorpha was redefined by Sterling Nesbitt to be a branch-based clade, defined by including reptiles closer to one group than to another. Under this definition, Dinosauromorpha included all reptiles closer to dinosaurs, rather than pterosaurs, ornithosuchids, or other pseudosuchians. Nesbitt's study supported the hypothesis that Pterosauromorpha was the sister group of Dinosauromorpha. Pterosauromorphs and dinosauromorphs together formed the group Ornithodira, which encompasses almost all avemetatarsalians.
Origins
Dinosauromorphs appeared by the Anisian stage of the Middle Triassic around 242 to 244million years ago, splitting from other ornithodires. Early Triassic footprints reported in October 2010 from the Świętokrzyskie Mountains of Poland may belong to a dinosauromorph. If so, the origin of dinosauromorphs would be pushed back into the Early Olenekian, around 249 Ma. The oldest Polish footprints are from a small quadrupedal animal named Prorotodactylus, but footprints belonging to the ichnogenusSphingopus that have been found from Early Anisian strata show that moderately large bipedal dinosauromorphs had appeared by 246 Ma. The tracks show that the dinosaur lineage appeared soon after the Permian-Triassic extinction event. Their age suggests that the rise of dinosaurs was slow and drawn out across much of the Triassic. Basal forms include Saltopus, Marasuchus, the perhapsidenticalLagosuchus, the lagerpetidLagerpeton from the Ladinian of Argentina and Dromomeron from the Norian of Arizona, New Mexico, and Texas, Ixalerpeton polesinensis and an unnamed form from the Carnian of Brazil, and the silesaurids, which include Silesaurus from the Carnian of Poland, Eucoelophysis from the Carnian-Norian of New Mexico, Lewisuchus and the perhaps identical Pseudolagosuchus from the Ladinian of Argentina, Sacisaurus from the Norian of Brazil, Technosaurus from the Carnian of Texas, Asilisaurus from the Anisian of Tanzania, and Diodorus from the Carnian to Norian of Morocco.
