Avemetatarsalia

Avemetatarsalia (meaning "bird metatarsals") is a clade name established by British palaeontologist Michael Benton in 1999 for all crown group archosaurs that are closer to birds than to crocodiles. It includes a similarly defined subgroup, Ornithodira. An alternate name is Panaves, or "all birds", in reference to its definition containing all animals, living or extinct, which are more closely related to birds than to crocodiles.

Members of this group include the Dinosauromorpha, Pterosauromorpha, and the genus Scleromochlus. Dinosauromorpha contains more basal forms like Lagerpeton and Marasuchus and more derived forms like dinosaurs, which according to most modern scientists the birds belong to as members of the theropods. Pterosauromorpha contains Pterosauria, which are the famous flying reptiles, and as far as is known the first vertebrates capable of true flight. Most researchers think pterosaurians had neither an S-curved neck, nor an upright gait: the clade Ornithodira is defined on basis of ancestry, not characters – and it is always possible later descendants show a change in characters.

Origin
Bird-line archosaurs appear in the fossil record by the Anisian stage of the Middle Triassic about 245 million years ago, represented by the dinosauriform Asilisaurus. However, Early Triassic fossil footprints reported in 2010 from the Świętokrzyskie (Holy Cross) Mountains of Poland may belong to a more primitive dinosauromorph. If so, the origin of avemetatarsalians would be pushed back into the early Olenekian age, around 249 Ma. The oldest Polish footprints are classified in the ichnogenus Prorotodactylus and were made by an unknown small quadrupedal animal, but footprints called Sphingopus, 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.

Classification
In 1986 Jacques Gauthier coined the name Ornithodira for a node clade, containing the last common ancestor of the dinosaurs and the pterosaurs and all of its descendants. Paul Sereno had in 1991 given a formal (and different) definition of Ornithodira, one in which Scleromochlus was explicitly added. It was thus a potentially larger group than the Ornithodira of Gauthier. However, at that point, there was no named clade that could encompass species with a basal position on the archosaurian branch leading to dinosaurs (as opposed to that leading to crocodiles). In 1999 Michael Benton concluded that Scleromochlus was indeed outside of Gauthier's original conception of Ornithodira, so he named a new branch-based clade for this purpose: Avemetatarsalia, named after the birds (Aves), the last surviving members of the clade, and the metatarsal ankle joint that was a typical character of the group. Avemetatarsalia was defined as: all Avesuchia closer to Dinosauria than to Crocodylia. In 2004 Benton gave a formal definition of Ornithodira sensu Gauthier. In 2005 Sereno stated the opinion that Ornithodira was not a useful concept, whereas Avemetatarsalia was.

In 2001, the same clade was given the name Panaves (meaning "all [pan in Greek] birds" [aves in Latin]), coined by Jacques Gauthier. He defined it as the largest and most inclusive clade of archosaurs containing Aves (birds, anchored on Vultur gryphus) but not Crocodylia (anchored on Crocodylus niloticus). Gauthier referred Aves, all other Dinosauria, all Pterosauria, and a variety of Triassic archosaurs including Lagosuchus and Scleromochlus to this group.

Cladogram after Nesbitt (2011):