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Avemetatarsalia (meaning "bird ") is a clade of diapsid Reptile containing all more closely related to than to . The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. Dinosaur were the largest terrestrial animals for much of the Mesozoic era, and one group of small feathered dinosaurs (Aves, i.e. birds) has survived up to the present day. Pterosaur were the first flying vertebrates and persisted through the Mesozoic before dying out at the Cretaceous-Paleogene (K-Pg) extinction event. Both dinosaurs and pterosaurs appeared in the Triassic period, shortly after avemetatarsalians as a whole. The name Avemetatarsalia was first established by British palaeontologist Michael Benton in 1999. An alternate name is Pan-Aves, or "all birds", in reference to its definition containing all animals, living or extinct, which are more closely related to birds than to crocodilians.
Although dinosaurs and pterosaurs were the only avemetatarsalians to survive past the end of the Triassic, other groups flourished during the Triassic. The most basal (earliest-branching) and plesiomorphic ("primitive") known avemetatarsalians were the Aphanosauria. Aphanosaurs were rare, four-legged carnivores which were only properly distinguished as a group in 2017. The split between dinosaurs and pterosaurs occurred just after aphanosaurs branched off the archosaur family tree. This split corresponds to the subgroup Ornithodira (Ancient Greek ὄρνις ( órnis, “bird”) + δειρή ( deirḗ, “throat”), defined as the last common ancestor of dinosaurs and pterosaurs, and all of its descendants. Until the discovery of aphanosaurs, Ornithodira and Avemetatarsalia were considered roughly equivalent concepts.
Pterosauromorpha includes all avemetatarsalians closer to pterosaurs than to dinosaurs. True non-pterosaur pterosauromorphs have been historically difficult to determine. Small, insectivorous archosaurs of the family Lagerpetidae may potentially be examples, alongside the similar genus Scleromochlus. Dinosauromorpha, on the other hand, includes all avemetatarsalians closer to dinosaurs than to pterosaurs. Probable non-dinosaur dinosauromorphs include the diverse and widespread Silesauridae, as well as more controversial and fragmentary taxa such as Marasuchus, Lagosuchus, Nyasasaurus, and Saltopus. Lagerpetids were also traditionally considered dinosauromorphs, though this has been more recently debated.
and other filamentary structures are known across the avemetatarsalians, from the downy pycnofibers of pterosaurs, to quill-like structures present in dinosaurs, such as Psittacosaurus and Tianyulong, to feathers in theropod dinosaurs and their descendants, birds.
Two clades of avemetatarsalians, pterosaurs and birds, independently evolved flight. Pterosaurs are the earliest known to have evolved powered flight. Their wings are formed by a membrane of skin, muscle, and other tissues stretching from the ankles to a dramatically lengthened fourth finger. Birds evolved flight much later. Their wings formed from elongated fingers and their arms, all covered with flight feathers.
Avemetatarsalians were generally more lightly built than pseudosuchia. They had smaller heads and usually a complete lack of .
In a 2005 review of archosaur classification, Phil Senter attempted to resolve this conflicting set of terminology by applying strict priority to names based on when and how they were first defined. Senter noted that Ornithosuchia, the earliest name used for the total group of archosaurs closer to birds than to crocodiles, should be the valid name for that group, and have precedence over later names with identical definitions, such as Avemetatarsalia and Pan-Aves. While this has been followed by some researchers, others have either continued to use Avemetatarsalia or Ornithodira, or have followed Senter only reluctantly. Mike Taylor (2007) for example noted that, while Senter is correct in stating that Ornithosuchia has priority, this is "undesirable" because it probably excludes the eponymous family Ornithosuchidae, and questioned the utility of using priority before the PhyloCode is implemented to govern it. In fact, the name Ornithosuchia may be "illegal" under the PhyloCode because it does not include its eponymous taxon as part of its definition.
Cladogram after Nesbitt et al. (2017), with clade names from Cau (2018).
Kammerer et al. (2020) and Ezcurra et al. (2020) supported an alternative hypothesis regarding the relationships of lagerpetids. They were interpreted as non-pterosaur pterosauromorphs. This phylogeny would shorten the morphological and chronological gap perceived between pterosaurs and other stem-birds, and explain the origin of this group. Bennett (2020) argued that Scleromochlus, a genus historically considered a relative of ornithodirans or even a basal pterosauromorph, was instead a non-archosaur archosauriform (possibly a Doswelliidae).
In 2023, Nesbitt et al. described Mambachiton as the earliest diverging avemetatarsalian, outside of the minimally inclusive clade containing aphanosaurs and ornithodirans. Preliminary analyses had considered Mambachiton to be a basal poposauroid (a clade of ), though the later recognition of aphanosaurs as basal avemetatarsalians corrected this view. The results of the phylogenetic analyses of Nesbit et al. (2023) are shown in the cladogram below:
Sources
Michael J. Benton (2025). 9780520242098, Zweite Auflage, University of California Press. . ISBN 9780520242098
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