Avemetatarsalia

Avemetatarsalians Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N (possible Early Triassic record if Prorotodactylus is part of the clade[1])
Clockwise from top-left: Dorygnathus banthensis (a pterosaur), Alamosaurus sanjuanensis (a sauropod), Edmontosaurus sp. (an ornithopod), Daspletosaurus torosus (a theropod), Styracosaurus albertensis (a ceratopsian) & Scolosaurus cutleri (an ankylosaurian), and Grus grus (an extant avian).
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Sauropsida
Clade:	Archosauria
Clade:	Avemetatarsalia Benton , 1999
Subgroups
†Incertovenator? †Mambachiton †Aphanosauria Ornithodira Gauthier, 1986 †Pterosauromorpha Aves )
Synonyms
Dracones Haeckel, 1895 Ornithosuchia Huene, 1908 Ornithotarsi Gauthier, 1986 Pan-Aves Gauthier and de Queiroz, 2001

Avemetatarsalia (meaning "bird

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.^[6] Dinosauromorpha, on the other hand, includes all avemetatarsalians closer to dinosaurs than to pterosaurs. Probable non-dinosaur dinosauromorphs include the diverse and widespread silesaurids, as well as more controversial and fragmentary taxa such as Marasuchus, Lagosuchus, Nyasasaurus, and Saltopus. Lagerpetids were also traditionally considered dinosauromorphs,^[7][3] though this has been more recently debated.^[8][6][9]

Description

The foundational characteristic is the "advanced mesotarsal" ankles, which are characterized by a large astragalus and a small calcaneum. This ankle orientation operated on a single hinge, allowing for better mobility. Probably as a result of this change, the common ancestor of the avemetatarsalians had an upright, bipedal posture, with their legs extending vertically, similar to that of mammals.

Feathers and other filamentary structures are known across the avemetatarsalians, from the downy pycnofibers of pterosaurs, to quill-like structures present in ornithischian 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

Avemetatarsalians were generally more lightly built than crocodile-line archosaurs. They had smaller heads and usually a complete lack of osteoderms.

Origin

Bird-line archosaurs appear in the fossil record by the

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.[14] 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.^[15] In fact, the name Ornithosuchia may be "illegal" under the PhyloCode because it does not include its eponymous taxon as part of its definition.^[15]

Cladogram after Nesbitt et al. (2017),^[16] with clade names from Cau (2018).^[17]

Archosauria	Pseudosuchia (crocodile-line archosaurs) Avemetatarsalia †Aphanosauria Ornithodira  †Pterosauromorpha † Pterosauria  Dinosauromorpha  †Lagerpetidae   Dinosauriformes   †Marasuchus   Dracohors   †Silesauridae   Dinosauria   †Ornithischia  Saurischia  †Sauropodomorpha Theropoda

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.^[8][6] 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 doswelliid).^[18]

In 2023, Nesbitt et al. described

Archosauria	Pseudosuchia Avemetatarsalia Mambachiton Aphanosauria Ornithodira Lagerpetidae Pterosauria Lagosuchus Silesauridae Dinosauria

References

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7. ^ ^{a b} Gauthier, Jacques (1986). "Saurischian monophyly and the origin of birds". In Padian, Kevin (ed.). The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Sciences. Vol. 8. San Francisco: California Academy of Sciences. pp. 1–55.
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14. ^ Senter, P. (2005). "Phylogenetic taxonomy and the names of the major archosaurian (Reptilia) clades". PaleoBios. 25 (3): 1–7.
15. ^ ^{a b} Taylor (2007). "Phylogenetic definitions in the pre-PhyloCode era; implications for naming clades under the PhyloCode" (PDF). PaleoBios. 27 (1): 1–6.
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