| Avemetatarsalians | |
|---|---|
 | |
| Seven avemetatarsalians (top left to bottom right): aGentoo penguin,Marasuchus,Kentrosaurus,Thalassodromeus (foreground) withAnhanguera (background),Diplodocus,Rhamphorhynchus | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Eucrocopoda |
| Clade: | Archosauria |
| Clade: | Avemetatarsalia Benton, 1999 |
| Subgroups | |
| Synonyms | |
Avemetatarsalia (meaning "birdmetatarsals") is aclade ofdiapsidreptiles containing allarchosaurs more closely related tobirds than tocrocodilians.[2] The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs.Dinosaurs were the largest terrestrial animals for much of theMesozoic Era, and one group of small feathered dinosaurs (Aves, i.e. birds) has survived up to the present day.Pterosaurs were the first flying vertebrates and persisted through the Mesozoic before dying out at theCretaceous-Paleogene (K-Pg) extinction event. Both dinosaurs and pterosaurs appeared in theTriassic Period, shortly after avemetatarsalians as a whole.[2][3] The name Avemetatarsalia was first established by British palaeontologistMichael Benton in 1999. An alternate name isPan-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.[4]
Although dinosaurs and pterosaurs were the only avemetatarsalians to survive past the end of the Triassic, other groups flourished during the Triassic. The mostbasal (earliest-branching) andplesiomorphic ("primitive") known avemetatarsalians were theaphanosaurs. Aphanosaurs were rare, four-legged carnivores which were only properly distinguished as a group in 2017.[5] The split between dinosaurs and pterosaurs occurred just after aphanosaurs branched off the archosaur family tree. This split corresponds to the subgroupOrnithodira (Ancient Greekὄρνις (órnis, "bird") +δειρή (deirḗ, "throat"), defined as thelast common ancestor of dinosaurs and pterosaurs, and all of its descendants. Until the discovery of aphanosaurs, Ornithodira and Avemetatarsalia were considered roughly equivalent concepts.[3]
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 familyLagerpetidae may potentially be examples, alongside the similar genusScleromochlus.[4]Dinosauromorpha, on the other hand, includes all avemetatarsalians closer to dinosaurs than to pterosaurs. Probable non-dinosaur dinosauromorphs include the diverse and widespreadsilesaurids, as well as more controversial and fragmentary taxa such asMarasuchus,Lagosuchus,Nyasasaurus, andSaltopus. Lagerpetids were also traditionally considered dinosauromorphs,[6][3] though this has been more recently debated.[7][4][8]
The foundational characteristic is the "advanced mesotarsal" ankles, which are characterized by a largeastragalus and a smallcalcaneum. 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 inornithischian dinosaurs, such asPsittacosaurus andTianyulong, to feathers in theropod dinosaurs and their descendants, birds.
Two clades of avemetatarsalians, pterosaurs and birds, independently evolved flight. Pterosaurs are the earliestvertebrates known to have evolved powered flight. Their wings are formed by a membrane of skin, muscle, and othertissues stretching from the ankles to a dramatically lengthened fourth finger.[9] Birds evolved flight much later. Their wings formed from elongated fingers and their arms, all covered withflight feathers.
Avemetatarsalians were generally more lightly built thancrocodile-line archosaurs. They had smaller heads and usually a complete lack ofosteoderms.
Bird-line archosaurs appear in the fossil record by theAnisian stage of theMiddle Triassic about 245 million years ago, represented by thedinosauriformAsilisaurus. However,Early Triassicfossil footprints reported in 2010 from theŚwiętokrzyskie (Holy Cross) Mountains ofPoland may belong to a more primitivedinosauromorph.[1] If so, the origin of avemetatarsalians would be pushed back into the earlyOlenekian age, around 249 Ma. The oldest Polish footprints are classified in theichnogenusProrotodactylus and were made by an unknown small quadrupedal animal, but footprints calledSphingopus, 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 thePermian–Triassic extinction event. Their age suggests that therise of dinosaurs was slow and drawn out across much of the Triassic.[1] However, other researchers have questioned the dinosauromorph affinities ofProrotodactylus and suggested that the tracemaker may have been a basal archosauriform such as theEuparkeriidae.[10][11] The primitive traits found in thequadrupedal aphanosaurTeleocrater shows that the earliest avemetatarsalians had many pseudosuchian-like features, and that the traits typical for the group evolved later.[12]
In 1986,Jacques Gauthier defined the nameOrnithosuchia (previously coined byHuene) for a branch-based clade including all archosaurs more closely related to birds than to crocodiles.[6] In the same year, Gauthier also coined and defined a slightly more restrictive node-based clade, Ornithodira, containing the last common ancestor of thedinosaurs and thepterosaurs and all of its descendants.Paul Sereno in 1991 gave a different definition of Ornithodira, one in whichScleromochlus was explicitly added.[13] It was thus a potentially larger group than the Ornithodira of Gauthier. In 1999Michael Benton concluded thatScleromochlus was indeed outside Gauthier's original conception of Ornithodira, so he named a newbranch-based clade for this purpose: Avemetatarsalia, named after the birds (Aves), the last surviving members of the clade, and the metatarsalankle joint that was a typical character of the group. Avemetatarsalia was defined as all Avesuchia closer to Dinosauria than toCrocodylia. 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" (lit. 'all birds', from Greekpan- + Latinaves), coined by Jacques Gauthier. He defined it as the largest and most inclusive clade of archosaurs containing Aves (birds, anchored onVultur gryphus) but not Crocodylia (anchored onCrocodylus niloticus). Gauthier referred Aves, all other Dinosauria, all Pterosauria, and a variety ofTriassic archosaurs, includingLagosuchus andScleromochlus, to this group.[14]
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.[15] 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 familyOrnithosuchidae, and questioned the utility of using priority before thePhyloCode is implemented to govern it.[16] In fact, the name Ornithosuchia may be "illegal" under the PhyloCode because it does not include its eponymous taxon as part of its definition.[16]
Cladogram after Nesbittet al. (2017),[5] with clade names from Cau (2018).[17]
| Archosauria |
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Kammereret al. (2020) and Ezcurraet 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.[7][4] Bennett (2020) argued thatScleromochlus, a genus historically considered a relative of ornithodirans or even a basal pterosauromorph, was instead a non-archosaur archosauriform (possibly adoswelliid).[18]
In 2023, Nesbittet al. describedMambachiton as the earliest diverging avemetatarsalian, outside of the minimally inclusive clade containingaphanosaurs and ornithodirans. Preliminary analyses had consideredMambachiton to be abasalpoposauroid (a clade ofpseudosuchians), though the later recognition of aphanosaurs as basal avemetatarsalians corrected this view. The results of thephylogenetic analyses of Nesbitet al. (2023) are shown in the cladogram below:[19]
| Archosauria |
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