Avemetatarsalia

File:AdvMesotarsal.png

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 vertebrates 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.{{cite journal |vauthors=Elgin RA, Hone DW, Frey E |title=The Extent of the Pterosaur Flight Membrane |journal=Acta Palaeontologica Polonica |volume=56 |issue=1 |pages=99–111 |year=2011 | doi=10.4202/app.2009.0145 |doi-access=free }} 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 crocodile-line archosaurs. They had smaller heads and usually a complete lack of osteoderms.

{{see|Origin of birds}}

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. However, other researchers have questioned the dinosauromorph affinities of Prorotodactylus and suggested that the tracemaker may have been a basal archosauriform such as the Euparkeriidae.{{cite journal|author1=Fichter J.|author2=Kunz R.|year=2013|title="Dinosauromorph" tracks from the Middle Buntsandstein (Early Triassic: Olenekian) of Wolfhagen, northern Hesse, Germany|journal=Comunicações Geológicas|volume=100|issue=1|pages=81–88}}{{cite journal |author1=Mujal, E. |author2=Fortuny, J. |author3=Bolet, A. |author4=Oms, O. |author5=López, J.A. |year=2017 |title=An archosauromorph dominated ichnoassemblage in fluvial settings from the late Early Triassic of the Catalan Pyrenees (NE Iberian Peninsula) |journal=PLOS ONE |volume=12 |issue=4 |pages=e0174693 |doi=10.1371/journal.pone.0174693 |pmid=28423005 |pmc=5396874 |bibcode=2017PLoSO..1274693M |doi-access=free }} The primitive traits found in the quadrupedal aphanosaur Teleocrater shows that the earliest avemetatarsalians had many pseudosuchian-like features, and that the traits typical for the group evolved later.[http://www.ibtimes.com/dinosaur-evolution-crocodile-ancient-cousin-teleocrater-rhadinus-confuses-scientists-2524858 Dinosaur Evolution: Crocodile-Like Ancient Cousin, Teleocrater Rhadinus, Confuses Scientists]

In 1986, Jacques Gauthier defined the name Ornithosuchia (previously coined by Huene) for a branch-based clade including all archosaurs more closely related to birds than to crocodiles. In the same year, Gauthier also coined and defined a slightly more restrictive node-based clade, Ornithodira, containing the last common ancestor of the dinosaurs and the pterosaurs and all of its descendants. Paul Sereno in 1991 gave a different definition of Ornithodira, one in which Scleromochlus was explicitly added.Sereno, P. C. 1991. Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate Paleontology Memoir 2, 11(4, Supplement):1–53. It was thus a potentially larger group than the Ornithodira of Gauthier. In 1999 Michael Benton concluded that Scleromochlus was indeed outside 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 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 Greek {{wikt-lang|grc-Latn|παν-|pan-}} + Latin {{wikt-lang|la|aves}}), 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.Gauthier, J. and de Queiroz, K. (2001). "Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name "Aves"". pp. 7–41 in Gauthier, J. and L. F. Gall (eds.), New Perspectives on the Origin and Early Evolution of Birds: Proceedings of the International Symposium in Honor of John H. Ostrom. New Haven: Peabody Museum of Natural History, Yale University. {{ISBN|0-912532-57-2}}.

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.{{cite journal |last1=Senter |first1=P. |year=2005 |title=Phylogenetic taxonomy and the names of the major archosaurian (Reptilia) clades. |journal=PaleoBios |volume=25 |issue=3 |pages=1–7}} 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.{{cite journal | last1 = Taylor | year = 2007 | title = Phylogenetic definitions in the pre-PhyloCode era; implications for naming clades under the PhyloCode | url =http://www.miketaylor.org.uk/dino/pubs/taylor2007/Taylor2007-pre-phylocode-pn.pdf | journal = PaleoBios | volume = 27 | issue = 1| pages = 1–6 }} 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).{{Cite journal|author=Andrea Cau |year=2018 |title=The assembly of the avian body plan: a 160-million-year long process |journal=Bollettino della Società Paleontologica Italiana |volume=57 |issue=1 |pages=1–25 |doi=10.4435/BSPI.2018.01 |doi-broken-date=2024-11-20 |url=https://www.paleoitalia.it/wp-content/uploads/2024/03/01_Cau_2018_BSPI_571.pdf }}

{{Clade| style=font-size:90%;line-height:85%

|label1=Archosauria

|1={{clade

|1=Pseudosuchia (crocodile-line archosaurs) 80px

|label2=Avemetatarsalia

|2={{Clade

|1=†Aphanosauria 80 px

|label2=Ornithodira

|2={{Clade

|label1=†Pterosauromorpha

|1=†Pterosauria 80 px

|label2=Dinosauromorpha

|2={{Clade

|1=†Lagerpetidae 80 px

|label2=Dinosauriformes

|2={{Clade

|1=†Marasuchus 80 px

|label2=Dracohors

|2={{Clade

|1=†Silesauridae 80 px

|label2=Dinosauria

|2={{Clade

|1=†Ornithischia 80px

|label2=Saurischia

|2={{Clade

|2=Theropoda 30 px

|1=†Sauropodomorpha 80 px

}} }} }} }} }} }} }} }} }}

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.{{Cite journal|last1=Kammerer|first1=Christian F.|last2=Nesbitt|first2=Sterling J.|last3=Flynn|first3=John J.|last4=Ranivoharimanana|first4=Lovasoa|last5=Wyss|first5=André R.|date=2020-07-28|title=A tiny ornithodiran archosaur from the Triassic of Madagascar and the role of miniaturization in dinosaur and pterosaur ancestry|journal=Proceedings of the National Academy of Sciences|language=en|volume=117|issue=30|pages=17932–17936|doi=10.1073/pnas.1916631117|issn=0027-8424|pmc=7395432|pmid=32631980|doi-access=free |bibcode=2020PNAS..11717932K }}{{Cite journal|last1=Ezcurra|first1=Martín D.|last2=Nesbitt|first2=Sterling J.|last3=Bronzati|first3=Mario|last4=Dalla Vecchia|first4=Fabio Marco|last5=Agnolin|first5=Federico L.|last6=Benson|first6=Roger B. J.|last7=Brissón Egli|first7=Federico|last8=Cabreira|first8=Sergio F.|last9=Evers|first9=Serjoscha W.|last10=Gentil|first10=Adriel R.|last11=Irmis|first11=Randall B.|date=2020-12-17|title=Enigmatic dinosaur precursors bridge the gap to the origin of Pterosauria|url=http://www.nature.com/articles/s41586-020-3011-4|journal=Nature|language=en|volume=588|issue=7838|pages=445–449|doi=10.1038/s41586-020-3011-4|pmid=33299179|bibcode=2020Natur.588..445E |s2cid=228077525 }} 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).{{Cite journal|last=Bennett|first=S. Christopher|date=2020-02-19|title=Reassessment of the Triassic archosauriform Scleromochlus taylori: neither runner nor biped, but hopper|journal=PeerJ|language=en|volume=8|pages=e8418|doi=10.7717/peerj.8418|pmid=32117608|pmc=7035874|issn=2167-8359|doi-access=free}}

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 pseudosuchians), 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:{{Cite journal |last1=Nesbitt |first1=Sterling J. |last2=Patellos |first2=Emily |last3=Kammerer |first3=Christian F. |last4=Ranivoharimanana |first4=Lovasoa |last5=Wyss |first5=Andre´ R. |last6=Flynn |first6=John J. |date=2023-07-25 |title=The earliest-diverging avemetatarsalian: a new osteoderm-bearing taxon from the Triassic (?Earliest Late Triassic) of Madagascar and the composition of avemetatarsalian assemblages prior to the radiation of dinosaurs |url= |journal=Zoological Journal of the Linnean Society |volume=199 |issue=2 |language=en |pages=327–353 |doi=10.1093/zoolinnean/zlad038 |issn=0024-4082|doi-access=free }}

{{clade

|1={{clade

|1=Pseudosuchia 70px

|label2=Avemetatarsalia

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|1=Mambachiton

|2={{clade

|1=Aphanosauria 70px

|label2=Ornithodira

|2={{clade

|1={{clade

|1=Lagerpetidae 55px

|2=Pterosauria 50px }}

|3={{clade

|1=Lagosuchus 60px

|2={{clade

|1=Silesauridae

|2=Dinosauria 35px

}} }} }} }} }} }}|label1=Archosauria}}

{{Reflist}}

• {{cite book |author=Michael J. Benton |chapter=Origin and relationships of Dinosauria |editor=David B. Weishampel |editor2=Peter Dodson |editor3=Halszka Osmólska (Hrsg.) |title=The Dinosauria |url=https://archive.org/details/dinosauriandedit00weis |url-access=limited |publisher=Zweite Auflage, University of California Press |location=Berkeley |year=2004 |pages=[https://archive.org/details/dinosauriandedit00weis/page/n25 7]–19 |isbn=0-520-24209-2|author-link=Michael J. Benton }}

{{Portal|Dinosaurs}}

{{Archosauromorpha|B.}}

{{Avemetatarsalia}}

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Category:Anisian first appearances

Category:Extant Middle Triassic first appearances

Category:Taxa named by Michael Benton