Averostra, or "bird snouts", is a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

that includes most

theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...

dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

, namely

Ceratosauria Ceratosaurs are members of the clade Ceratosauria, a group of dinosaurs defined as all theropods sharing a more recent common ancestor with ''Ceratosaurus'' than with birds. The oldest known ceratosaur, ''Saltriovenator'', dates to the earliest ...

and

Tetanurae Tetanurae (/ˌtɛtəˈnjuːriː/ or "stiff tails") is a clade that includes most theropod dinosaurs, including megalosauroids, allosauroids, tyrannosauroids, ornithomimosaurs, compsognathids and maniraptorans (including birds). Tetanurans are ...

, and represent the only group of post-Early Jurassic theropods. Both survived into the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

period. When the

Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...

occurred, ceratosaurians,

megaraptorans Megaraptora is a clade of carnivorous tetanuran theropod dinosaurs with controversial relations to other theropods. Its derived members, the Megaraptoridae are noted for their elongated hand claws and proportionally large arms, which are usual ...

an ''incertae sedis'' group within Tetanurae, and two groups of tetanurans within the clade

Coelurosauria Coelurosauria (; from Greek, meaning "hollow tailed lizards") is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. Coelurosauria is a subgroup of theropod dinosaurs that includes compsognathids, tyrann ...

, the

Tyrannosauroidea Tyrannosauroidea (meaning 'tyrant lizard forms') is a superfamily (or clade) of coelurosaurian theropod dinosaurs that includes the family Tyrannosauridae as well as more basal relatives. Tyrannosauroids lived on the Laurasian supercontinent b ...

and

Maniraptoriformes Maniraptoriformes is a clade of dinosaurs with pennaceous feathers and wings that contains ornithomimosaurs and maniraptorans. This group was named by Thomas Holtz, who defined it as "the most recent common ancestor of '' Ornithomimus'' and bird ...

, were still extant. Only one subgroup of Maniraptoriformes,

Aves Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

, survived the extinction event and persisted to the present day. One important diagnostic feature of Averostra is the absence of the fifth metacarpal. Other

saurischia Saurischia ( , meaning "reptile-hipped" from the Greek ' () meaning 'lizard' and ' () meaning 'hip joint') is one of the two basic divisions of dinosaurs (the other being Ornithischia), classified by their hip structure. Saurischia and Ornithis ...

ns retained this bone, albeit in a significantly reduced form.

Definition

Averostra was named by

Gregory S. Paul Gregory Scott Paul (born December 24, 1954) is an American freelance researcher, author and illustrator who works in paleontology, and more recently has examined sociology and theology. He is best known for his work and research on theropod dino ...

in 2002 as an apomorphy-based

defined as the group including the

Dromaeosauridae Dromaeosauridae () is a family of feathered theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek ('), meaning ...

and other

Avepoda Neotheropoda (meaning "new theropods") is a clade that includes coelophysoids and more advanced theropod dinosaurs, and is the only group of theropods that survived the Triassic–Jurassic extinction event. All neotheropods became extinct by the ...

with (an ancestor with) a promaxillary fenestra (''fenestra promaxillaris''), an extra opening in the front outer side of the

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

, the bone that makes up the upper jaw. It was later re-defined by

Martin Ezcurra Martin may refer to: Places * Martin City (disambiguation) * Martin County (disambiguation) * Martin Township (disambiguation) Antarctica * Martin Peninsula, Marie Byrd Land * Port Martin, Adelie Land * Point Martin, South Orkney Islands Austra ...

and

Gilles Cuny The Gilles are the oldest and principal participants in the Carnival of Binche in Belgium. They go out on Shrove Tuesday from 4 am until late hours and dance to traditional songs. Other cities, such as La Louvière and Nivelles, have a traditi ...

in 2007 as a node-based clade containing ''

Ceratosaurus nasicornis ''Ceratosaurus'' (from Ancient Greek, Greek κέρας/κέρατος, ' meaning "horn" and wikt:σαῦρος, σαῦρος ' meaning "lizard") was a carnivorous Theropoda, theropod dinosaur in the Late Jurassic Period (geology), period (Kim ...

'', ''

Allosaurus fragilis ''Allosaurus'' () is a genus of large carnosaurian theropod dinosaur that lived 155 to 145 million years ago during the Late Jurassic epoch ( Kimmeridgian to late Tithonian). The name "''Allosaurus''" means "different lizard" alludi ...

'', their

last common ancestor In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...

and all its descendants. Mickey Mortimer commented that Paul's original apomorphy-based definition may make Averostra a much broader clade than the ''Ceratosaurus''+''Allosaurus'' node, potentially including all of Avepoda or more. The clade formed by Ceratosauria and Tetanurae while excluding Coelophysoidea was first named Neotheropoda by

Bakker Bakker is a common Dutch surname equivalent to English Baker. In 2007 it was the seventh most common name in the Netherlands (55,273 people).clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

that also contains Coelophysoidea and is similar in content to Avepoda. A 2020 monograph supported the classification of very early large Jurassic theropods such as ''

Dilophosaurus ''Dilophosaurus'' ( ) is a genus of theropod dinosaurs that lived in what is now North America during the Early Jurassic, about 193 million years ago. Three skeletons were discovered in northern Arizona in 1940, and the two best preserved ...

'' as not members of Averostra (as defined by Ezcurra and Cuny), but related to basal members. Marsh, A., & Rowe, T. (2020)
A comprehensive anatomical and phylogenetic evaluation of Dilophosaurus wetherilli (Dinosauria, Theropoda) with descriptions of new specimens from the Kayenta Formation of northern Arizona.
Journal of Paleontology, 94(S78), 1-103. doi:10.1017/jpa.2020.14

References

{{Taxonbar, from=Q4828332 Neotheropods Taxa named by Gregory S. Paul