Avialae

Avialae

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Avialae is a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 of dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

s containing their only living representatives, bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s (Aves
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

), and the most immediate extinct relatives of birds.

Competing definitions


Systematic studies of the Avialae have produced different results depending on the specimens included and the definitions used. Including or excluding Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...

from the group Aves has a large effect on the subgroups of Avialae. Archaeopteryx, from the late Jurassic Period, may be the earliest known theropod dinosaur which may have had the capability of powered flight. If Archaeopteryx is defined as an avian, then there are few non-avian avialans.

Character-based definition


Avialae is traditionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Jacques Gauthier
Jacques Gauthier
Jacques Armand Gauthier is a vertebrate paleontologist, comparative morphologist, and systematist, and one of the founders of the use of cladistics in biology....

 named Avialae in 1986, and first defined it in 2001 as all dinosaurs that possessed feathered wing
Wing
A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...

s used in flapping flight
Bird flight
Flight is the main mode of locomotion used by most of the world's bird species. Flight assists birds while feeding, breeding and avoiding predators....

, and the birds that descended from them.

Avialae vs. Aves


Gauthier (page 34) identified four conflicting ways of defining the term "Aves", which is a problem because the same biological name is being used four different ways. Gauthier proposed a solution, number 4 below, which is to reserve the term Aves only for the crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...

, the last common ancestor of all living birds and all of its descendants. He assigned other names to the other groups.
  1. Aves can mean those advanced archosaur
    Archosaur
    Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...

    s with feathers (alternately Avifilopluma
    Avifilopluma
    Avifilopluma is a clade containing all animals with feathers. Unlike most clades, which are defined based on relative relationships, Avifilopluma is defined based on an apomorphy, that is, a unique physical characteristic shared by one group and not found outside that group .-Definition:The clade...

    )
  2. Aves can mean those that fly (alternately Avialae)
  3. Aves can mean all reptiles closer to birds than to crocodile
    Crocodile
    A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

    s (alternately Panaves)
  4. Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "crown group"). (alternately Neornithes)


Under the fourth definition Archaeopteryx is an avialan, and not a member of Aves.

Branch-based definition


Several authors have use a similar, but branch-based, definition; "all theropods closer to birds than to Deinonychus
Deinonychus
Deinonychus was a genus of carnivorous dromaeosaurid dinosaur. There is one described species, Deinonychus antirrhopus. This 3.4 meter long dinosaur lived during the early Cretaceous Period, about 115–108 million years ago . Fossils have been recovered from the U.S...

."

The cladogram below follows the results of a phylogenetic study by Gao and colleagues in 2002. Note that these authors used the more inclusive, node-based definition of Aves. The placement of scansoriopterygids follows Zhang et al., 2008.

Non-Avian Avialans


There are three taxa that are not members of Aves, but are avialans, in any definition of Avialae listed above.

Senter (2007) placed Scansoriopteryx
Scansoriopteryx
Scansoriopteryx is a genus of avialan dinosaur. Described from only a single juvenile fossil specimen found in Liaoning, China, Scansoriopteryx is a sparrow-sized animal that shows adaptations in the foot indicating an arboreal lifestyle. It possessed an unusual, elongated third finger...

as a sister group to Archaeopteryx within Avialae. The age of the Daohugou Beds
Daohugou Beds
The Daohugou Beds are a series of fossil-bearing rock deposits located in northeastern China, with the type locality around Daohugou village of Ningcheng County south of Chifeng, Inner Mongolia, but extending into southwest Liaoning. The rocks are grey, finely bedded, lacustrine, sandy mudstones...

 where Scansoriopteryx was collected may be Middle Jurassic
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176-161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger....

; older than Archaeopteryx. A study by Zhang et al. (2008) confirmed that Scansoriopteryx
Scansoriopteryx
Scansoriopteryx is a genus of avialan dinosaur. Described from only a single juvenile fossil specimen found in Liaoning, China, Scansoriopteryx is a sparrow-sized animal that shows adaptations in the foot indicating an arboreal lifestyle. It possessed an unusual, elongated third finger...

was within Avialae and also added a new taxon, Epidexipteryx
Epidexipteryx
Epidexipteryx is a genus of small avialan dinosaur, known from one fossil specimen in the collection of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. Epidexipteryx represents the earliest known example of ornamental feathers in the fossil record. The type specimen is...

.

See also

  • Bird flight
    Bird flight
    Flight is the main mode of locomotion used by most of the world's bird species. Flight assists birds while feeding, breeding and avoiding predators....

  • Bird
    Bird
    Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

  • Feathered dinosaurs
    Feathered dinosaurs
    The realization that dinosaurs are closely related to birds raised the obvious possibility of feathered dinosaurs. Fossils of Archaeopteryx include well-preserved feathers, but it was not until the early 1990s that clearly non-avialan dinosaur fossils were discovered with preserved feathers...