| Avialans | |
|---|---|
 | |
| Fossil specimen ofJeholornis prima | |
 | |
| Collage of four extantbirds. Clockwise from top-left:Spanish imperial eagle (Aquila adalberti),common ostrich (Struthio camelus),mallard (Anas platyrhynchos), andcommon kingfisher (Alcedo atthis) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Pennaraptora |
| Clade: | Paraves |
| Clade: | Avialae Gauthier, 1986 |
| Subgroups | |
Avialae ("bird wings") is aclade containing the only livingdinosaurs, thebirds, and their closest relatives. It is usually defined as alltheropod dinosaurs more closely related to birds (Aves) than todeinonychosaurs, though alternative definitions are occasionally used (see below).
Archaeopteryx lithographica, from the lateJurassic PeriodSolnhofen Formation ofGermany, is usually considered the earliest known avialan which may have had the capability of powered flight;[4] a minority of studies have suggested that it might have been adeinonychosaur instead.[5] Several older (but non flight-capable) possible avialans are known from the late JurassicTiaojishan Formation ofChina, dated to about 160 million years ago.[6][7]
Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used a definition similar to "alltheropods closer to birds than toDeinonychus."[8][9] A nearly identical definition, "the theropod group that includes all taxa closer toPasser than toDromaeosaurus", was used by Agnolín and Novas (2013) for their cladeAveraptora, operating under the assumption thattroodontids and birds were more closely related to each other than to dromaeosaurs. They also redefine Avialae as the smallest clade containingArchaeopteryx and modern birds.[10]
Additionally, beginning in the late 2000s and early 2010s, several groups of researchers began adding the genusTroodon as an additional specifier in the definition of Avialae.Troodon had long been considered a close relative of the dromaeosaurids in the larger group Deinonychosauria, though some contemporary studies found it and other troodontids more closely related to modern birds, and so it has been specifically excluded from Avialae in more recent studies.[11]
Avialae is also occasionally defined as anapomorphy-based clade (that is, one based on derived characteristics that were not present among lineage predecessors).Jacques Gauthier, who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed featheredwings used in flappingflight, and the birds that descended from them.[12][13] The cladeAvialae was given a formal phylogenetic definition in thePhyloCode by Juan Benito and colleagues in 2022 as "the largest clade containingVultur gryphus, but notDromaeosaurus albertensis andSaurornithoides mongoliensis".[14] This definition ensures that bothdromaeosaurids andtroodontids are excluded fromAvialae.[14]
Gauthier and de Queiroz[13] (page 34) identified four conflicting ways of defining the term "Aves", which is a problem since the same biological name is being used four different ways. They proposed a solution, number 4 below, which is to reserve the termAves only for thecrown group, the last common ancestor of all living birds and all of its descendants. Other definitions of Aves found in literature were reassigned to other clade names.
Under the fourth definitionArchaeopteryx is an avialan, and not a member of Aves. Gauthier's proposals have been adopted by many researchers in the field of paleontology and bird evolution, though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is sometimes used synonymously with the vernacular term "bird" by these researchers.[11]
| Cladogram following the results of a phylogenetic study by Wanget al., 2016.[15] |
The earliest known avialans come from theTiaojishan Formation ofChina, which has been dated to the lateJurassic period (Oxfordian stage), about 160 million years ago.[11] The avialan species from this time period includeAnchiornis huxleyi andAurornis xui.Xiaotingia zhengi used to be considered a member, but was later classified within the cladeDromaeosauridae. The well-knownArchaeopteryx dates from slightly later Jurassic rocks (about 155 million years old) fromGermany. Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds, but were later lost during bird evolution. These features include enlarged claws on the second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering.[16] It is also thought that early avialans were either cranially akinetic or had otherwise limitedcranial kinesis.[17][18]
Avialans diversified into a wide variety of forms during theCretaceous Period.[19] Many groups retainedprimitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves). While the earliest forms, such asArchaeopteryx andShenzhouraptor, retained the long bony tails of their ancestors,[19] the tails of more advanced avialans were shortened with the advent of thepygostyle bone in the groupPygostylia. In the late Cretaceous, around 95 million years ago, the ancestor of all modern birds also evolved a better sense of smell.[20]
The following cladogram is based on the analysis by Hartmanet al. (2019), which found flight likely evolved five separate times among paravian dinosaurs, two of those among Avialae (in Scansoriopterygids and other avialans).Archaeopteryx and "anchiornithids" were placed in Deinonychosauria, Avialae's sister group.[5]
In a study conducted in 2020,Archaeopteryx was recovered as an avialan.[21]
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