Avialan

Avialans Temporal range: Late Jurassic–Present, 160–0 Ma PreЄ Є O S D C P T J K Pg N
Fossil specimen of Archaeopteryx lithographica
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Dinosauria
Order:	Saurischia
Suborder:	Theropoda
Clade:	Eumaniraptora
Clade:	Avialae Gauthier, 1986
Subgroups
†Anchiornis? †Archaeopteryx †Balaur? †Chongmingia? †Jeholornis †Rahonavis †Xiaotingia? †Aurornis? Euavialae †Jixiangornis Avebrevicauda †Sapeornis Pygostylia

Avialae ("bird wings") is a clade of flying dinosaurs containing their only living representatives, the birds. It is usually defined as all theropod dinosaurs more closely related to modern birds (Aves) than to deinonychosaurs, though alternate definitions are occasionally used (see below).

Archaeopteryx lithographica, from the late Jurassic Period Solnhofen Formation of Germany, is the earliest known avialan which may have had the capability of powered flight. However, several older avialans are known from the late Jurassic Tiaojishan Formation of China, dated to about 160 million years ago.

Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used a definition similar to "all theropods closer to birds than to Deinonychus." A nearly identical definition, "the theropod group that includes all taxa closer to Passer than to Dromaeosaurus", was used by Agnolín and Novas (2013) for their clade Averaptora.

Additionally, beginning in the late 2000s and early 2010s, several groups of researchers began adding the genus Troodon 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.

Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Jacques Gauthier, who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight, and the birds that descended from them.

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