Avialae ("bird wings") is a clade containing the only living dinosaurs, the birds. It is usually defined as all theropod dinosaurs more closely related to birds (Aves) than to deinonychosaurs, though alternative definitions are occasionally used (see below).
Archaeopteryx lithographica, from the late Jurassic Period Solnhofen Formation of Germany, is possibly the earliest known avialan which may have had the capability of powered flight,[2] though it might have been a deinonychosaur instead.[3] Several older (but non flight-capable) avialans are known from the late Jurassic Tiaojishan Formation of China, dated to about 160 million years ago.[4][5]
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."[6][7] 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, operating under the assumption that troodontids and birds were more closely related to each other than to dromaeosaurs. They also redefine Avialae as the smallest clade containing Archaeopteryx and modern birds.[8]
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.[9]
Avialae is also occasionally defined as an apomorphy-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 feathered wings used in flapping flight, and the birds that descended from them.[10][11]
Gauthier and de Queiroz[11] (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 term Aves only for the crown 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.