Archaeopteryx lithographica, from the late Jurassic Period Solnhofen Formation of Germany, 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 a deinonychosaur instead.

"[8][9] 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.

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.

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.

Avialae, initially proposed to replace the traditional fossil content of Aves, is sometimes used synonymously with the vernacular term "bird" by these researchers.

[11] †Anchiornis †Archaeopteryx †Rahonavis †Jixiangornis †Jeholornis †Sapeornis †Confuciusornis †Chongmingia Ornithothoraces The earliest known avialans come from the Tiaojishan Formation of China, which has been dated to the late Jurassic period (Oxfordian stage), about 160 million years ago.

[20] The following cladogram is based on the analysis by Hartman et al. (2019), which found flight likely evolved five separate times among paravian dinosaurs, two of those among Avialae (in Scansoriopterygids and other avialans).