Airplanes

Most researchers define avial as a nodal clade, although definitions are different. Many authors used a definition similar to "all theropods, which are closer to the birds than to Deinonych " ^{[6] [7]} . Almost the same definition: “a group of theropods, including all taxa that are closer to sparrows than to dromeosaurus” was used by Agnolina and Novas (2013) to define the clade of Averaptora ^[8] .

In addition, beginning in the late 2000s - early 2010s, several groups of researchers began to add the genus Troodon as an additional qualifier in the definition of the clade Avialae. Troodon has long been considered a close relative of dromaeosaurids in a larger group of deinonychosaurs, although some modern studies have shown that it and other troodontids are more closely related to modern birds. Therefore, in later studies, troodon was excluded from the avial ^[9] .

Aviales are also sometimes referred to as apomorphic treasure. Jacques Gautier , who introduced Avialae to systematics in 1986, derived another group definition in 2001: “all dinosaurs that had feathered wings that were used for flapping flight and birds that came from them” ^{[10] [11]} .

J. Gautier ^[11] derived 4 controversial ways of defining the term "Aves", which caused the problem, since the same biological name is used in four different ways. Gauthier proposed a solution (below, under No. 4), which was to reserve the term Aves only for a crown group - the last common ancestor of all living birds and its descendants. He assigned other names to other groups.

1. Aves can mean all reptiles closer to birds than crocodiles (alternative to Avemetatarsalia [= Pan-aves]).
2. Aves can mean advanced archosaur with feathers (alternative to ).
3. Aves can mean feathered dinosaurs that could fly (alternative to Avialae).
4. Aves can mean the last common ancestor of all living birds and all its descendants (crown group) (an alternative to Neornithes ).

According to the fourth definition, Archeopteryx is a member of the Avialae treasure, but is not a representative of Aves. Gauthier’s proposals were accepted by many researchers in the field of paleontology and the evolution of birds, although they were recognized as contradictory. The clade Avialae, originally proposed to replace the taxonomy of traditional fossil residues Aves, is sometimes used as a synonym for the common name of birds ^[9] .

The earliest avial fossils originate from the Chinese Tiaojishan formation, which dates back to the Oxford stage of the Jurassic era, about 160 Ma ^[9] . The taxa found here are Anchiornis huxleyi , Xiaotingia zhengi and Aurornis xui ^[13] . Archeopteryx , whose fossils date back to a slightly earlier epoch (approximately 150–145 Ma) was found in Germany ^[14] . Many of these early members of the clade had unusual anatomy features that modern birds could inherit, but lost them in the process of evolution. These features include an enlarged claw on the thumb, as well as long feathers, or "rear wings", which covered the legs and feet and could be used for air maneuvering ^[13] .

During the Cretaceous period, the airs diversified into a wide variety of forms ^[15] . Many groups retained primitive characteristics, such as wings with fingers and teeth, although the latter were lost independently in a number of groups, including modern birds. While the earliest forms, such as Archeopteryx and Dzhoholornis , retained the long bony tails of their ancestors ^[15] , the tails of the more advanced members of the clade were shortened with the appearance of pygostyle in the Pygostylia group. In the Late Cretaceous, about 95 million years ago, the ancestor of modern birds also improved their sense of smell ^[16] .