[7][6] Fossils also suggest that legs and feet covered with feathers was an ancestral condition, possibly having originated in the Coelurosauria, even if this trait was later lost in more advanced birds.

An increasingly asymmetric wrist joint, a trend that can be traced back to primitive coelurosaurs, allowed the forelimbs to elongate and an elaboration of their plumage, traits that made the evolution of flapping flight possible.

This apparent paradox was addressed by later studies which showed that early paravians like Microraptor were capable of flapping flight and powered launching from the ground into the air without relying on climbing.

While this characteristic claw and its associated modifications to the anatomy of the foot (such as a shortened metatarsus in eudromaeosaurs) had been known since the mid-20th century, their possible functions were the subject mainly of speculation, and few actual studies were published.

This makes it likely that advanced dromaeosaurids also used their claws to puncture and grip their prey to aid in pinning it to the ground, while using shallow wing beats and tail movements to stabilize themselves.

[19] Turner et al. (2007) suggested that extreme miniaturization was ancestral for the clade, whose common ancestor has been estimated to have been around 65 centimetres (26 in) long and 600–700 grams (21–25 oz) in mass.

In Eumaniraptora, both Dromaeosauridae and Troodontidae went later through four independent events of gigantism, three times in dromaeosaurids and once in troodontids, while the body mass continued to decrease in many forms within Avialae.

[20] Fossils show that all the earliest members of Paraves found to date started out as small, while Troodontidae and Dromaeosauridae gradually increased in size during the Cretaceous period.

[25][2] Since the 1960s, the dromaeosaurids and troodontids have often been classified together in a group or clade named the Deinonychosauria, initially based primarily on the presence of a retractable second toe with sickle-claw (now also known to be present in some avialans).

The name Deinonychosauria was coined by Ned Colbert and Dale Russell in 1969, and defined as a clade (all theropods closer to dromaeosaurids than to birds) by Jacques Gauthier in 1986.

[27] The clade containing avialans, microraptorians, unenlagiids, Anchiornis, and Xiaotingia to the exclusion of Eudromaeosauria was named Averaptora by Agnolín and Novas (2013), defined as all animals closer to Passer than to Dromaeosaurus.

[28] Averaptora additionally contains troodontids according to Cau, Beyrand, Voeten et al. (2017)[22] and other phylogenies in which find Eudromaeosauria to be an outgroup to a clade including Troodontidae and Avialae.

[22] In 2015 Chatterjee created Tetrapterygidae in the second edition of his book The Rise of Birds: 225 Million Years of Evolution, where he included Microraptor, Xiaotingia, Aurornis, and Anchiornis; together they were proposed to be the sister group of the Avialae.

[31] Other than the crown group of modern birds, which are direct descendants in the stem lineage of Paraves, there are no extant survivors or genetic material, so their entire phylogeny is inferred only from the fossil record.