Drepanosaurs (members of the clade Drepanosauromorpha) are a group of extinct reptiles that lived between the Carnian and Rhaetian stages of the late Triassic Period, approximately between 230 and 210 million years ago.

[1] The various species of drepanosaurs were characterized by a bird-like skull, a barrell shaped body, and a horizontally narrow tail.

[4] Fossils of drepanosaurs have been found in North America (Arizona, New Mexico, New Jersey, Utah) and Europe (England and northern Italy).

The name is taken from the family's namesake genus Drepanosaurus, which means "sickle lizard," a reference to their strongly curved claws.

Above the shoulders of most species was a specialized "hump" formed from fusion of the vertebrae, possibly used for advanced muscle attachments to the neck, and allowing for quick forward-striking movement of the head (perhaps to catch insects).

Some individuals of Megalancosaurus (possibly exclusive to either males or females) had a primate-like opposable toe on each foot, perhaps used by one sex for extra grip during mating.

"[6][13] Within Avicephala, Senter named the group Simiosauria ("monkey lizards") for the extremely derived tree-dwelling forms.

Longisquama and Coelurosauravus were not found to be closely related to drepanosaurids, but instead were recovered as non-neodiapsid diapsids as in Senter's analysis.

One of them is the increased angulation of the zygapophyses in the posterior dorsal vertebrae; Evans noted that this character is also present in the skeletons of lizards belonging to the modern genus Draco "and is likely to be functional (and thus potentially convergent)."

The other synapomorphy, the enclosed thyroid fenestra in the pelvis, "may be variable in the British kuehneosaurs and remains unknown in Pamelina," according to Evans.

[1] Hypuronector Vallesaurus Dolabrosaurus Drepanosaurus Megalancosaurus Renesto et al. (2010) used modified versions of the matrices from the earlier analyses of Laurin (1991)[18] and Dilkes (1998)[10] in order to determine the phylogenetic position of Drepanosauromorpha within Diapsida.

Renesto et al. (2010) concluded that avicephalan synapomorphies proposed by Senter (2004) are merely evolutionary convergences caused by common lifestyle shared by drepanosaurids, coelurosauravids and Longisquama.

The authors' phylogenetic analysis recovered Drepanosauromorpha as allokotosaurian archosauromorphs, specifically as the sister group of trilophosaurids.

This study found that Drepanosauromorpha was one of the earliest diverging groups of diapsids in the analysis, even more basal than weigeltisaurids (such as Coelurosauravus), tangasaurids (such as Hovasaurus), and younginids (such as Youngina).

Although drepanosaurs are only known from the late Triassic, this new finding suggests that the first members of the drepanosauromorph lineage may have evolved much earlier, in the Permian (about 260 million years ago).