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Drepanosaurs (members of the clade Drepanosauromorpha) are a group of extinct that lived between the Carnian and Rhaetian stages of the late Triassic Period, approximately between 230 and 210 million years ago. The various species of drepanosaurs were characterized by a bird-like skull, a barrel-shaped body, and a horizontally narrow tail. A number of drepanosaurs had specialized grasping limbs and often prehensile tails similar to those of . Drepanosaurs are generally thought to have been arboreal (tree-dwelling), and probably . Some studies have alternately suggested fossorial (digging) and aquatic lifestyles for some members. 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.
Some studies have included Drepanosaurs within the group Avicephala, which also includes the gliding Weigeltisauridae, but the close relationship between the two groups has been doubted by other authors. Their phylogenetic position has been disputed, with some studies considering them to be members of Archosauromorpha (and thus more closely related to modern birds and crocodilians than to lizards), while other studies have considered them to be basal Neodiapsida that are not related to any modern reptiles.
Aerial locomotion has been attributed to at least two drepanosaur genera: Megalancosaurus and Hypuronector. The first was originally suggested by Ruben et al. 1998 on the basis of bird-like characters and limb proportions. While the suggestion has not been ruled out entirely, it has since been largely dismissed, due to Megalancosaurus
Within Avicephala, Senter named the group Simiosauria ("monkey lizards") for the extremely derived tree-dwelling forms. Simiosauria was defined as "all taxa more closely related to Drepanosauridae than to Coelurosauravus or Sauria." However, Renesto and colleagues (see below) found drepanosaurids to lie within Sauria, which would make the clade Simiosauria obsolete. Senter found that Hypuronector, originally described as a drepanosaurid, actually lies just outside that clade, along with the primitive drepanosaur Vallesaurus. He also recovered a close relationship between the drepanosaurids Dolabrosaurus and Megalancosaurus.
The following cladogram was proposed by Senter in his 2004 analysis:
A clade containing drepanosaurids, Longisquama, and Coelurosauravus (as well as Wapitisaurus) was also recovered in a 2003 analysis conducted by John Merck; however, in Merck's analysis this clade was nested within Neodiapsida as the sister taxon of Sauria. In a 2006 study, Renesto and Binelli found that when pterosaur Eudimorphodon was added to Senter's original matrix, it was found to be a member of Avicephala. The authors also conducted a second analysis, this time based on a character set and matrix updated by scoring additional characters previously reported as unknown and by adding a few relevant characters. This analysis recovered drepanosaurids as the sister taxon of Eudimorphodon; the clade containing pterosaurs and drepanosaurids was recovered as the sister taxon of Archosauriformes. 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. However, it is feasible that this arrangement might be a result of poor knowledge of Longisquama rather than a reflection of its true phylogenetic position. The authors did note that there are similarities in the structure of the forelimb and shoulder regions of Longisquama and all or some drepanosaurids (e.g. the humerus of both Longisquama and Vallesaurus is as long as the fourth digit of the Hand). They stressed that they could not rule out the possibility that at least some of the similarities are convergent due to a similar behaviors, and that they did not examine Longisquama firsthand. Therefore, further studies of drepanosaurids should take the hypothesis that Longisquama might be a drepanosaurid into consideration.
In a later study, Renesto et al. demonstrated that Senter's 2004 cladogram was based on poorly defined characters and data. The resulting phylogeny was therefore very unusual compared to any other previous study on drepanosaurs or related taxa. The new cladogram proposed in this last study abandoned both Avicephala (because it was polyphyletic) and Simiosauria. Senter's definition of Simiosauria included Sauria as an external specifier, causing the clade to become obsolete in Renesto et al.'s study (where drepanosaurs nested within Sauria). Renesto and colleagues instead defined a new clade, Drepanosauromorpha, as the least inclusive clade containing Hypuronector limnaios and Megalancosaurus preonensis. A more inclusive taxon, Elyurosauria ("lizard with coiled tail"), was erected in order to include all the drepanosaurs with coiled tails. Vallesaurus is thus more derived than Hypuronector (as shown by its morphology). Drepanosaurus and Megalancosaurus were also placed in a new taxon named Megalancosaurinae.
The alternative cladogram presented in Renesto et al. (2010).
Renesto et al. (2010) used modified versions of the matrices from the earlier analyses of Laurin (1991) and Dilkes (1998) in order to determine the phylogenetic position of Drepanosauromorpha within Diapsida. The analyses using Laurin's matrix recovered drepanosaurs either as the sister group of the clade containing Prolacerta, Trilophosaurus and Hyperodapedon, (Archosauromorpha), or in unresolved polytomy with Archosauromorpha and Lepidosauromorpha. The analyses using Dilkes' matrix recovered drepanosaurs either as "Protorosauria" archosauromorphs and the sister taxon to the clade containing Tanystropheus, Langobardisaurus and Macrocnemus, or in unresolved polytomy with Lepidosauromorpha, Choristodera and several archosauromorph clades. 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 did not rule out the possibility that drepanosaurs and Longisquama might really be close relatives.
The phylogenetic study published by Buffa et al. (2024) did not recover "avicephalans" as closely related. The authors' phylogenetic analysis recovered Drepanosauromorpha as archosauromorphs, specifically as the sister group of Trilophosauridae.
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