Avicephala

Avicephala ("bird heads") is a potentially polyphyletic grouping of extinct diapsid reptiles that lived during the Late Permian and Triassic periods characterised by superficially bird-like skulls and arboreal lifestyles. As a clade, Avicephala is defined as including the gliding weigeltisaurids and the arboreal drepanosaurs to the exclusion of other major diapsid groups. This relationship is not recovered in the majority of phylogenetic analyses of early diapsids and so Avicephala is typically regarded as an artificial or unnatural grouping. However, the clade was recovered again in 2021 following a redescription of Weigeltisaurus, raising the possibility that the clade may be valid after all, although subsequent analyses have not supported this result.

==Description== thumb|Skeletal diagram of the drepanosaur Megalancosaurus|left Avicephalans were named in reference to their pointed, lightly constructed, superficially bird-like skulls. The resemblance is especially striking in some drepanosaurs such as Megalancosaurus which possess long, pointed snouts and expanded, rounded craniums. The three-dimensionally preserved Avicranium has even been proposed to have been toothless and had forward-facing eyes, although the construction of its skull is debated. These superficial similarities to birds led some scientists to hypothesise that birds were derived from various avicephalans, although the well established relationship of birds to theropod dinosaurs indicates that these similarities arose only through convergent evolution.