The
phylogenetic relationships of both drepanosaurs and weigeltisaurs, as well as
Longisquama, have historically been difficult to pin down. Drepanosaurs and weigeltisaurids were first suggested to form a clade together by John Merck in an abstract presented to the 2003 annual meeting of the
Society of Vertebrate Paleontology. The same conclusion would be reached by Phil Senter in a paper in 2004, who named this clade Avicephala and found it to be the sister taxon to
Neodiapsida (defined wherein as the clade containing "Younginiforms" and all living diapsids). Within Avicephala, Senter found two clades, one that he named Simiosauria (equivalent to Drepanosauromorpha), and another containing
Coelurosauravus and
Longisquama. The
cladogram below depicts the result of his analysis: }} Later research, including Renesto and Binelli (2006) and Renesto et al. (2010), argued that the phylogenetic analysis of Senter (2004) was flawed due to the absence of available data for drepanosaur skulls at the time and the exclusion of particular diapsid groups, such as
pterosaurs. Renesto and colleagues (2010) recommended abandoning Avicephala for these reasons (as well as defining a new clade, Drepanosauromorpha, to replace Simiosauria following the guidelines of the
PhyloCode). They instead found drepanosauromorphs to be
archosauromorphs, possibly related to
Prolacertiformes, and unrelated to weigeltisaurids or other early diapsids. Drepanosauromorphs would later be argued to be early-diverging diapsids once again following the description of the three-dimensionally preserved skull of
Avicranium in 2017, interpreted as possessing archaic features of their skull anatomy. Still, drepanosaurs and weigeltisaurids remained as separate lineages in the study's phylogenetic analysis, with each being successively closer to living diapsids. '' Avicephala would not be recovered again until 2021 following the redescription of
Weigeltisaurus by Adam Pritchard and colleagues, wherein the phylogenetic analysis found a clade comprising drepanosauromorphs and weigeltisaurids. They identified four unambiguous
synapomorphies (shared unique traits) supporting Avicephala: the absence of both
cervical and
dorsal intercentra, a length/height ratio for the
scapula between 0.4 and 0.25, and no outer process on the fifth
metatarsal. A study by Valentin Buffa and colleagues published in 2024 set out to test the revived monophyly of Avicephala following their reinterpretation of the skull of the drepanosaur
Avicranium. Like most previous analyses they did not recover a monophyletic Avicephala, with weigeltisaurids once again recovered as
stem-saurians (albeit more crownward than typical of prior analyses) and drepanosauromorphs likewise as archosauromorphs, although uniquely close relatives of
Trilophosauridae within
Allokotosauria, a novel position for them. The simplified results of this analysis are shown in the right cladogram below, highlighting 'avicephalans'.
Pritchard et al. (2021): Monophyletic Avicephala }}
Buffa et al. (2024): Non-monophyletic Avicephala 'Avicephalans' }} ==References==