Possible paraphyletic stem group of other amniotes. Autapomorphies of diapsid clades . Loss of lower temporal bar, elaborate squamosal frill. Seems to overlap with Prolacertiformes, q.v. Domed skull of Amniotes permits muscles to run vertically.
Vertical muscles allow strong static pressure when jaws are closed or almost closed.
Increase in muscle bulk would strain periosteal covering of bone. Fenestrae may have evolved as unossified attachment points. Note no obvious change in jaw musculature in primitive forms. LCA Lanthanosuchids and pareiasaurs. Since turtles are not part of the clade as used here, it might be better to use the more traditional name, Pareiasauriformes.