Paleothyris was an early sauropsid , yet it still had some features that were more labyrinthodont-like than reptile-like . Its skull lacked fenestrae , holes found in the skulls of most reptiles and mammals . Paleothyris was an early sauropsid , yet it still had some features that were more primitive , more labyrinthodont-like than reptile-like , especially its skull , which lacked fenestrae , holes found in the skulls of most modern reptiles and mammals .