Megalancosaurus, although it differs from Megalancosaurus by being completely toothless. The frontal, postfrontal, and postorbital bones (above and behind the eyes) project outwards, forcing the orbits to face forwards. The frontal and parietal bones are both broad and thick, creating a high and domed cranium. According to the digital endocast of the specimen, this domed cranium held an enlarged brain with large optic lobes. On the other hand, the skull also possesses many primitive features. The squamosal is tall and anteroposteriorly broad, and possesses bony plates which frame the quadrate from the side and rear. This is similar to the condition in captorhinids and Araeoscelis, but unlike that of neodiapsids, which only have the quadrate framed from the side. The quadrate itself is also stouter than that of neodiapsids and lacks a concave rear edge and a dorsal extension which articulates with the squamosal. The occipital condyle of the braincase possesses a deep pit, while the foramen ovale is very large. The stapes is massive, similar to the condition in other early reptiles. Combined with a straight quadrate without a tympanic crest, this indicates that Avicranium (and presumably other drepanosaurs) lacked a tympanic membrane (eardrum), and that the drepanosauromorph lineage evolved before more advanced diapsids (such as saurians) acquired tympanic membranes. Avicranium can be distinguished from Hypuronector, Megalancosaurus, and Vallesaurus'' (the only other drepanosaurs with preserved skulls) due to the following features: The complete absence of teeth. A tall and triangular retroarticular process. Cervical neural spines which are approximately as wide as they are long. Paleobiology The binocular eyes and enlarged brain of Avicranium are likely adaptations to an arboreal lifestyle, as such a lifestyle requires advanced sensory processing, such as depth perception and three-dimensional navigation. It is very likely that these features convergently evolved with those of pterosaurs and modern birds, which would require similar sensory processing during flight. An arboreal lifestyle has been previously proposed for other members of the drepanosauromorph lineage. References Drepanosaurs Late Triassic reptiles of North America Fossil taxa described in 2017 Prehistoric reptile genera