How do rhynchocephalians differ from squamates?

Although rhynchocephalids (order Rhynchocephalia) and squamates (order Squamata) are sister groups that share several traits—such as a transverse cloacal opening, superficial tooth attachment, tail vertebrae fracture planes, and the periodic shedding of skin—they exhibit distinct anatomical differences.1

Key differences include:

• Skull Structure: Rhynchocephalids possess enclosed temporal fossae (depressions) in the skull and an immovable quadrate bone. In contrast, squamates have evolved increasingly kinetic skulls; for instance, lizards have lost the lower temporal bar, which frees the quadrate bone and allows for greater jaw mobility.123
• Dentition: Rhynchocephalids are characterized by a unique, beaklike extension of the premaxillary bones that replaces premaxillary teeth.1
• Skeletal Features: Rhynchocephalids possess gastralia, or abdominal ribs, which are not found in squamates. Additionally, rhynchocephalids are noted for having biconcave vertebrae.12

While both groups are part of the superorder Lepidosauria and possess kinetic skulls, the rhynchocephalid skull is limited to a basisphenoidal-pterygoidal joint, whereas squamates have developed significantly more complex and mobile skull structures.3