The tuatara has gastralia, rib-like bones also called gastric or abdominal ribs,[REF] the presumed ancestral trait of diapsids.
In the early tetrapods, the gastralia and ribs with uncinate processes, together with bony elements such as bony plates in the skin (osteoderms) and clavicles (collar bone), would have formed a sort of exoskeleton around the body, protecting the belly and helping to hold in the guts and inner organs.