Gastralium

Gastralia (singular gastralium) are dermal bones found in the ventral body wall of crocodilian and Sphenodon species. They are found between the sternum and pelvis, and do not articulate with the vertebrae. In modern animals, they provide support for the abdomen and attachment sites for abdominal muscles.

These bones may have been derived from the ventral scales found in animals like rhipidistians, labyrinthodonts, and Acanthostega, and may be related to ventral elements of turtle plastrons. Similar but not homologous cartilagenous elements are found in the ventral body walls of lizards and anurans. The terminology for these groups of structures is confused; both types, along with sternal ribs (ossified costal cartilages), have been called abdominal ribs, a term which should be avoided.

Gastralia are also present in a variety of extinct animals, including theropod and prosauropod dinosaurs, pterosaurs, plesiosaurs, champsosaurs and some primitive pelycosaurs. In dinosaurs, the elements articulate with each other in a sort of zig-zag along the midline and may have aided in respiration. Although they were thought to be present in some basal ornithischian dinosaurs, and sauropods (most notably Eobrontosaurus), the perceived occurrences have been shown to be mistaken.