gastralium

thumb|250px|Crocodile|Crocodiles are among the few modern animals with gastralia 250px|thumb|Tyrannosaurus gastralia Gastralia (: gastralium) are dermal bones found in the ventral body wall of modern crocodilians and tuatara, and many prehistoric tetrapods. They are found between the sternum and pelvis (collectively forming the gastral basket), and do not articulate with the vertebrae. In these reptiles, gastralia provide support for the abdomen and attachment sites for abdominal muscles. thumb|Diagram of the dinosaur Torvosaurus, with the gastral basket labelled "20" The possession of gastralia may be ancestral for Tetrapoda and were possibly derived from the ventral scales found in animals like rhipidistians, labyrinthodonts, and Acanthostega, Similar, but not homologous cartilagenous elements, are found in the ventral body walls of lizards and anurans. These structures have been referred to as inscriptional ribs, based on their alleged association with the inscriptiones tendinae (the tendons that form the six pack in humans). However, the terminology for these gastral-like structures remains confused. Both types, along with sternal ribs (ossified costal cartilages), have been referred to as abdominal ribs, a term with limited usefulness that should be avoided. The posterior part of the plastron (lower part of the shell) of turtles is thought to be formed of heavily modified gastralia.

Gastralia are also present in a variety of extinct animals, including theropod and prosauropod dinosaurs, pterosaurs, plesiosaurs, choristoderes and in some early synapsids. In dinosaurs, the elements articulate with each other in a sort of zig-zag along the midline and may have aided in respiration. Gastralia are known to be present in primitive ornithischian and sauropodomorph dinosaurs. However gastralia are only known from heterodontosaurid ornithschians, and gastralia are lost in eusauropodan sauropods.