Gastralia (sg.:gastralium) aredermal bones found in theventral body wall of moderncrocodilians andtuatara, and many prehistorictetrapods. They are found between thesternum andpelvis, and do not articulate with thevertebrae. In these reptiles, gastralia provide support for theabdomen and attachment sites for abdominal muscles.
The possession of gastralia may be ancestral for Tetrapoda and were possibly derived from the ventral scales found in animals likerhipidistians,labyrinthodonts, andAcanthostega, and may be related to ventral elements ofturtleplastrons.[1][2]Similar, but nothomologouscartilagenous elements, are found in the ventral body walls oflizards andanurans. These structures have been referred to as inscriptional ribs,[2] based on their alleged association with the inscriptiones tendinae (the tendons that form thesix pack in humans). However, the terminology for these gastral-like structures remains confused. Both types, along with sternal ribs (ossifiedcostal cartilages), have been referred to as abdominal ribs, a term with limited usefulness that should be avoided.[2]Gastralia are also present in a variety ofextinct animals, includingtheropod andprosauropoddinosaurs,pterosaurs,plesiosaurs,choristoderes and some primitivepelycosaurs. In dinosaurs, the elements articulate with each other in a sort of zig-zag along themidline and may have aided inrespiration.[2] Gastralia are known to be present in primitiveornithischian andsauropodomorph dinosaurs. However gastralia are only known fromheterodontosaurid ornithschians, and gastralia are lost ineusauropodansauropods.[3][4]
Discoveries about how the gastralia fit together in the skeleton ofSue the T. rex have led to an understanding thatTyrannosaurus bodies were more barrel-chested – and heavier – than previously thought.[5]
The term "gastralia" was proposed byGeorg Baur in 1898.[6] They had previously been termed "abdominal ribs",[7] but because the term "abdominal ribs" has been applied to various structures, and the gastralia are not true ribs, this is not considered an appopriate term.[6][2]
In turtles, where the gastralia are incorporated into theplastron, each pair of gastralia gets a distinct name: the hyoplastra, hypoplastra, xiphiplastra, and in some taxa the mesoplastra.[8]
Gastralia were ancestrally present in amniotes, but have been lost in many groups. Among extant taxa, they are only present incrocodylians and thetuatara, and in modified form as part of the plastron of turtles.[2] Gastralia are rarely preserved intherapsids, but have been identified in somedinocephalians andgorgonopsians and severalanomodonts. However, they were probably genuinely absent in somedicynodonts,therocephalians, andcynodonts.[9] Mostornithischiandinosaurs lacked gastralia, butheterodontosaurids, one of the earliest-branching lineages of ornithischians, retained them.[10] Sauropods have been considered to lack gastralia.[2][11] Elements interpreted as gastralia have been rarely found in sauropods,[12] but it has been argued that these elements are more convincingly interpreted as sternal ribs.[11] Modern birds lack gastralia, but they were present in early lineages of birds, as in other theropods.[13]
TheAllosaurus fragilis specimenUSNM 8367 contained several gastralia which preserve evidence of healed fractures near their middle. Some of the fractures were poorly healed and "formed pseudoarthroses." An apparent subadult maleAllosaurus fragilis was reported by Laws to have extensive pathologies. The possible subadultA. jimmadseni[14] specimenMOR 693 also had pathological gastralia.[15] The leftscapula andfibula of anAllosaurus fragilis specimen catalogued asUSNM 4734 are both pathological, both probably due to healed fractures.[16]
The holotype ofNeovenator salerii had many pathologies, includingpseudoarthrotic gastralia and a deviation to the right of the third and fourth neural spines of the neck vertebrae.[16]
An immaturedromaeosaurid specimen (which had not been described in the scientific literature as of 2001) fromTugrugeen Shireh was observed to have a "bifurcated" gastralium.[16]
In theGorgosaurus libratusholotype (NMC 2120) the 13th and 14th gastralia have healed fractures. AnotherG. libratus specimen catalogued asTMP94.12.602 bears multiple pathologies, including apseudoarthortic gastralium.[16]
The unidentified tyrannosaurid specimenTMP97.12.229 had a fractured and healed gastralium.[16]