Gastralium

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

Crocodiles have the abdominal ribs modified into gastralia
Tyrannosaurus gastralia

Gastralia 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.[1][2] 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,[2] 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. [2] 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.[2] Although they were thought to be present in some basal ornithischian dinosaurs,[3] and sauropods (most notably Eobrontosaurus), the perceived occurrences have been shown to be mistaken.[2]

Pathology

The Allosaurus fragilis specimen USNM 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 male Allosaurus fragilis was reported by Laws to have extensive pathologies. The possible subadult A. jimmadseni[4] specimen MOR 693 also had pathological gastralia [5]. The left scapula and fibula of an Allosaurus fragilis specimen catalogued as USNM 4734 are both pathological, both probably due to healed fractures.[6]

The holotype of Neovenator salerii had many pathologies, including pseudoarthrotic gastralia and a deviation to the right of the third and fourth neural spines of the neck vertebrae.[6]

An immature dromaeosaurid specimen (which hadn't been described in the scientific literature as of 2001) from Tugrugeen Shireh was observed to have a "bifurcated" gastralium.[6]

In the Gorgosaurus libratus holotype (NMC 2120) the 13th and 14th gastralia have healed fractures. Another G. libratus specimen catalogued as TMP94.12.602 bears multiple pathologies, including a pseudoarthortic gastralium.[6]

The unidentified tyrannosaurid specimen TMP97.12.229 had a fractured and healed gastralium.[6]

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References
  1. Kardong, Kenneth V. (2002). Vertebrates: Comparative Anatomy, Function, Evolution (3rd ed.). New York: McGraw-Hill. pp. 291–293. ISBN 0-07-290956-0.
  2. Claessens, Leon P.A.M. (March 2004). "Dinosaur gastralia: origin, morphology, and function". Journal of Vertebrate Paleontology. 24 (1): 89–106. doi:10.1671/A1116-8.
  3. Claessens, Leon (1995-11-28). "Dinosaur gastralia and their function in respiration". Dinosaur Mailing List. Retrieved 2007-07-08.
  4. Chure, D.J. and Loewen, M.A. 2020. Cranial anatomy of Allosaurus jimmadseni, a new species from the lower part of the Morrison Formation (Upper Jurassic) of Western North America. PeerJ 8: e7803.
  5. Hanna, R.R. 2002. Multiple injury and infection in a sub-adult theropod dinosaur Allosaurus fragilis with comparisons to allosaur pathology in the Cleveland-Lloyd Dinosaur Quarry Collection. Journal of Vertebrate Paleontology 22 (1): 76–90.
  6. Molnar, R. E., 2001, Theropod paleopathology: a literature survey: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, p. 337-363.
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