Skull of the sauropoddinosaurSmitanosaurus in posterior view, with (left) and without (right) the proatlases in place
A number of different interpretations have been made of the proatlas.[2][3] The most common interpretation is that it is the vestigial neural arch of a vertebra that is otherwise fully incorporated into their skull, but the development shows some differences from other vertebrae that present difficulties for this hypothesis.[1]
The proatlas was not present in early finned tetrapodomorphs, but is present in the limbed stem-tetrapod Greererpeton.[4] It was probably widely present across early tetrapods, and is retained in some modern reptiles, such as the tuatara.[5] In crocodylians, the left and right proatlases fuse into a single V-shaped midline element.[1][3] Lissamphibians, mammaliaforms, squamates, turtles, and birds all lack proatlases.[4]
The proatlas was first recognized in dinosaurs by Othniel Marsh, who initially termed them the "post-occipital bones",[7] but their homology with the proatlas of other reptiles was subsequently recognized by Charles W. Gilmore.[8]
^ abcKorneisel, Dana E.; Vice, Rebekah; Maddin, Hillary C. (2021-11-30). "Anatomy and development of skull–neck boundary structures in the skeleton of the extant crocodylian Alligator mississippiensis". The Anatomical Record. 305 (10). doi:10.1002/ar.24834. eISSN1932-8494. ISSN1932-8486.
^Baur, G. (1886). "The proatlas, atlas and axis of the Crocodilia". The American Naturalist. 20 (3): 288–293. doi:10.1086/274205. ISSN0003-0147.
^ abMook, Charles C. (1921). "Notes on the postcranial skeleton in the Crocodilia". Bulletin of the American Museum of Natural History. 44: 67–100.
^ abKorneisel, Dana E.; Maddin, Hillary C. (2025-07-22). "Review of the tetrapod skull–neck boundary: implications for the evolution of the atlas–axis complex". Biological Reviews. doi:10.1111/brv.70053.
^Romer, Alfred Sherwood (1956). Osteology of the Reptiles.
^Muhleman, Mitchel; Charran, Ordessia; Matusz, Petru; Shoja, Mohammadali M.; Tubbs, R. Shane; Loukas, Marios (2012). "The proatlas: a comprehensive review with clinical implications". Child's Nervous System. 28 (3): 349–356. doi:10.1007/s00381-012-1698-8. eISSN1433-0350. ISSN0256-7040.
