Proatlas

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The proatlas is a paired bone in the skeleton of many vertebrates that occurs between the skull and the first cervical vertebra. It ossifies endochondrally. [1]

Skull of the sauropod dinosaur Smitanosaurus in posterior view, with (left) and without (right) the proatlases in place Smitanosaurus skull from the back.jpg
Skull of the sauropod dinosaur Smitanosaurus 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]

A proatlas can occur pathologically in humans. [6]

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]

The proatlas plays a role in the circulation of cerebrospinal fluid in crocodylians. [9]

References

  1. 1 2 3 Korneisel, 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. eISSN   1932-8494. ISSN   1932-8486.
  2. ↑ Baur, G. (1886). "The proatlas, atlas and axis of the Crocodilia". The American Naturalist. 20 (3): 288–293. doi:10.1086/274205. ISSN   0003-0147.
  3. 1 2 Mook, Charles C. (1921). "Notes on the postcranial skeleton in the Crocodilia". Bulletin of the American Museum of Natural History. 44: 67–100.
  4. 1 2 Korneisel, 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.
  5. ↑ Romer, Alfred Sherwood (1956). Osteology of the Reptiles.
  6. ↑ 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. eISSN   1433-0350. ISSN   0256-7040.
  7. ↑ Marsh, O. C. (1883). "Principal characters of American Jurassic dinosaurs. Part VI: restoration of Brontosaurus" (PDF). American Journal of Science. 3. 26 (152): 81–85. doi:10.2475/ajs.s3-26.152.81. ISSN   0002-9599.
  8. ↑ Gilmore, Charles W. (1907). "The type of the Jurassic reptile Morosaurus agilis redescribed, with a note on Camptosaurus". Proceedings of the United States National Museum. 32 (1519): 151–165. doi:10.5479/si.00963801.32-1519.151. ISSN   0096-3801.
  9. ↑ Swords, Annelise; Cramberg, Michael; Parker, Seth; Scott, Anchal; Sopko, Stephanie; Taylor, Ethan; Young, Bruce A. (2024). "The Crocodylian proatlas functions to redistribute venous blood and cerebrospinal fluid". Journal of Morphology. 285 (3): –21683. doi:10.1002/jmor.21683. ISSN   1097-4687. PMID   38424675.


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