Proatlas

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

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]