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.
File:Smitanosaurus skull from the back.jpg|thumb|Skull of the sauropod dinosaur Smitanosaurus in posterior view, with and without the proatlases in place
A number of different interpretations have been made of the proatlas. 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.
The proatlas was not present in early finned tetrapodomorphs, but is present in the limbed stem-tetrapod Greererpeton. It was probably widely present across early tetrapods, and is retained in some modern reptiles, such as the tuatara. In crocodylians, the left and right proatlases fuse into a single V-shaped midline element. Lissamphibians, mammaliaforms, squamates, turtles, and birds all lack proatlases.
A proatlas can occur pathologically in humans.
The proatlas was first recognized in dinosaurs by Othniel Marsh, who initially termed them the "post-occipital bones", but their homology with the proatlas of other reptiles was subsequently recognized by Charles W. Gilmore.
The proatlas plays a role in the circulation of cerebrospinal fluid in crocodylians.