Proatlas

The proatlas is a paired bone in the skeleton of some living and many extinct terrestrial vertebrates (tetrapods) that occurs between the skull and the atlas, the first cervical vertebra. It ossifies endochondrally.^[1]

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 of dinosaurs was first recognized by Othniel Marsh, who initially termed them the "post-occipital bones",^[4] but their homology with the proatlas of other reptiles was subsequently recognized by Charles W. Gilmore.^[5]

The proatlas was not present in early finned tetrapodomorphs, but are present in the limbed stem-tetrapod Greererpeton. Proatlases were probably widely present in early tetrapods, with records in aïstopods, temnospondyls, embolomeres and seymouriamorphs. They were widely present among non-mammalian synapsids, but were lost in the mammaliaform ancestors of mammals. They were also widely present among extinct reptiles, including dinosaurs.^[6] The proatlas is absent as a separate element in birds, perhaps as a consequence of fusion with the braincase, with Archaeopteryx the closest relative of modern birds known to retain a separate proatlas.^[7] They have also been lost in modern amphibians (Lissamphibia), turtles and squamates (lizards and snakes), but are present in the stem-squamate Huehuecuetzpalli.^[6] They are present in the tuatara,^[6] though they have not yet been identified in extinct rhynchocephalians.^[8] They are also present in living crocodilians, in which the left and right proatlases fuse into a single V-shaped midline element.^[1][3] The proatlas plays a role in the circulation of cerebrospinal fluid in crocodylians.^[9]

A proatlas can occur pathologically in humans.^[10]