The vertebral column is commonly divisible into the usual five regions, but in the Ophidia, Ichthyosauria, and Amphisbaenidae among Lacertilia, only into caudal and precaudal regions. In the Chelonia there are no lumbar vertebrae.
The form of the vertebral centra is very variable. A large proportion of extinct reptiles,—several entire orders,—and the earlier and more primitive forms in some of the other groups have amphicoelous vertebrae. Vertebrae of this type occur in the Theromorpha, Ichthyosauria, most Sauropterygia and Rhynchocephalia, and many Dinosauria, also in some of the early Crocodilia such as Belodon, Teleosaurus and Goniopholis, and the Geckonidae among Lacertilia.
The majority of living reptiles have procoelous vertebrae. Thus they occur in the Lacertilia (excluding the Geckos), the Ophidia, and the Crocodilia, also among extinct forms in the Pterosauria and many Dinosauria. On the other hand some Dinosauria such as Iguanodon have opisthocoelous cervical vertebrae, while others have opisthocoelous thoracic vertebrae. The vertebrae of the Ceratopsidae and some Sauropterygia, the thoracic vertebrae of Iguanodon, and the sacral vertebrae of Crocodilia have flat centra. The first caudal vertebra of modern Crocodilia is biconvex, and in the Chelonia all types of vertebral centra are found. The cervical vertebrae of Sphenodon are noticeable for the occurrence of a small pro-atlas, which may represent the neural arch of a vertebra in front of the atlas.
In most reptiles the vertebrae are fully ossified, but in some of the more primitive forms the notochord persists in the centre of the vertebra (i.e. intervertebrally), this is the case for instance in many of the Theromorpha and Rhynchocephalia, and also in the Geckos. In other reptiles it persists longest intravertebrally.
The centrum of each of the caudal vertebrae of most Lacertilia is traversed by an unossified septum along which it readily breaks.
Chevron bones occur below the caudal vertebrae in Lacertilia, Chelonia, Ichthyosauria, many Dinosauria, and Sphenodon, articulating with quite the posterior part of the centrum which bears them. In Lacertilia and Crocodilia (fig. 41, 3) the axis has a well-marked odontoid process. The ventral portions of the intervertebral discs are sometimes ossified, forming wedge-shaped inter centra, as in Geckos, and the cervical vertebrae of Sphenodon.
In snakes, Theropod Dinosaurs, and the iguanas among lizards, the neural arches are provided with zygosphenes, and zygantra.
The neural arches are usually firmly ankylosed to the centra, but in the Crocodilia and some Chelonia, Sauropterygia, and Dinosauria, the suture between the centrum and neural arch persists at any rate till late in life. In the Ichthyosauria the neural arches were united to the centra by cartilage only.
The thoracic vertebrae of some of the Theromorpha (Dimetrodon) are remarkable for the extraordinary development of the neural spine, and those of Chelonia for the absence of transverse processes.
In living reptiles the number of sacral vertebrae is nearly always two, but in the Theromorpha, Dinosauria, and Pterosauria, as many as five or six bones may be ankylosed together in the sacral region. In Crocodiles the two halves of the pelvis sometimes articulate with different vertebrae. The vertebrae of some of the great Sauropoda are remarkably hollowed out, having a large vacuity on each side of the centrum communicating with a series of internal cavities. The whole structure of these vertebrae shows a combination of great strength with lightness.