A biomechanical transformation model for the evolution of semi-spheroidal articulations between adjoining vertebral bodies in crocodilians

With the exception of the biplanar articulation between the two sacral vertebrae, the adjoining vertebral bodies of extant crocodilians and other eusuchians form synovial, semi-spheroidal articulations. On each of the cervical, thoracic and lumbar vertebrae, the semi-spheroidal-shaped condyle is situated at the caudal end of the vertebral body, while the socket, in which the condyle of the following vertebra articulates, is at the cranial end; a condition that is described as procoelous. The body of the first caudal vertebra is biconvex, having a condyle at both ends. All the other caudal vertebrae are procoelous. As is the case in all crocodilians, both the cranial end of the body of the first sacral vertebra and caudal end of that belonging to the second sacral vertebra are concave. In contrast to eusuchians, the non-sacral vertebrae of most other crocodilians are amphicoelous, meaning that the vertebral body is concave at both ends. Procoelous vertebrae are also known to occur in a some Cretaceous mesosuchians, namely Bernissartia fagesii, Gilchristosuchus palatinus and at least two species of atoposaurids. Additionally, in B. fagesii the body of the first caudal vertebra is biconvex, whereas in the atoposaurid Theriosuchus pusillus it is convex cranially and concave terminally; a condition that is described as opisthocoelous.