Opisthocoely

Opisthocoely refers to a specific anatomical condition found in some vertebrates, particularly reptiles, where the centra of the vertebrae are concave at the posterior end and convex at the anterior end. This arrangement is the opposite of what is found in most tetrapods, which exhibit procoely (concave anteriorly, convex posteriorly).

In opisthocoelous vertebrae, the convex anterior articular surface of one vertebra fits into the concave posterior articular surface of the preceding vertebra. This type of vertebral articulation is thought to provide greater flexibility and range of motion in the vertebral column, allowing for more complex movements.

Opisthocoely is characteristic of several groups of extinct and extant reptiles. It is notably found in chelonians (turtles and tortoises) and in some long-necked sauropod dinosaurs. The specific arrangement of opisthocoelous vertebrae in these animals is believed to have been an adaptation for their particular lifestyles and locomotive needs. For instance, the flexible necks of sauropods may have been facilitated by this type of vertebral articulation. While common in certain reptilian lineages, opisthocoely is absent in mammals, birds, and amphibians, which typically exhibit different vertebral articulations such as amphicoely or procoely. The evolutionary significance of opisthocoely lies in its contribution to the diverse anatomical adaptations observed within the vertebrate fossil record and in living species.