amphicoely

Amphicoely refers to a specific type of vertebral articulation found in many fish and some extinct reptiles. In an amphicoelous vertebra, both the anterior (front) and posterior (back) surfaces of the vertebral body, also known as the centrum, are concave, forming cup-like depressions. This unique shape allows for a significant degree of flexibility between adjacent vertebrae.

This type of articulation permits a wide range of motion, which is particularly advantageous for aquatic locomotion. The ability for vertebrae to move freely against each other facilitates the undulating movements of the body and tail that many fish use to propel themselves through water. The cushioning effect provided by the space between the concave surfaces can also help to absorb shock during movement.

Amphicoely is a common feature in the vertebral columns of Actinopterygii, the ray-finned fishes, which constitute the vast majority of living fish species. It was also present in some early tetrapods and certain groups of extinct marine reptiles, such as ichthyosaurs. In contrast, many terrestrial vertebrates, including mammals and birds, have less flexible vertebral articulations, often procoelous (concave anteriorly, convex posteriorly) or opisthocoelous (convex anteriorly, concave posteriorly), which provide greater support for the body against gravity. The presence and degree of amphicoely can vary among different fish groups, reflecting adaptations to diverse lifestyles and swimming styles.