Heterocoelous vertebrae are unique to birds (class Aves). The articular surfaces of the centra are saddle-shaped — concave in one direction and convex in the perpendicular direction (like a horse saddle). This configuration allows flexibility in multiple planes while providing strong interlocking joints, supporting the demands of flight. Amphicoelous (concave on both ends) is found in fish; procoelous (concave anteriorly) in frogs and reptiles; acoelous (flat) in mammals.