Amphicoelian

Amphicoelian is a term used in paleontology to describe a type of vertebral centrum, the main body of a vertebra. Specifically, amphicoelous vertebrae are concave on both the anterior (front) and posterior (rear) surfaces of the centrum. This shape creates a biconcave disc, similar to two saucers placed rim to rim. This articulation allows for a significant degree of flexibility and shock absorption within the vertebral column.

Amphicoelous centra are characteristic of many early vertebrates, including fish and early amphibians. In these animals, the vertebral column often needs to accommodate significant movement, such as swimming or leaping. The biconcave structure permits a greater range of motion between adjacent vertebrae compared to other types of vertebral centra. For example, in many fish, the amphicoelous vertebrae contribute to the undulating motion used for propulsion through water.

While common in basal vertebrate groups, the amphicoelous condition is less prevalent in later, more derived terrestrial vertebrates like mammals and advanced reptiles. As animals evolved to rely more on weight-bearing and different forms of locomotion, other vertebral centrum shapes, such as procoelous (concave in front, convex in back) or opisthocoelous (convex in front, concave in back), became more advantageous for stability and specific types of movement. However, amphicoelous vertebrae can still be found in some modern amphibians and certain fish species, reflecting their continued utility in specific ecological niches.