fin-fold theory

The fin-fold theory proposes that paired fins in ancestral vertebrates originated from a continuous fold of skin running along each side of the body. This fold is thought to have fused into a single fin behind the anus. The theory suggests that localized persistence of parts of this fold led to the development of paired fins, which initially served as stabilizers to prevent rolling and pitching movements.1

These paired fins are considered the evolutionary precursors to the limbs of land vertebrates. The skeletal structure within these fins, including the basal supporting elements and limb girdles, evolved from cartilage and later ossified in bony fishes and land vertebrates. The pectoral girdle, associated with the anterior appendages (pectoral fins/forelimbs), is located behind the gill region, while the pelvic girdle, for posterior appendages (pelvic fins/hind limbs), is situated in the trunk region.1