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an asymmetrical caudal fin found on a number of aquatic vertebrates; the axial skeleton continues within the upper (epicercy) or lower (hypocercy) lobe of this fin. A hypocercal tail is characteristic of fossil Agnatha (Anaspida, Heterostraci) and ichthyosaurs; an epicercal tail is characteristic of many fossil and modern fishes (sharks, true sturgeons). When in motion a heterocercal fin creates buoyancy and helps the animal’s stability and balance. Upon development of active control with the aid of paired sculling fins, the heterocercal fin lost its function of regulator and was transformed into a homocercal or diphycercal fin.