oar-shaped extremities that function as organs of locomotion and as depth and steering rudders in vertebrate animals that have secondarily transferred to aquatic living. Among reptiles, flippers are found in sea turtles, in whom they originated from the anterior and posterior extremities. Many fossil aquatic reptiles also had two pairs of flippers. Among birds, the fore-limbs of penguins were converted to flippers. Among mammals, the pinnipeds have two pairs of flippers; the cetaceans and sireni-ans, whose ancestors had two pairs of flippers, have one pair. The reduction of the posterior flippers in them was caused by the transfer of the locomotive function to the caudal section of the body.

All the skeletal elements of flippers are flattened and located in one plane. The segments of the forearm and lower limb and also of the arm and femur are as a rule greatly foreshortened. The hand and foot are elongated (usually owing to an increase in the number of phalanges). The digits in almost all animals that have flippers lack claws (except pinnipeds). An externally similar form of flippers originated independently in various animals and serves as a classic example of convergence.