right|thumb|[[Humboldt penguin swimming. Penguin wings evolved into short, strong flippers after acquiring flightlessness.
Animals with flippers include penguins (whose flippers are also called wings), cetaceans (e.g., dolphins and whales), pinnipeds (e.g., walruses, earless and eared seals), sirenians (e.g., manatees and dugongs), and marine reptiles such as the sea turtles and the now-extinct plesiosaurs, mosasaurs, ichthyosaurs, and metriorhynchids.
Usage of the terms "fin" and "flipper" is sometimes inconsistent, even in the scientific literature. However, the hydrodynamic control surfaces of fish are always referred to as "fins" and never "flippers". Tetrapod limbs which have evolved into fin-like structures are usually (but not always) called "flippers" rather than fins. The dorsal structure on cetaceans is called the "dorsal fin" and the large cetacean tails are referred to primarily as flukes but occasionally as "caudal fins"; neither of these structures are flippers.
Some flippers are very efficient hydrofoils, analogous to wings (airfoils), used to propel and maneuver through the water with great speed and maneuverability (see Foil). Swimming appendages with the digits still apparent, as in the webbed forefeet of amphibious turtles and platypus, are considered paddles rather than flippers.
The flippers of penguins became flatter, denser, and smaller while being modified for hydrodynamic properties. The functionality of the penguin hydrofoil comes from the wing bones themselves as opposed to feathers in a volant bird. Their bones have the thickest cortical bone of any bird, a very small medullary cavity, and are considered largely osteosclerotic. There is also fusing of digits and joints within the penguin wing that prevent most flexion and extension.
Hydrodynamics
Cetacean flippers may be viewed as being analogous to modern engineered hydrofoils, which have hydrodynamic properties: lift coefficient, drag coefficient and efficiency. Flippers are one of the principal control surfaces of cetaceans (whales, dolphins and porpoises) due to their position in front of the center of mass, and their mobility which provides three degrees of freedom.
thumb|The [[Tubercle effect|tubercles on the flippers of humpback whales improve the hydrodynamics of the flipper at their size. Breaking up channels of fast-moving water allows humpbacks to retain their "grip" on the water, and turn at sharper angles even at low velocities.
The foreflippers used by the pinnipeds act as oscillatory hydrofoils. Both fore and hind flippers are used for turning.
Evolution of flippers
Marine mammals have evolved several times, developing similar flippers. The forelimbs of cetaceans, pinnipeds, and sirenians presents a classic example of convergent evolution. There is widespread convergence at the gene level. Distinct substitutions in common genes created various aquatic adaptations, most of which constitute parallel evolution because the substitutions in question are not unique to those animals.
Digit processes
thumb|Close up skeletal of [[fin whale flipper]]
Whales and their relatives have a soft tissue flipper that encases most of the forelimb, and elongated digits with an increased number of phalanges. Hyperphalangy is an increase in the number of phalanges beyond the plesiomorphic mammal condition of three phalanges-per-digit.
thumb|Illustration of limbs of aquatic reptiles: the [[mosasaurs Platecarpus and Clidastes, and the ichthyosaurs Ophthalmosaurus and Platypterygius. From The Osteology of the Reptiles (1925)]]
Cetaceans are the sole mammals to have evolved hyperphalangy. Though the flippers of modern cetaceans are not correctly described as webbed feet, the intermediate webbed limbs of ancient semiaquatic cetaceans may be described as such. The presence of interdigital webbing within the fossils of semi-aquatic Eocene cetaceans was probably the result of BMP antagonists counteracting interdigital apoptosis during embryonic limb development. Modifications to signals in these tissues likely contributed to the origin of an early form of hyperphalangy in fully aquatic cetaceans about 35 million years ago. The process continued over time, and a very derived form of hyperphalangy, with six or more phalanges per digit, evolved convergently in rorqual whales and oceanic dolphins, and was likely associated with another wave of signaling within the interdigital tissues.
Although toothed cetaceans have five digits, most baleen whales have four digits and even lack a metacarpal. In the latter (mysticetes), the first digit ray may have been lost as late as 14 million years ago.
Evers et al. identified characters related to the pectoral girdle and forelimb that are related to the modification of sea turtle arms and hands into flippers. Based on presumed limb use in ancestral turtles, these behaviors may have occurred as long ago as 70 million years.
See also
- Fish fin
- Homology (biology)