One of the fundamental questions in developmental and evolutionary biology concerns the origin of novel structures. Do appendages evolve de novo (from the beginning) or from pre-existing structures? The adaptive evolution of terrestrial vertebrates to aquatic environments has been well documented through the fossil record. The transition from life on land to life in the ocean was made possible by the appearance of new types of limbs, often accompanied by the loss of certain appendages.
Here, I examine the origin and diversification of appendages in cetaceans from a terrestrial ancestor. Cetaceans are marine mammals descended from land mammals. This will be a general overview of the evolution of the pectoral fins (commonly known as flippers) in dolphins. The pectoral fins are the forelimbs that allow dolphins to maneuver their locomotion, mainly through steering.
As revealed in the diagrams above, pectoral fins have a skeletal structure that resembles that of the human forelimb (i.e., the arm, wrist, and hand). Like land mammal forelimbs, flippers have a humerus, radius, ulna, phalanges, and a ball and socket joint. This internal bone structure of the dolphin forelimb is one of the strong indicators that cetaceans arose from a terrestrial ancestor that had frontal appendages that aided in forward movement. However, the skeletal elements of the flipper are foreshortened and modified. Pectoral fins are made of cartilage and bone, but unlike the human forelimb, they are very rigid and stuff, preventing movement at the elbow joint.
Another interesting deviation from the land mammal forelimb bone structure is that dolphins display hyperphalangy, in which the number of phalanges (finger bones) in the forelimb is greatly increased from the standard number of 3 phalanges per finger. Cetaceans are the only mammals in evolutionary history to undergo hyperphalangy. It is important to note that 3 phalanges per finger do development during embryonic development in dolphins. However, this process does not cease in late embryonic development as in land mammals; the process persists into the fetal period until 9-13 phalanges develop in some fingers.
Currently, there is a debate over which ancient group of animals gave rise to the cetacean lineage. One theory suggests that cetaceans are descendants of the mesonychid, a terrestrial dog-like animal that lived 55-95 million years ago. However, more recent analyses based on genetics and molecular studies provide more support for the 2nd major theory, which proposes that cetaceans share a common ancestor with the modern-day hippopotamus.
Based on fossil evidence, the mesonychid was a terrestrial animal that had front and hind limbs containing bones to support its heavy body weight, as well as hoofed toes. Scientists believe that it went into the water to seek food, and over millions of years, it become more adapted to life in the water, leading to the structural changes in its forelimbs. The prevailing theory is that their hind legs reduced in size until disappearing altogether. Thus, the pectoral fins in present-day dolphins and whales are remnants of its ancestor’s life on land, contributing to the aerodynamic shape that allows for efficient swimming.
Of course, it is impossible to determine the exact evolutionary steps from the “missing link” ancestor to today’s cetaceans, but more recent fossil evidence has bolstered support for the alternative theory that dolphins share a common ancestor with hippos. Many scientists believe that the recent astonishing discovery of a bottlenose dolphin with an extra set of flippers is living proof of the theory. The abnormal bottlenose dolphin is the first of its kind ever to be found, possessing an additional pair of stubby fins near its tail. According to evolutionary biologists, the extra fins are an example of an “atavistic trait,” a genetic trait that appears to be an evolutionary throwback to the ancestral land-dwelling days. For some unknown reason, the dolphin’s extra fins may be the remnants of a pair of hind legs, adding to the theory that dolphins descended from terrestrial four-footed mammals.
1. Lovett, Richard. “Dolphin With Four Fins May Prove Terrestrial Origins.” National Geographic News. 8 Nov. 2006. National Geographic Society. <http://news.nationalgeographic.com/news/2006/11/061106-dolphin-legs.html>
2. Dolphin Research Center. 2007. Dolphin Research Center. 1 March 2012. <http://www.dolphins.org/marineed_physiology.php>
3. SeaWorld Animals. 2011. SeaWorld Inc. 1 March 2012. <http://www.seaworld.org/animal-info/info-books/bottlenose/physical-characteristics.htm>
4. American Museum of Natural History. "Getting A Leg Up On Whale And Dolphin Evolution: New Comprehensive Analysis Sheds Light On The Origin Of Cetaceans." ScienceDaily. 24 Sep. 2009. <http://www.sciencedaily.com/releases/2009/09/090924185533.htm>