Sunday, March 4, 2012

Evolution of Pectoral Flippers

     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.


Sources:
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> 

2 comments:

  1. One way the evolutionary biologists approach the topic of finding "missing links" nowadays is through genomic phylogeny. Perhaps from studying their sequences (given that there is enough DNA sample to amplify), we might be able to find the link. Or at least to confirm that the bottlenose dolphin is the answer.

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  2. I think it's very interesting how the development of aquatic species appendages came about. You mention that researchers believe land animals were in search for food within the water leading to the development of this species. I wonder what types of ecological conditions contributed to the decision for such species to enter the water? For instance, is it possible that maybe the energy many animals needed to invest in order to obtain resources such as food during warm climates were too large of an expenditure on the body's energy stores? Consequently, maybe a cooling effect of water provided animals a way to obtain food without expending as much energy?

    I think it would be interesting to understand exactly what the "tipping" factors/variables were in leading to the terrestrial animals to all of a sudden explore the water habitats.

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