The amphibians are the simplest Tetrapods, and are the first class of the vertebrate to invade the land. The Tetrapods includes approximately 18100 existing species of amniotes (birds and their dinosaur ancestors, reptiles and mammals) and approximately 300 existing species of amphibians (frogs, salamanders and caecilians). The term Tetrapoda refers to species with ‘four feet’ and forms the largest group of the terrestrial vertebrates.
Most of the Tetrapods lived in terrestrial ecosystem, but, others still live in the aquatic environment, where their distant ancestors lived. Aquatic Tetrapods include various salamanders (sirenids, cryptobranchids, proteids, etc.), frogs (pipids), some caecilians (typhlonectids), leatherback turtles, sea snakes, pinnipeds (seals and walruses), and whales. Some Tetrapods are capable of flight (birds and bats), while others glide, such as flying squirrels, dermopterans (sometimes called "flying lemurs", even though they are not primates), and the flying dragons (Draco Volans).
Indeed, some of these animals have lost their limbs. However, their common ancestor had two limbs in front and two in back, where fins once flicked instead. When we talk about the limb we are referring to a leg, wing or arm of an animal. To understand where the limb originates, this could bring us to two major hypotheses that were discovered, namely, the fin-fold hypothesis and the gill arch hypothesis.
Some of the known animals, which lost their limbs or limbless animals includes snakes and whales. The whales are any range of marine mammals of the order Cetacea, and they are have the common form of a fish with forelimbs adapted to form flippers, a tail with horizontal flukes and a single blowhole for breathing. The whales open its blowhole and start to breathe out before reaching the water surface and hold its breathing when they are below the water surface.
Reasons were suggested on why the snake became limbless. It was suggested that the snake might have ‘burrowed underground searching for food in small crevices’. I turn to agree with this suggestion, because even to date snakes spent much of their times in small holes (hibernate). Most of the times, snake are seen during summer times, when they are searching for foods and sometimes when they are feeling hot in their habits. It is suggested that snakes lost their limbs and wide shoulders and pelvis that supported their limbs when they were struggling to enter the small holes.
The loss of limbs is very common in all lineages of vertebrates. Both the fishes and the amphibians evolved several times and they both move by using the limbs. The axial muscular mammals and the archosaurs are not able to support the animals, so if they can lose their limbs they will not make it. At least one group of mammal (whales) has ever lost either of the limbs. They survived only because they developed a larger, broad tail which is significantly strengthened their axial muscular. The loss of the limbs of these animals might also be caused by changes in environmental conditions.
1. [http://www.buschgardens.org/infobooks/KillerWhale/adapaqkw.html, accessed on 09/05/2006, 10:00].
2. [http://www.scientificamerican.com/print_version.cfm?articleID=000DC8B8-EA15-137C-AA1583414B7F0000 , accessed on 10/05/2006, 12:30].
3. [http://www.tolweb.org/Terrestrial_vertebrates, accessed on 10/05/2006, 12:30].