REASON WHY AMPHIBIANS AFTER EVOLVING LIMBS THEY BECAME LIMBLESS

Amphibians are grouped in phylum chordate, subphylum of vertebrate and class amphibian. These Vertebrates comprise of many living chordates, and they have evolved an enormous variety of forms. This is divided into eight classes; four are aquatic which is grouped into super class Pisces, while four are terrestrial. They are grouped as super class Tetrapods or four-footed animals.

They have different movement aquatic species like fish moves by weaving of the trunk and tails, "most of them have sets of paired fins pelvic and pectoral they do not have limbs".(1) The terrestrial species move in the form of wriggling which is similar to those of fish even though it has limbs that support the movement.

Some have limbs that make movement simple, like Frogs and toads use their muscular hind legs for jumping. While other amphibians have lost their limbs and most of that species are burrowing forms. (1)

"As evolution are taking place basic forms of structure evolve to perform number of task, example tetrapods, forelimbs becomes used for grasping climbing, flying, burrowing and swimming as well as terrestrial locomotion. In amphibians, limbs improved for locomotion. These evolve into upright position which is different to what amphibian have of sprawling, and have some tendency towards bipedalism among reptiles and birds". (1)


All these changes in limbs evolve from fish and the vertebrates that start to evolve on land it was suggested by scientist that many of limb and muscles developments needed for walking on land before fish and limbed animals split.


When Tetrapods moved onto land they lose connection between the skull and the pectoral girdle as they move on land. The pelvic girdle became closely involved with vertebral column using the sacral vertebrae. While in fish the pelvic is made up of one and basiterygium (paired bone). In amphibians, the basiterygium divided into the ilium and ischium. Because the pectoral girdles are not attached to the head of terrestrial animal no force of walking can be send directly into skull which shocks the brain. Amphibians did not obtain the strong girdles like fish, because they evolved from fish.

Loss of limbs is very common in all lineages of vertebrates, this because fishes and amphibians, has evolved many times and they all move by the use of the limbs. While if mammals or archosaurs were to lose their limbs, their axial muscular would still be weak and unable to support the animal. (2)


Only one group of mammal (whales) have ever lost either of the limbs. Whales develop a larger, broad tail which is considerably strengthened their axial muscular. "Whales and snakes are animals which have lost limbs and are still retain some element of the girdles".(2)


In conclusion changes in adaptation tend to evolve in the change of structural patterns of the species, this result because some of the species live in both environment and they would like to have features that will allow them to move. so evolving of limbs in amphibians and vanishing of limbs are also influenced by the environment that they choose to live in,


Reference:

1 Limb evolution [internet] [cited 2006-May-09] Available from: http://sellers.sbc.man.ac.uk/~wis/lectures/limb-evolution/LimbEvolution.html


2. Amphibians [internet] [cited 2006-may-09] Available from: http://web6.duc.auburn.edu/academic/classes/zy/0301/Topic9/topic9.pdf


Mr Elelwani Muanalo
NISL- Ecological Informatics Student
CSIR Pretoria
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