Fish have various groups, which is made up of three living classes that hold an important place in evolution history and modern ecology. Fish are the first known vertebrate (animals with backbones) and also the first species that evolve in land –walking vertebrates, they are the first known true chordate that has been found. (1)


It was suggested that fish evolve before the Devonian periods which was referred as the Age of fishes, fishes where aquatic species. Then at the end of Devonian periods tetrapods (vertebrate) which evolved legs which they can be used to walk on land evolve, and this evolution led to the adaptation to land in the form of amphibians.(1)


Ostracoderms was the first fish to evolve in the period of Cambrian about 510 million years ago and became extinct at the end of the Devonian periods it was jawless fishes found in the aquatic environment, it was covered by scales this Ostracoderms were less than 30cm long and it was placed in the class of Agnathans. These jawless fishes were the first vertebrates. Jawless fish was having circular mouth with a sharp spine tongue which is used for sucking and filtering food. Most of the species under this class agnath was not having fins except for a fringe around the tail, And they possesses the beginning of the backbone in a form of cartilaginous. (3)


In agnath group they were other small fish like animal which possessed heavy protective covering –plating, this was originated from the deposit of salts from food they eat. This plating was suggested that it tend to harden the gill arches to become the upper jaw and the bottom gill into lower jaws. And this evolution of jaws on fishes was important because it permit the fish to feed on variety of foods. Having jaws allowed them to be vigorous hunters as opposed to passive filter feeder. Being vigorous hunters led the fish to have large diversity of adaptations.(3)


This salt deposit marks the first presents of bone in the fishes which influence the evolution of vertebrates. The bony plate provides protection against sea scorpions that co-existed at the same time.(1)


Some of the bony scales in the skin around the mouth enlarge and became first teeth, while the lateral flaps of skin evolved into true fins which improve the swimming. Because of the food they eat with salts, lots of features evolved other develop some back bones. Species such as Acanthodii develop internal bony skeleton running longitudinal through the body. And this type of species was likely the ancestor of the bony fish we know today. (3)


Bony fish are often regarded as "true" fish. This group includes most of the fish we are familiar with to day. They are characterised by short jaw with lower jaw articulates with vertical quadrate it also have symmetrical caudal fin for swimming. They also have bony skeleton and single pair gill opening this are the advance bony fish we have today in the class of vertebrate. (2)


In general there is much to be learned from fish, they have evolved to fill every aquatic and inland environment with variety of adaptation. Fishes are important in evolutionary steps they are the first known vertebrate and also influence the evolution of all living vertebrate on land.

Reference:


1. Kagle R.1997.The Evolutionary Steps Of Fish [internet] [cited 2006- May -08] Availablefrom:http://serendip.brynmawr.edu/biology/b103/f97/projects97/Kagle.html


2. Fish Distribution [Internet] [cited 2006-May-08] Available from: http://www.lookd.com/fish/distribution.html


3. The Major Radiation of Fishes;[Internet][ cited 2006-May-08] Available from:http://users.tamuk.edu/kfjab02/Biology/Vertebrate%20Zoology/b3405_ch08.htm


4. Wikipedia contributors. Osteichthyes. [Internet] Wikipedia, the Free Encyclopedia; 2006 May 07. [Cited 2006-May-08]Available from: http://en.wikipedia.org/wiki/Osteichthyes



Mr Elelwani Muanalo
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CSIR Pretoria
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