Chapter 12 : THE FISHES VERTEBRATE SUCCESS IN WATER

THE FISHES VERTEBRATE SUCCESS IN WATER

EVOLUTIONARY PERSPECTIVE

Importance of water: Water is a buoyant medium. It resists rapid fluctuations in temperature. Water covers over 70% of the earth’s surface. Life began in water. The living tissues of the organism are mostly made up of water. Therefore, life is impossible without water.

Adaptations in fishes: The fishes are adapted to aquatic. No other animal is adapted to aquatic environment like fishes. A variety of beautiful fishes is present every where. A variety of evidence of adaptive radiation in fishes. The adaptive radiations in fishes started more than 500 million years ago. These radiations are still continuing. Fishes dominant many watery environments. They are also ancestors of all other members of the subphylum Vertebrata.

PHYLOGENETIC RELATIONSHIPS

Fishes are members of the chordate subphylum Vertebrata. They have vertebrae. These vertebrates surround the spinal cord. Vertebral column provides the primary axial support. Fish also have a skull. It protects the brain. Zoologists do not know about the first vertebrate. Molecular evidences are gathered by comparing gene of cephalochordates.

These evidences suggest that the vertebrate lineage goes back to 750 million years. This date can not be confirmed by fossil evidence. Cladistic analysis indicates that hag fishes are the most primitive vertebrates. Two key vertebrate characteristics develop connection between this lineage and other vertebrates. These characteristics are brain and bone.

1.Evolution of brain

Chinese researchers have discovered the oldest vertebrate fossils. It was 530 million year-old animal. It is a small lancelet shaped animal. Its characteristics suggest that these animals have active predatory lifestyle. A brain is present in them. It processed sensory information from the pair of eyes. Muscle blocks were present along the body wall. It suggests that they were active swimming existence. These evidences show that these animals locate prey by sight and then follow it in prehistoric seas.

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Fig: A Conodont Reconstruction

2. Evolution of bones

The origin of bone in vertebrates is also not clearly known. There are two hypotheses about the origin of bone:


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(i)   Origin from dentine: The fossils of the group of ancient animal conodonts were discovered. Conodonts were eel-like animals. They were present 510 million years ago. They were placed in different phyla. But now zoologists have accepted them as full- fledged vertebrates due to fossil evidences. They have two large eyes and a mouth. The mouth is filled with root like structures made of dentine. Dentine is found in the vertebrate skeleton. These structures show the presence of bone in old vertebrates.

(ii)   Origin from denticles: Other hypotheses suggest that the bone was arised from the denticles in the skin. Denticles were used for mineral storage like calcium phosphate. Bone was well developed by 500 million years ago. Bones were present in the bony armor fishes called ostracoderms. Ostracoderms were inactive filter feeders. It lived on the bottom of prehistoric lakes and seas. They lack jaws and paired appendages. Then evolution of fishes took place from ostracoderms. The fishes developed jaws, paired appendages and many other structures.

Evolution of fishes in marine or fresh water

It is difficult to know that whether the first vertebrate was marine or fresh water. The ancient deutrostome phyla were all marine. Therefore, the first vertebrates were also marine. However, vertebrates were adapted to freshwater very early. Much of the evolution of fishes took place there. There was back and forth movement between marine and freshwater environments during early vertebrates. Majority of the evolutionary history of some fishes took place in ancient seas. Most of the evolutionary historyof other fishes occurred in freshwater. Freshwater habitat is only a small percentage (0.0093% by volume) of the earth’s water resources. But 41% of all fish species are freshwater. It shows the importance of fresh water in the evolution of fishes.

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