Origin and evolution of fish M.Sc. sem. IV (M U) Dr. Ramesh Pathak Fish are very important creature for mankind. They are one of the prime food for them. They provide good quality of protein and important liver oil which is used as medicine in the treatment of certain diseases. The first fish appeared during the Cambrian explosion around 530mya(million years ago).Early fish from the fossil record are presented by a group of small jawless, armored fish known as Ostracoderm. Jaw less fish lineages are almost extinct. The lampreys may approximate the ancient pre jaw fish. The first jaw fish was Placoderm fossilis . The diversity of jawed vertebrates may indicate the evolutionary advantages of jawed mouth. Fish may have evolved from an animal similar to sea squirt whose larvae resembled primitive fish in many important ways. The first ancestor of fish may have descended from the larval form into adult (paedogenesis). The first ancestors of the fish that were probably Pikaia, Haikouichthys, Myllokunmingia. These three genera all appeared around 530 mya. Pikaia had a leaf like body and a primitive notochord and a well defined head and tail. it lacked jaw and relied on filter feeding close to sea bed. These three genera are almost similar in their structure, habitat and feeding and represented most basic plan of the vertebrate life. These were followed by indisputable fossil vertebrate in the form of heavily armored fish discovered in the rocks from Ordovician period 500- 430 mya .The Devonian period (395- 345 mya) was marked by giant Placoderm Dunkleosteus which can grow up to 7 meters long and were the early air breathing fish that could remain on land for extended period. The first jawed vertebrate appeared in the late Ordovician and became common in Devonian which is known as the age of fishes. The jaw developed from the first anterior gill arch. The Devonian period is known for the diminish of all virtually jawless fish except Lampreys and Hag fish as well as Placodermii that dominated much of the Silurian period Placoderms are extinct prehistoric fish which appeared about 450 mya in the middle early Silurian. They were mostly wiped out in late Devonian. Those which survived made a slight recovery before dying out entirely at the close of Devonian 360 mya Spiny sharks which shared features both with bony and cartilaginous fish evolved in the sea at the beginning of Silurian period .They were not true shark because the true shark appeared 50 mya later than the spiny shark. Their competition with bony fish was fatal for them and they become extinct in Permian 250mya Cartilaginous fish which consists of sharks ,rays and chimera appeared around 350 mya in the middle Devonian. The class consists of subclass Holocephali (chimera)and elasmobranchii (sharks and rays).The bony fish appeared in the late Silurian about 490 mya.The recent discovery of entelognathus strongly suggests that the bony fish evolve from early placoderms. In subclass of osteichthyes,the ray finned fishes have become the dominant group of fishes in the post palaeozoic and modern world with some 30,000 species. Lobed finned fishes such as lung fish were the most diverged group of bony fishes in the Devonian(410-397 mya).The lobed fin fishes split into two main lineages: (i) the Coelacanth (ii) the Rhipidistian. The former never left the ocean and their days were Devonian and Carboniferous from 285-299 mya.Coelocanth still live today in ocean and are represented by genus Latimaria .The Rhipidistians whose ancestors probably lived in oceans migrated into fresh water habitats.The lung fish evolved the first protolungs and first protolimbs. .