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Fish are cold-blooded aquatic: vertebrates which breathe through gills.
The earliest fish are Upper Ordovician and belong to a group called the Agnatha. These had no jaws - their mouths were simple openings on the underside of their heads - and many had a well developed armour of bony plates. Good fossil specimens have been found in Silurian and Devonian rocks in Scotland. The jawless fishes virtually died out at the end of the Devonian, but a couple of types, lampreys and hagfish, survived to the present day.
The next fish to appear as fossils are the Ptacoderms, which lived between the Upper Silurian and the Permian Periods. They had primitive jaws and extensively armoured bodies. Most were quite small, between 5 and 20 centimetres long, but Dinichthys from the Devonian, reached a length of 10 metres and must have been a ferocious predator.
The Chondrichthyes, which include the sharks and rays, appeared in the Devonian, are still with us today, and occasionally star in movies. Their skeletons are made of cartilage, which is soft and rarely fossilised, but their teeth (which occur in great numbers) are very durable and are abundant in some British Cretaceous and Tertiary beds. Some teeth from Miocene rocks in the U.S.A. are l5cm long and must have come from fish 15 metres in length - giant relatives of the modern White Shark.
The most advanced fishes are the Bony Fishes, which appeared in the Devonian and include most of the modern forms we eat or put in aquariums. As their name suggests, they have bony skeletons which are often preserved as fossils. Palaeozoic and early Mesozoic types usually had coverings of thick glossy scales, which are often found loose as fossils. From the Cretaceous onwards bony fish tended to have much thinner scales, more like those of today.
Probably the most interesting of the bony fish are the lungfish and coelacanths which, though still alive today, were most abundant in the Devonian Period. Many had primitive lungs and fleshy fins which enabled them to survive for brief periods on land.
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