Many ancient fish, together with their surviving relative the lungfish, developed simple lungs which they used instead of their gills when out of the water. Modern lungfish hibernate in a mud cocoon when the pools in which they live dry up. Lungfish can also migrate overland to other pools, using their leg-like fins to help pull them along.
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The main reasons for an animal to change its habits are to obtain fresh food supplies and to escape from its predators. Many modern fish, such as those commonly found around tidal rock pools, leave the water briefly for these reasons. Their ancient ancestors probably also did this, spending more and more time on land as they evolved.
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Around 400 million years ago, ancestors of the coelacanth began to creep out of the water onto the land. Many fish are able to wriggle along on land, but in order to lift their body clear of the ground ordinary fins are not strong enough. The relatives of the coelacanth had leg-like fins reinforced with bones, which allowed them to slither along like a modern crocodile even though they did not walk properly. They were still fish, however, and had to return to the water to breed. It is thought that these early four-legged animals, known as tetrapods, lived in shallow freshwater pools, like modern lungfish. They needed to develop lungs in order to breathe when oxygen levels in the stagnant water fell.
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In 1909 a fossil bed was found in Canadian rocks high up in the Rocky Mountains. Examination of the tiny fossils in this rock revealed very small creatures. The fossils found here are unique and no animals like these have been discovered anywhere else. Special conditions in the shallow seas where these animals lived allowed their delicate structures to be preserved, buried in mud. Many of these animals crept about on the sea-bed, feeding on small organisms in the mud. Some were active predators. Anomalocaris was about 60 cm long and a powerful swimmer. It was armed with grasping claws and a circular mouth lined with cutting blades, enabling it to make short work of its far smaller prey.
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The modern white shark reaches a length of 6 m, making it one of the most feared predators of the seas. But an extinct shark may have been 12 m long, and weighed around ten times as much as the white shark. This shark, called Carcharocles, lived about 15 million years ago, and its bulk was the size of a bus. Only its huge teeth have survived, allowing palaeontologists (scientists who study fossils) to calculate its actual size. The jaws of the mega tooth shark would have been large enough to swallow a car.
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The earliest fish were very small. They were heavily armoured to protect themselves from attacks by the invertebrate predators that were common in the seas 450 million years ago. Most of these fish seem to have lived by feeding on small animals and algae in the mud on the sea-bed. Some of the earliest forms had a third eye and all of them differed from modern fish because they had no jaws. Their mouth was a simple hole or slit, which limited the types of food they could eat. The descendants of these primitive fish are modern lampreys and hagfish. These eel-like creatures resemble their ancestors in that they have no jaws. However, they are either parasites living off other fish or scavengers feeding on dead fish.
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