The first fish evolved later in the Cambrian Period, probably about 500 million years ago. The fish had neither fins nor jaws. They swam with their mouths held open all the time, sucking in things to eat. Many of these jawless fish had bony armour on their bodies. This would help to protect them from attack by other marine creatures.

Trilobites

Trilobites first appeared in the Cambrian seas. They became very numerous animals for the next 250 million years. Their bodies were divided into three lengthwise strips, or lobes. They used their legs to carry food to their mouths. Some kinds of trilobite swam in open water, while others crawled along the sea bed. To protect themselves from predators, some kinds of trilobite burrowed into the mud of the sea floor, while others were covered in long spines and rolled themselves into a ball.

Fish with jaws

Millions of years later, fish with fins evolved. Their fins allowed them to steer more easily through the water and swim faster. Some fish also developed jaws and teeth. They could now feed on other sea creatures.

Dunkleosteus was a 9-metre-long (30 feet) predator. Instead of teeth, it had huge plates of bone in its jaws. These acted like slicer blades.