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A 50-million-year-old fossil has revealed that fish may have been swimming in schools for centuries.

The fossil shows that Erismatopterus levatus - a type of fish which lived at least six million years ago and is now extinct - may have swam together in large groups much like fish today.

A total of 259 fish have been identified by researchers who studied the fossil, and the thimble-sized animals all appear to be travelling in the same direction.

It has been suggested that animal groups moving together may be the result of certain behavioural rules that affect how they interact with one another.

These rules have led to very specific group behaviour, but little is known about where they actually came from.

The study of this particular fossil has allowed researchers to identify two rules for social interaction, which they believe are still used by fish today.

Fish are attracted to their neighbours which are further away, but purposely keep a reasonable distance from close individuals to avoid collisions.

Nobuaki Mizumoto, a behavioural ecologist who carried out the research, told Science News magazine: "Because collective behaviour is seen in so many animals, including the flocking of birds or swarming of insects, scientists believed such behaviour evolved long ago. But there has been scant evidence in extinct species."

It is still not known how the fish school structure was preserved in the fossil, but this study may help with future research exploring how extinct animals once communicated with one other.