Ammonites were free-swimming molluscs of the ancient oceans, living around the same time as dinosaurs.

BBC: Children discover 'rare' fossils at Cotswold Water Park

With no direct counterpart today, it has been difficult to nail down the ammonites' true place in the ancient food chain.

BBC: Ammonite diet revealed in X-rays

But the analysis by Kruta and her colleagues indicates their ammonites would have dined on small organisms floating in the water, such as zooplankton, tiny crustaceans, and even other petite ammonites.

BBC: Ammonite diet revealed in X-rays