With no direct counterpart today, it has been difficult to nail down the ammonites’ true place in the ancient food chain.
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.
In one of the specimen’s mouths the team found the remains of what was probably the animal’s last meal, diced up by the jaws and the teeth-laden radula.
If plankton provided the major part of the ammonite diet then this could help explain their extinction 65 million years ago, the team believes.
The asteroid or comet impact widely implicated in the dinosaurs’ demise would also have damaged plankton production in the oceans.