A sea creature killed just before it could defaecate has given new insight into the feeding habits of plesiosaurs.

The fossilised contents of its lower intestine show the long-necked marine reptile had a fondness for clams and snails - food items from the sea floor.

Plesiosaurs existed in dinosaur times and were thought to be hunters of fish, squid and other free-swimming prey.

But a research team tells Science magazine that the discovery suggests plesiosaur diets were far more varied.

"Traditionally, from the day these creatures were first found, it was quite reasonably assumed plesiosaurs were fish and squid eaters," said team member Dr Steve Wroe, from the University of Sydney, Australia.

"We're not saying they weren't but it's pretty clear they had a far more generalised diet and this Australian specimen found near Cairns was eating a lot of benthic, or bottom-dwelling, animals."

The team actually examined the fossilised remains of two elasmosaurids, the most extreme form of plesiosaur that had necks longer than their bodies and tails combined.

They were both unearthed in Queensland, which 100-110 million years ago would have been covered by a sea.

When they were alive, the plesiosaurs would have been 5-6m in length and about a tonne in weight.

One of the specimens contained a bromalite, a fossilised mass of food waste.

"The indigestible parts of the prey were compacted together, just prior to being expelled, and the result was a solid lump of digested food composed entirely of broken shells from bottom living animals," explained Colin McHenry, from the University of Newcastle.

"That raised the question - how was this supposedly specialised fish-eater digesting such hard-shelled prey? As far as anyone knows, elasmosaurs did not have teeth that were capable of crushing hard-shelled animals."

The clue to answer, the team says, came in the form of large polished pebbles found within the stomach region. Both specimens had them.

Explained Dr Wroe: "The role of these gastroliths, or stomach stones, has been an area of contention for many years.

"In marine animals, there's certainly a theoretical advantage with respect to buoyancy control or ballast; but with these plesisaours, these stones would have been very useful for crushing up clam shells and snail shells."