Researchers find 'Nessie' also ate clams, snails and crabs

Friday 7 October 2005

Researchers have found that the bizarre long-necked plesiosaurs - one of the most recognisable animals from the Dinosaur era and inspiration of the Loch Ness monster myth - actually ate a wide variety of food, including bottom living hard shelled clams and snails, rather than just being the specialist hunters of free-swimming prey they were traditionally thought to be.

In a paper published this week in the international journal Science, Colin McHenry from the University of Newcastle, Dr Alex Cook from the Queensland Museum and Dr Steve Wroe from the University of Sydney, describe two fossil plesiosaurs from the elasmosaurid family, collected from the rocks of the Great Artesian Basin in Queensland.

"The elasmosaurs were the ultimate long-necked plesiosaurs - the neck was longer than the body and tail put together. In many ways, the story that they ate fish and squid made sense," says Colin. "The neck was strong and flexible, and it is easy to imagine them using the extra reach to catch small fast prey."

So the researchers were somewhat surprised when, while excavating a 110 million year old elasmosaur from Northern Queensland during the 1990s, they found bits of broken clam and snail shell in the stomach region of the animal.

"The most amazing thing was the food mass from the lower intestine that we found with it. 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."

"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," says Colin.

Colin believes that the answer to how this plesisaur digested its meals lies with the large polished pebbles also found within the stomach region of the fossil.

"We knew the animal must have swallowed these because you don't get pebbles like that in the rocks where we found this fossil. The role of these gastroliths (stomach stones) - has been argued for years, but we have little doubt that these stones would have helped the plesiosaur grind up the clams and snails after it had swallowed them."

Stones were also found in another specimen, collected from central Queensland by the Queensland Museum many years earlier, which also had well preserved stomach contents, including parts of a crab and 135 gastroliths. The stomach stones in this second elasmosaur contained minerals which were traced to a region of volcanic rocks near Townsville - at least 300 kms from where the fossil was found.

The significance of these finds, says Colin, are two-fold. "The Queensland specimens demonstrate that stomach stones probably were very useful for digestion, which doesn't completely sink the alternative theory - that they helped control buoyancy - but means that at the least gastroliths had a dual role."

However, Colin is more impressed with the versatility of the elasmosaurs' feeding equipment. "We're not suggesting that free-swimming fish and squid weren't important parts of the diet in these animals. But these finds suggest the elasmosaur neck may not have been the specialised fish catching structure that we'd assumed. It seems that they could use the neck to help catch a much wider variety of prey than previously believed".

This observation, Colin believes, may help explain the extraordinary success of the long-necked plesiosaurs, a dominant marine reptile group for 135 million years. "The idea that the long neck was useful as a generalised feeding tool is consistent with this success - specialised species tend to die out more quickly. In the end, it took the mass extinction that also killed the dinosaurs, when marine ecosystems were almost completely disrupted, to kill the plesiosaurs off."

For media interviews: Colin McHenry, University of Newcastle, on (02) 4921 5404.

FAQ

How old were the two fossil plesiosaurs reported in the Science paper?

The two specimens are from rocks that were laid down at the bottom of a shallow inland sea about 100 million years ago, during the Cretaceous Period, when the dinosaurs still roamed the land.

Are plesiosaurs a kind of dinosaur?

No. Although they lived during the Age of the Dinosaurs - known to scientists as the Mesozoic Era and made up of the Triassic, Jurassic, and Cretaceous periods - plesiosaurs were a completely different type of reptile. Despite rumours of sightings in various Scottish lakes, plesiosaurs are now thoroughly extinct.

Is an 'elasmosaur' the same thing as a 'long necked plesiosaur'?

Elasmosaurs were one family of long-necked plesiosaur - there were at least two other families of plesiosaurs with long necks, but the elasmosaurs were the group with the longest necks of all. Elasmosaurus itself, after which the family was named, was more than 10 metres long and had a 7 metre neck containing 72 vertebrae. In contrast, most mammals - including giraffes - have only seven neck vertebrae.

Is the first time stomach contents have been found in elasmosaurs?

No, stomach contents have been found before, in particular from North American specimens. These show that the North American elasmosaurs also swallowed stomach stones, and apparently travelled distances of several hundred kilometres to find them. But the prey items preserved in these fossilised stomach contents have mainly been fish and squid-like animals, with benthic molluscs (snails and claims living on the sea bed) apparently being a very rare part of the diet in these elasmosaurs.

Were plesiosaurs the only animals that swallowed stomach stones?

No. Modern animals that swallow gastroliths include penguins, sea-lions, and crocodiles. Birds that eat tough plant matter also use gastroliths, although in this case it's clear that the stomach stones are helping the herbivore to grind its food, and it's thought that some plant-eating dinosaurs did this as well. But the role of gastroliths in the water-living animals has always been controversial.

What was the controversy about the role of gastroliths in plesiosaurs?

Essentially, there are two theories of gastrolith function in marine animals. One theory is that the stones help air breathing animals to control their buoyancy in water. The problem with that idea is that it's not certain that a couple of kilograms of pebbles is going to have much effect on the buoyancy of a two-tonne reptile. The other theory was that the stones helped them digest their food, but this theory had its problems as well - until now, all the modern and extinct marine animals that use(d) gastroliths are mainly fish and squid predators, and fish and squid are not exactly the most difficult things to digest.