"The almost impossibly long neck of an elasmosaur was one of the most successful structures in the history of life in the sea," he added.
The team's discovery provides hard evidence for a theory first suggested as long ago as the 1920s, according to plesiosaur expert Richard Forrest of the New Walk Museum in Leicester, England.
"Some plesiosaurs have rather long and slender teeth, which it has been suggested were for straining [the ocean floor] rather than biting," he said.
Forrest says "enigmatic traces" of such feeding behavior have been found in ancient seabed sediments in Switzerland.
"There are grooves in the sediments where it looks as if something has swept through it, and it's been suggested that this was done by plesiosaurs," Forrest said.
Yet in England, where numerous plesiosaur fossils have been unearthed, no evidence for bottom-feeding has been found.
Forrest says fossilized gut remains found in the region are "highly consistent with plesiosaurs living on things that swam past, such as fish, belemnites [squid-like carnivores], and even other plesiosaurs."
The paleontologist adds that around 60 percent of the plesiosaur bones found in Oxford Clay, a fossil-rich marine deposit in central England, have bite marks on them.
"Some of those are very obviously from the teeth of big plesiosaurs," he said.
Forrest suggests that differences in feeding habits might be because the two Australian elasmosaurs lived during the early Cretaceous period (144 to 127 million years ago), around 90 million years later than the English specimens.
"When you start coming to the Cretaceous you have mosasaurs taking the top marine predator role," he said.
Mosasaurs grew to a huge size and were armed with massive jaws and teeth.
Forrest says competition from larger marine reptiles may have forced plesiosaurs to adopt different feeding strategies.
"The big plesiosaurs seem to have pretty well vanished by the [late] Cretaceous," he added. "Instead we have very big mosasaurs which had big, sharp, pointy teeth for crunching other animals."
The Australian finds may also help solve the riddle of why fossilized plesiosaurs are often found with large polished pebbles in their stomachs.
The function of the pebbles, called gastroliths, has been debated for years. The leading theory is that they were swallowed as ballast to help plesiosaurs hunt at deeper depths and to stop them from floating to the surface.
One of the two Queensland elasmosaurs contained 135 gastroliths.
Given the food remains found inside the two fossil skeletons, McHenry of the University of Newcastle thinks the main role of these stones is now obvious: Gastroliths would have been a useful digestive aid, helping the sea reptiles to crush up clams and crustaceans.
"An animal with stones in its stomach is going to make short work of a shellfish meal," he said.
"This doesn't necessarily sink the alternative theorythat they helped control buoyancybut means that at the least gastroliths had a dual role," he added.
Angela Milner, associate keeper of paleontology at the Natural History Museum in London, says this idea makes sense.
"I don't think it has been suggested before that [gastroliths] might have acted as a gastric mill, but there is no real reason why not," she said.
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