The fossilized amphibian is also in exactly the right position to suggest it had been eaten—it was lying tail-first along the shark's digestive tract, according to Kriwet.
"Also, the fish remains are fully enclosed within the amphibian's outer covering of scales," he added. That confirms that it was indeed eaten by the amphibian and not the shark.
Before the shark ate it, the amphibian had caught a young fish known as an acanthodian, which was covered in bony spines.
"The fish was swallowed side on, otherwise the spines could have got stuck in the amphibian's mouth or throat," Kriwet said.
"The fish is situated in quite the correct area of digestive tract of the amphibian," said said study co-author Ulriche Heidtke, a paleontologist from the National History Museum of the Palatinate in Bad Dürkheim, Germany.
"It clearly shows the hallmarks of digestion, [such as] disintegration," he added.
If the shark had eaten the fish first and then the amphibian, they would be placed one after the other in the shark's stomach, he explained.
John Maisey, a curator of paleontology at the American Museum of Natural History in New York, was not involved in this study.
"Well-documented examples of predator-prey relationships such as this are very rare," he said.
Such fossils allow scientists to reconstruct parts of extinct food chains, Maisey added.
"Three tiers are exceptional—if [only] we could find a four-tier example."
The study appeared online last month in the journal Proceedings of the Royal Society B.
Unlike their ancient ancestors, no modern-day sharks are fully adapted to living in fresh water.
"Today we find some rays and skates—close relatives of sharks—living in fresh water, but sharks invade lakes and rivers only for a short time," said study co-author Kriwet.
"We don't understand why this is," he said.
Another odd difference is that none of the sharks that swim through the oceans today are known to eat amphibians.
"There are no reports of sharks eating amphibians, even in the tropics, where there are large amphibians living close to the lakes and rivers that sharks temporarily enter," Kriwet said.
These ancient amphibians—known as temnospondyls—were reminiscent of modern-day crocodiles but lived in a world that was still crocodile free. (Related news: "Ancient Amphibians Bit Instead of Sucking, Skull Study Says" [April 16, 2007].)
"The amphibians had a short legs, long snouts, big teeth, and a long tail that they used as a rudder, much like crocodiles today," Kriwet said.
"Before the Permian extinction event, amphibians and sharks were the main top predators," he said.
The Permian extinction, Earth's most extreme die-off, occurred 251 million years ago.
"But by the end of the Triassic [199.6 million years ago]," Kriwet said, "there was a shift to crocodiles and bony fish being the top predators."
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