An ancient four-metre-long reptile has been found well-preserved in the stomach of a slightly larger marine carnivore that lived around 240 million years ago.

Scientists say the fossil, which was discovered in southwestern China, could be the first evidence of megapredation – a large animal preying on another large animal.

In a case of biting off more than it could chew, the five-metre-long ichthyosaur is thought to have died almost immediately after consuming its prey, possibly fatally injuring itself while trying to eat an enormous meal.

The findings have been described in the journal iScience.

Ichthyosaurs, which are also reptiles, are dolphin-like creatures which dominated the seas around 250 to 90 million years ago.

While the shape of their blunt teeth suggests these animals may have been at the top of their food chain, there is little direct evidence of the kind of prey they consumed.

The specimen was first discovered in the Guizhou province in 2010 but it took scientists a decade to excavate the ichthyosaur fossil and notice the “large bulge” in its abdomen.

Dr Ryosuke Motani, professor of Earth and planetary sciences at the University of California, Davis, in the US, and one of the authors on the paper, said: “We always guessed from tooth shape and jaw design that these predators must have fed on large prey but now we have direct evidence that they did.”

The prey in question, a thalattosaur – a lizard-shaped reptile almost as long as the ichthyosaur but much skinnier.

The thalattosaur remains show little evidence of being broken down by digestive juices, which according to the scientists, suggests the ichthyosaur died shortly after consuming its final meal.

The thalattosaur’s limbs were found to be partially attached to its body inside the ichthyosaur’s stomach, while the creature’s tail was found nearby, leading the researchers to believe the appendage was ripped off and left behind.

The team also found the ichthyosaur’s body and head were detached from one another, indicating it may have died of a broken neck.

Dr Motani said: “We now have a really solid articulated fossil in the stomach of a marine reptile for the first time.

“Before, we guessed that they must have eaten these big things, but now, we can say for sure that they did eat large animals.

“This also suggests that megapredation was probably more common than we previously thought.”