Relative to a body as long as some whales, the ichthyosaur — a long-snouted marine reptile that lived millions of years ago — had tiny teeth. But this apparent anatomical limitation might not have stopped it from locking prey between its jaws, jerking and flailing until the creature’s brain turned to mush, and its head and tail were ripped from the trunk.

Because of their relatively small, blunt teeth, ichthyosaurs — or “fish lizards,” etymologically — were thought to prey only on small cephalopods such as ancient squids. But according to a study published Thursday in the journal iScience, fossils discovered in southwestern China’s Guizhou province suggest that these extinct reptiles might once have sat comfortably atop the food chain.

During the Mesozoic era some 240 million years ago, Guizhou was a vast ocean. In 2010, paleontologists discovered a nearly complete 5-meter-long skeleton of an ichthyosaur in the now-mountainous province. For years, a bulging bunch of bones in the skeleton’s abdominal region confused scientists: Was it a fetus, undigested food, or another animal that had died in the immediate vicinity?

After nearly a decade of cleaning and preparation, a team of scientists led by Jiang Dayong of Peking University determined the bones were from a thalattosaur — another marine reptile — that had been inside the ichthyosaur’s stomach.

“We were so thrilled because you really don’t come across fossilized stomach contents from millions of years ago that are this well-preserved,” Jiang told Sixth Tone.

The scientist added that the ichthyosaur may have died shortly after devouring the thalattosaur, which would explain why the larger animal’s stomach acid did not break down its meal. “Our findings may revolutionize what we know about this ancient reptile,” he said.

The researchers estimated that the thalattosaur was around 4 meters long — large enough to suggest that, far from feasting only on small creatures, ichthyosaurs could have been apex predators that hunted and killed larger food.

But precisely how the thalattosaur ended up in the ichthyosaur’s stomach will remain a mystery for now. It’s possible that the larger reptile wasn’t the killer, but rather a savvy scavenger that chanced upon and ate an already-dead thalattosaur.

Knowing the ichthyosaur’s position in the food chain — whether it was a king or a street sweeper — would be an important step toward understanding the Mesozoic marine ecosystem and reconstructing the era’s history.

“Either scenario is possible, but based on our understanding of what would happen to a corpse in the ocean, we lean toward the predator hypothesis,” Jiang said.

Compared with the head, which is attached to the body by a vertebral column, muscles, and other durable connective tissue, an animal’s hands and feet are typically the first parts that rot and slough off after it dies in the ocean, the scientist said. The thalattosaur fossil had a full set of limbs but no head or tail — a phenomenon related to the hunting habits of modern crocodiles, according to Jiang. Despite not having particularly sharp teeth, crocodiles latch onto their prey and shake violently until the extremities are ripped clean off.

“But we could be wrong. We have limited evidence on hand, so it’s very difficult to reconstruct what happened 240 million years ago,” he said. “That’s the beauty of paleontology: You keep looking for the truth, hoping to find it. Maybe you will one day, or maybe you won’t.”

Editor: David Paulk.

(Header image: A general view of the ichthyosaur fossil in Guizhou, its stomach contents visible as a bolus protruding from the rock face. Courtesy of Ryosuke Motani)