Scientists Uncover Massive Prey in the Belly of an Ancient Sea Monster

Finding suggests "megapredators" may have been more common in Mesozoic Era than previously thought.
Image
ichtyosaur fossil stomach

The stomach area of an icthyosaur fossil, where researchers found a bundle of bones belonging to a massive thalattosaur. 

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Charles Q. Choi, Contributor

(Inside Science) -- Inside the giant dolphin-shaped body of an extinct marine reptile, scientists unexpectedly discovered the nearly equally long corpse of its prey, suggesting this carnivore was indeed a "megapredator," a new study finds.

When dinosaurs ruled the land, air-breathing marine reptiles such as dolphinlike ichthyosaurs dominated the seas. These sea monsters, which evolved from land reptiles just as modern whales did from land mammals, roamed the oceans for 160 million years, apparently going extinct about 90 million years ago, roughly 25 million years before the end of the Mesozoic Era, or Age of Reptiles.

In 2010, study lead author Da-yong Jiang, a paleobiologist at Peking University in Beijing, and his colleagues excavated a nearly complete 4.8-meter-long specimen of the ichthyosaur Guizhouichthyosaurus from a site in the southwestern Chinese city Xingyi. The 240-million-year-old carnivore would have lived in warm, shallow seas populated with other marine reptiles, fish, sea lilies and nautiluslike cousins of squid known as ammonites.

Closer analysis of a bulge of bones in the predator's belly surprisingly revealed they did not come from the ichthyosaur, but from the remains of a victim. Although the prey's fossil lacked its skull and tail, the researchers successfully identified the bones as belonging to Xinpusaurus xingyiensis, a kind of lizardlike marine reptile with four paddling limbs known as a thalattosaur. (A fossil of what may have been the prey's tail was found nearby.)

The scientists estimated the thalattosaur was roughly 4 meters in length, almost as long as the ichthyosaur, although the predator's wide body likely made it roughly seven times more massive than its prey.

Scientists often assume apex predators possess large teeth with sharp edges. Guizhouichthyosaurus had relatively small, peglike teeth likely adapted for grasping small, soft targets such as squid, which might suggest this ichthyosaur was not a top predator, "so it was a surprise to find such large prey," Jiang said. The researchers suggest Guizhouichthyosaurus used its teeth to grip its victims, perhaps breaking the spine with the force of its bite, and then ripped or tore the prey apart, a tactic used by modern predators with blunt teeth, such as killer whales and crocodiles.

It remains uncertain whether the ichthyosaur scavenged or preyed upon the thalattosaur. Still, previous research suggested this was not a case of scavenging -- if left to decay, the thalattosaur's limbs would likely disintegrate and detach before the tail, whereas the opposite was true with these fossils.

The fossils are the oldest direct evidence that marine reptiles were megapredators -- that is, that they consumed animals larger than humans. These bones also set the record for the largest prey captured by marine reptiles during the Mesozoic, with the previous record only 2.5 meters long.

Many Mesozoic marine reptiles possessed grasping teeth similar to those of Guizhouichthyosaurus. These findings suggest "we are probably underestimating the prevalence of megapredation back in time," said study co-author Ryosuke Motani, a paleobiologist at the University of California, Davis. "Other predators with similar dentition could also have been megapredators,"

The scientists detailed their findings online Aug. 20 in the journal iScience.

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Charles Q. Choi is a science reporter who has written for Scientific American, The New York Times, Wired, Science, Nature, and National Geographic News, among others.