For its last meal, an ancient marine reptile called an ichthyosaur may have bitten off more than it could chew.

The dolphinlike creature was nearly 5 meters long, about the length of a canoe. And its belly contained the remains of a lizardlike reptile called a thalattosaur that was almost as long: 4 meters. This is the longest known prey of a marine reptile from the dinosaur age, and may be the oldest direct evidence of a marine reptile eating an animal larger than a human, researchers report August 20 in iScience. In fact, this particular thalattosaur may have been such a big meal that the ichthyosaur died after stomaching it.

The ichthyosaur’s blunt teeth suggest it should have favored small, soft prey like cephalopods (SN: 10/3/17). “Now we have really solid evidence saying these [blunt] teeth can be used to eat something big,” says Ryosuke Motani, a paleobiologist at the University of California, Davis. “That means the other species with similar teeth we discounted before … may be megapredators too.”

Motani and colleagues examined the nearly complete skeleton of an adult ichthyosaur that was unearthed in southwestern China in 2010. The reptile, from the genus Guizhouichthyosaurus, lived during the Triassic Period about 240 million years ago. Upon closer inspection of a big lump of bones in the creature’s belly, Motani’s team discovered that the last thing the ichthyosaur ate was the body of a thalattosaur, sans head and tail. The thalattosaur remains show little evidence of being degraded by stomach acid, suggesting the ichthyosaur died shortly after its enormous meal.

These fossils provide “pretty good evidence that the bigger animal ate the smaller one,” says vertebrate paleontologist Steve Brusatte of the University of Edinburgh, who was not involved in the study. “If this really is the case, it’s quite stunning,” because the predator was not much larger than its prey — at least in terms of length. The ichthyosaur is thought to have been roughly seven times more massive than the whip-thin thalattosaur.

ichthyosaur illustration
A 5-meter-long ichthyosaur chowed down on the body of a 4-meter-long thalattosaur, minus the prey’s head and tail, as seen in this illustration. The previous record-holder for longest prey of a dinosaur-age marine reptile was a mereD.-Y. Jiang et al/iScience 2020

The researchers believe the ichthyosaur most likely hunted, rather than scavenged, its meal. For one thing, it would have been unusual to come across a whole dead animal that no other predator had gobbled up, and the ichthyosaur would have had to shovel down the huge meal on the seafloor — tough for an air-breathing creature.

Plus, the thalattosaur’s limbs were still at least partially attached to its body, while its tail was uncovered about 20 meters away. Studies of how bodies decompose underwater suggest that if the thalattosaur was a carcass when the ichthyosaur found it, the prey’s limbs would have rotted off before its tail, the authors argue. 

Motani suspects that killing and eating the thalattosaur may have spelled the ichthyosaur’s demise. The ichthyosaur’s fossilized body and head, while well preserved, are detached from one another, hinting that the animal may have died of a broken neck. The ichthyosaur could have injured its neck while holding the thalattosaur in its jaws and thrashing its head, which is how crocodiles and killer whales rip up their food without particularly sharp teeth.

The ichthyosaur also could have hurt itself while swallowing such large prey. “This is not a snake that’s adapted to swallow something really big, so it has to swallow just like dolphins and crocodiles do,” Motani says. That means swimming against its prey to shove the food down its throat, or sticking its head above water and using gravity to gulp the meal down. “It could easily damage its neck doing this.”

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