Prehistoric dolphin-like reptile's stomach record hints it hunted large preys, shows evidence of megapredation

The 15-feet long dolphin-like marine reptile ichthyosaur preyed on a 12 feet long, lizard-like aquatic reptile around 240 million years ago


                            Prehistoric dolphin-like reptile's stomach record hints it hunted large preys, shows evidence of megapredation
Ichthyosaur fossil with its stomach contents (Ryosuke Motani)

A 15-feet long dolphin-like marine reptile that roamed the oceans around 240 million years ago may have had an appetite for large animals. A fossil record of the animal found in southwestern China revealed its last meal: 12 feet long, lizard-like aquatic reptile, a new study has found.

The dolphin-like reptile is ichthyosaur. It lived during the time of dinosaurs, with bodies similar to the modern-day tuna and mackerel. The species lacked gills and breathed air, like whales and dolphins. They also gave birth to live young ones. The last of the species died out in the Cretaceous period — several million years before the last dinosaurs died out. However, what caused their extinction is unknown.

The fossil in question was discovered in 2010. Its appearance puzzled scientists after they found an unusual bulge of bones within the stomach. Further investigation traced these bones to another marine reptile named Xinpusaurus xingyiensis, which belonged to a group called thalattosaurs. It had a more lizard-like appearance than its hunter, ichthyosaur. The study shows evidence of megapredation -- large animal eating a large prey -- in the prehistoric times and that ichthyosaurs could be apex predators.

The stomach of this 240-million-year-old fossil ichthyosaur contains the mid-section of another marine reptile that in life would have been only slightly smaller. It's the first direct evidence of ancient megapredation -- one large animal eating another (Da-Yong Jiang, et al, iScience)

"We now have a really solid articulated fossil in the stomach of a marine reptile for the first time," Dr Ryosuke Motani, a professor of paleobiology at the University of California, Davis, and the co-author of the study, says. "Before, we guessed that they must have eaten these big things, but now, we can say for sure that they did eat large animals," he explains, adding that megapredation was probably more common than thought.

The reptile may have met its end shortly after eating its prey. "At first, we just didn't believe it, but after spending several years visiting the dig site and looking at the same specimens, we finally were able to swallow what we were seeing," Dr Motani notes. To learn about the feeding habits of Ichthyosaur, Dr. Motani and his colleagues from China studied the tooth and jaw shape of the marine reptile. Typically, apex predators have sharp teeth. Ichthyosaur, on the other hand, had blunt teeth. Still, it did not seem to have hindered the animal's hunting abilities. "It's pretty clear that this animal could process this large food item using blunt teeth," Motani explains. The megapredator likely used its teeth to break the preys' spine, ripping the body apart. It ingested the middle portion, throwing the tail end away. 

This image shows the ichthyosaur's teeth, with the broken white line indicating the approximate gum line of the upper jaw. (Jiang et al./iScience)

"It's been suggested before that maybe a cutting edge was not crucial [for preying], and our discovery really supports that. It's pretty clear that this animal could process this large food item using blunt teeth," he explains. However, the study has one limitation: scientists are not entirely sure that Ichthyosaur preyed upon large animals. It could have been a scavenger, but decomposition studies show the scenario is unlikely. 

The team continues to excavate the site, which has now transformed into a museum. "We've been digging in that particular quarry for more than ten years now, and still, new things are coming out," Dr Motani says. "At this point, it's beyond our initial expectations, and we'll just have to see what we'll discover next."

The study is published in iScience.

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