Surprise Inside Fossil Ichthyosaur Stomach Reveals It Went Big For Dinner Before It Died

With a belly that size it's easy to picture these marine behemoths taking down some sizable prey. Shutterstock/Daniel Eskridge

Paleontologists digging in a quarry in southwestern China were surprised when they recovered the near-complete skeleton of a 5-meter (16-foot) ichthyosaur only to find it was a Russian doll of fossils. Inside the enormous dolphin-like marine animal was another almost equally large aquatic reptile. Investigations revealed the ichthyosaur had eaten a thalattosaur not long before dying, meaning both animals were committed to the fossil record as a set. The findings were published in the journal iScience.

Stomach contents are rarely recovered from marine fossils, meaning a lot of what we think we know about marine animals’ diets is based on their tooth and jaw morphology. Ichthyosaurs had blunt teeth, so it was assumed that they most likely fed on small prey such as cephalopods. However, this discovery paints a new picture of these marine giants as apex predators who, like crocodiles that share their blunt tooth morphology, were capable of taking down enormous animals in the pursuit of a tasty supper.

It was thought that apex predators had sharp teeth but its possible ichthyosaurs used their blunt teeth like crocodiles. Jiang et al./iScience

The discovery is one of the longest fossils ever found inside the stomach of a marine reptile, but researchers don’t yet know if the thalattosaur was scavenged from the sea bed or hunted while it was still alive. It could represent the first evidence that Triassic marine reptiles such as ichthyosaurs were actually apex megapredators rather than cephalopod snackers.

The ichthyosaur specimen on display with its stomach contents visible as a block that extrudes from its body. Ryosuke Motani

"If you look across all the similar marine reptiles that lived in the age of dinosaurs, we've actually never found something articulated like this in the stomach," said co-author Ryosuke Motani, a professor of paleobiology at the University of California, in a statement.

"Our ichthyosaur's stomach contents weren't etched by stomach acid, so it must have died quite soon after ingesting this food item. 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."

The 240-million-year-old ~5-meter ichthyosaur contained the mid-section of a 4-meter thalattosaur that in life would have been only slightly smaller. Da-Yong Jiang, et al, iScience

While not conclusive, the fact that the thalattosaur’s limbs were still attached to its body while its tail wasn’t indicates that it hadn’t started decomposing before it was swallowed. The tail was found yards away from the ichthyosaur specimen, which is consistent with the reptile having been attacked and the tail being ripped off in the fray.

"Now, we can seriously consider that they were eating big animals, even when they had grasping teeth," said Motani. "It's been suggested before that maybe a cutting edge was not crucial, and our discovery really supports that. It's pretty clear that this animal could process this large food item using blunt teeth."

 

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