About 240 million years ago, a massive marine reptile swallowed another, slightly less massive reptile and died shortly thereafter. The larger creature—a dolphin-like reptile known as an ichthyosaur—then fossilized with the smaller animal in its belly.
The two reptiles remained locked in stone until 2010, when scientists in southwestern China began excavating the fossil. Now, the scientists say that much of what we thought we knew about life and death in the prehistoric ocean could be upended by this sea monster turducken. (For the uninitiated: A turducken is a culinary oddity made by cooking a deboned chicken that’s inside a deboned duck that’s inside a deboned turkey.)
In the unique fossil, the smaller creature in the ichthyosaur’s belly was a thalattosaur, an ancient marine reptile with a long, skinny body that looked more like a lizard than a fish. When Ryosuke Motani, a paleontologist at the University of California, Davis, realized there was a nearly complete torso from a 13-foot-long thalattosaur bulging from inside the 16-foot-long ichthyosaur’s stomach, he knew his team was onto something groundbreaking. A study describing the fossil was published today in the journal iScience.
Ichthyosaurs breathed air and gave birth to live young. While some species grew to lengths that approached the vastness of a blue whale, early ichthyosaurs like the Guizhouichthyosaurus that Motani examined were smaller, perhaps 13 to 19 feet in length. These ancient swimmers are thought to have preyed on slippery, squid-like cephalopods, using mouths full of dull, grabby teeth to snatch their meals out of the water. In fact, none of the aquatic animals alive during this time were thought to have tackled large prey; that sort of apex sea monster was not believed to have evolved until later.
But according to Motani, the newly described fossil suggests that early ichthyosaurs were among the Mesozoic era’s first “megapredators,” or large animals that prey on other large animals. “They were feeding on animals bigger than humans,” he says.
A prehistoric cold case
Piecing together an event that took place hundreds of millions of years ago comes with several challenges. For starters, Motani and his team needed to prove that the ichthyosaur actually ate the thalattosaur, rather than the smaller marine reptile simply fossilizing on top of the ichthyosaur by sheer happenstance.
“Thankfully, in this case, there’s a way to tell,” he says. The ichthyosaur’s rib cage wraps on top of the prey animal, indicating the thalattosaur was in fact a meal. But what kind of meal is another important question. The ichthyosaur might have scavenged the carcass of a thalattosaur that died by other means.
Inside the ichthyosaur, however, Motani and his colleagues found what they believe are two long, intact sections of thalattosaur vertebrae. These fossilized bones suggest the spine was still held together by connective tissues and was not ingested piecemeal as a mushy, rotten mess.
The thalattosaur’s skull and tail are missing from the stomach contents. The team did discover a piece of thalattosaur tail about 65 feet away from the ichthyosaur—and though there’s no way to prove that one belongs to the other, “the thing is, the size of this tail is just right,” Motani says.
The team’s best estimation is that the ichthyosaur attacked and killed the thalattosaur, likely at the water’s surface. Then the predator would have gone to work on the carcass, trying to swallow it whole or in very large chunks, like an alligator ingesting its prey. Through some combination of chomping and thrashing, the thalattosaur’s skinny neck and tail could have detached and drifted away while the ichthyosaur focused on the largest, meatiest chunks of the kill.
Since we can’t go back and watch prehistoric mealtimes, scientists have often looked at fossil teeth to determine what an ancient animal might have been munching. In the case of the early ichthyosaurs, their blunt, cone-shaped teeth hinted at a preference for more delicate meals, as opposed to the sharp, serrated teeth traditionally associated with apex predators.
But the newfound fossil suggests scientists can’t always rely on tooth shape alone to predict what a given species might have been eating, says Stephen Brusatte, a paleontologist at the University of Edinburgh in the United Kingdom who was not involved in the study. Rather than only picking off squishy cephalopods, some early ichthyosaurs may have been bold enough to go for more substantial meals.
“Sometimes the details of a prehistoric crime scene tell us a weapon was a lot more capable of damage than we thought,” Brusatte says.
An ichthyosaur’s last supper
Fossilized stomach contents are extremely rare, says Jessica Lawrence Wujek, a geologist and paleontologist at Howard Community College in Maryland who was not involved in the study. Lawrence Wujek has looked at hundreds of ichthyosaur specimens, and she says perhaps one or two had fossilized stomach contents, which are called bromalites.
“It’s just not very often you get stomach contents preserved, especially big things in the stomach like this,” Lawrence Wujek says. “It’s an awesome fossil.”
Because the thalattosaur bones show no visible signs of being digested, it seems likely the ichthyosaur met its own demise almost immediately after finishing its meal. The tail fragment that settled and fossilized nearby is about the same age as the ichthyosaur, providing another clue that the animal died shortly after its large meal.
While the thalattosaur was nearly as long as the ichthyosaur, Motani estimates that it was just one-eighth as heavy. Still, it was an animal that could fight back.
“It’s total speculation, but maybe during that fight a part of [the ichthyosaur’s] neck was damaged to some extent,” he says. Though we may never know for sure, that injury might have gotten even worse as the predator jerked and twisted in an attempt to swallow its hard-won meal.
Life finds a way
On top of the allure of ancient sea beasts locked in mortal battle, the fossil also paints a picture of how quickly ecosystems can recover, says Aubrey Jane Roberts, a paleontologist studying ancient marine reptiles at the University of Oslo in Norway who was not involved in the new study.
“There was a huge mass extinction, particularly in the marine realm, about 252 million years ago,” says Roberts, who is also a National Geographic Explorer. “Ninety percent of all marine species died out.” Considering the magnitude of that loss, it’s amazing that life was able to rebound and diversify as much as it did within just a few million years, Roberts says. But it’s particularly impressive that megapredator behavior like that displayed by the ichthyosaur would have shown up so soon after the extinction, as scientists think apex predators are among the last animals to develop when a food chain rebuilds.
“That’s why this paper is so important,” Roberts says. “It tells a story about how marine seas recovered from basically complete destruction to having a fully formed ecosystem.”
Editor's note: This story has been updated to include Aubrey Jane Roberts' affiliation as a National Geographic Explorer.