How Did an Ancient Sea Turtle End Up Under a Dinosaur’s Foot?

Since the invention of the internal combustion engine, it has not been uncommon for broken turtle shells to turn up by the sides of highways, bashed by passing cars and trucks whose operators may not have even noticed. What is more striking is to find a version of this familiar story from 150 million years ago.

This much we know: One day toward the end of the Jurassic Period, a long-necked, long-tailed sauropod dinosaur about the size of an elephant lumbered over a tidal flat in what is now Switzerland, leaving footprints wider than beach balls.

Maybe it heard a crunch. Because much later, near these footprints, a team of paleontologists found a damaged carapace from an extinct sea turtle species called Plesiochelys bigleri halfway pressed into the sediment. They argue, in a paper soon to be published in the Swiss Journal of Geosciences, that animal was stepped on.

“The evidence is pretty clear,” said Daniel Marty, a paleontologist at the Natural History Museum in Basel, Switzerland, who participated in the study. “It’s kind of a funny thing, and it also shows that these two animals were in the same paleoenvironment.”

The research was led by Christian Püntener, a paleontologist with the Swiss canton, or state, of Jura, where the specimen was found. If the scientists’ interpretation is right, the smashed turtle experienced a rare indignity in the fossil record.

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Crushed seashells are sometimes found in dinosaur tracks, and scientists also find dinosaur bones scoured with marks where other dinosaurs walked over them. But cases where footprints and damaged bones appear together are rare, the team said. One of the few other known examples involves a camel that is believed to have stepped on a flamingo two million or so years ago. The evidence was preserved in a fossil lake bed near Puebla, Mexico.

The turtle shell was found during an extensive dig in the Jura Mountains, which gave the Jurassic Period its name. In the 2000s, before a new highway, the A16, sliced through the range, paleontologists excavated rock that would otherwise have been discarded.

In one spot in the path of the new highway, archaeologists found a Roman road. Below that, paleontologists uncovered dinosaur tracks. “It’s a real time-crossroad,” Dr. Marty said. In total, the team found more than 15,000 dinosaur footprints and about 100 sea turtle fossils, including several new species, as well as marine crocodiles that would have snacked on the turtles from offshore.

But one turtle, dug up in 2007 near a small village called Courtedoux, looked odd. The paleontologists found it farther from the water than most of the other turtles, and it looked as if had been pressed down through layers of sediment. Once they took it out of plaster back at the lab, they noticed that many of its shell bones were crushed and twisted.

It could have died at sea and washed ashore. But the more likely explanation, the team argues, is that it crawled across a tidal flat, became stranded as it searched for a way back out to sea or a place to lay eggs, and died.

The massive sauropod dinosaurs crossing the same flat might have been looking for a less cluttered way to a new area than passing through a thick inland forest. And then came the putative stomping.

“It’s a wonderfully unique situation,” said Tristan Stayton, an evolutionary biologist at Bucknell University who studies the structural properties of turtle shells, among other things, but was not involved in the discovery of this particular shell. “There was definitely some large force that caused this turtle shell to fail.”

More recently there have been recorded cases of elephants crunching turtles underfoot, offering a modern analogy. But large predators like jaguars can also break apart a sea turtle shell in a way that can make it look as if it has been crushed, Dr. Stayton said.

Another explanation, said Jordan Mallon, a paleontologist at the Canadian Museum of Nature who also was not involved in the study, is that the fossil was simply crushed under the weight of the rocks above it over the millions of years it was buried. “I think they tell a plausible story, but it’s not a dead ringer,” he said.

Still, “it’s important to document fossils like this where you can try and show two species lived together,” Dr. Mallon said. “It’s only by doing that that we’re able to reconstruct ancient ecosystems.”

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