Standing in a farmer's home in China's Henan Province in the summer of 2018, paleontologists Fenglu Han and Haishui Jiang peered down into a box of rounded lumps of rock. The farmer had collected the trove near his home in Neixiang County, which is renowned for its dinosaur eggs. One stony orb in particular caught the scientists' eyes. About the size and shape of a billiard ball, the fossil was unlike any dinosaur egg they'd seen before.
Han and Jiang, who are based at the China University of Geosciences in Wuhan, initially thought the egg might have come from a new dinosaur species. But careful analysis revealed something even rarer. Entombed in the egg's rocky confines lay the remains of a giant extinct turtle.
The newfound fossil belongs to an extinct group of land-dwelling turtles known as the nanhsiungchelyids, according to a new study published in Proceedings of the Royal Society B. This group grew to momentous sizes and walked the Earth alongside the dinosaurs during the Cretaceous, a period that spanned from 145 to 66 million years ago. The turtle that laid the fossil egg—which is among the largest known from this time—was exceptionally big and likely sported a shell about as long as an average person is tall, the team estimates.
"These were not small turtles by any stretch," says Darla Zelenitsky, an author of the new study and a paleontologist at the University of Calgary in Alberta, Canada.
Discovering fossil embryos from any creature is not common. The delicate tissues and bones of developing animals readily break down over time. Turtle embryos are even less common than those of dinosaurs, perhaps partially because most turtle eggs are tiny and have thin shells, Zelenitsky says. Only a few fossil turtle embryos have ever been discovered, none of which are preserved well enough for scientists to place them in the turtle family tree.
This latest fossil embryo helped the team identify other turtle eggs that belong to the same group, providing a window to their ancient nesting behaviors and evolutionary adaptations.
While only so many conclusions can be drawn from a single fossil, the discovery of this one ancient turtle embryo is a promising hint that there are more waiting to be found, says Tyler Lyson, associate curator of vertebrate paleontology at the Denver Museum of Nature and Science, who was not part of the study team. "It's only a matter of time."
Reconstructing the tiny turtle
When Han and Jiang first saw the fossil egg, a pair of spindly bones that poked out from a crack on one side was the only hint of the treasure within. The farmer agreed to let the scientists take the egg for study, and he led them to the place where he found the strange egg. They spotted several others, but those fossils hadn't held up well through the millennia, Han says via email.
Back in their lab, the researchers scanned the farmer’s egg with micro-computed tomography (CT), which uses X-rays to peer beneath the fossil's smooth, rocky surface. The CT images revealed a tangle of disjointed bones within the egg. To make sense of the jumble, the team reconstructed each bone in three dimensions and then virtually assembled the tiny skeleton.
Overall, the embryo is strikingly similar to modern turtles, says Raul Diaz, a reptile evolutionary biologist specializing in embryos at California State University, Los Angeles. He points to the embryo's flat ribs, which would have hardened and spread as the turtle grew to form the underlying structure of its protective shell. "It's almost—in my head—indistinguishable from what I would see in the lab," says Diaz, who was not part of the new study.
However, there were a few key features that helped identify the ancient turtle's specific group. The upper jawbone, for example, bears a strong resemblance to nanhsiungchelyids, Zelenitsky says, due to its slightly square shape and serrated back edge.
Perhaps the most striking feature of the egg was its sturdy shell, which at two millimeters thick differs from the paper-thin shells common among turtles. Modern turtles have a variety of eggshell thicknesses, from the leathery orbs of sea turtles to the tough eggs of the Galápagos giant tortoises. But the newfound egg's shell measures about four times thicker than those of Geochelone elephantopus, one of the Galápagos giants, according to the study team.
The exact purpose of the ancient turtle’s tough eggshells is uncertain. The thickness may be an adaptation to the arid climate that is believed to have existed at the time, inferred from plant life found in the same rock formation as the egg. A thick shell would have limited the amount of water that escaped from the egg. Alternatively, the shell could have prevented the eggs from breaking if the turtles dug deep nests underground.
Regardless of the thick shell’s purpose, Zelenitsky says, "I don't know how they got out." The newborn turtles must have had to rigorously flex and extend their limbs in their attempts to hatch.
Wiped out with the dinosaurs
The fact that nanhsiungchelyid turtles lived and nested on land may have contributed to their demise. The group died out alongside all non-avian dinosaurs some 66 million years ago, when a colossal asteroid hurtled into Earth. The impact sent out a blast of energy that flung sizzling hot rock into the skies and ignited vast tracts of land. "Anything that was on the surface got boiled," Lyson says.
But "most turtles sail right through" the extinction, he says. This includes aquatic river turtles that were relatives of the nanhsiungchelyids, whose underwater lifestyle may have buffered them from the asteroid's blast. Diet may have also played a role in the turtles’ undoing, as nanhsiungchelyids were strictly plant-eaters, and such a limited diet would have made it tough for the turtles to find food in the post-impact world.
Turtle eggshells like the nanhsiungchelyids’ were not seen again after the impact, and the researchers suggest that perhaps the thick shells were unsuited to the dramatic shift in the environment. But more information is necessary to figure out exactly why the thick shells disappeared.
The new analysis is an important reminder of how far paleontology has come, says Emma Schachner, an evolutionary biologist at Louisiana State University, New Orleans, who was not part of the study team. Without destroying the fossil, scientists in the past could only study its exterior, but now, there's a whole world of digital reconstruction available. "The model is definitely what makes it special, in my opinion," she says of the new study.
Yet the work also shows how much there is still to learn about ancient turtles. Far fewer researchers devote their time to studying ancient turtles than charismatic dinosaurs, Lyson says. But turtles offer plenty of intrigue. "They just have this completely different body plan than any other animal," he says.
He hopes that finds like this fossilized turtle embryo will help inspire a new generation to work on untangling how these curious creatures came to be. What we need, he says, is "more good fossil turtle workers."