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When paleontologist Alexander Kellner first saw the 35-square-foot block of sandstone filled with pterosaur fossils, he couldn’t believe his eyes. “I was just speechless,” he says. That was in 2015, at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. Now, two years later, that 120-million-year-old block of fossilized sand has yielded 215 pterosaur eggs, 16 of which still have embryos inside. The unprecedented discovery can help us understand how pterosaurs laid their eggs, how those eggs developed, and even how pterosaur parents cared for their young.
The batch of eggs, described in a study published today in Science, confirms what scientists had suspected: pterosaurs laid soft-shell eggs like today’s lizards, rather than hard-shell eggs like birds. It also shows that flocks of pterosaurs may have nested together, laying their eggs relatively close to each other and returning to nest in the same spots over time. Based on the development of certain embryos, the researchers also think that newborn pterosaurs couldn’t fly, which suggests that parents had to take care of them. But several experts disagree about the stage the embryos were in, which has implications about how pterosaurs behaved.
“There’s a real danger that you can over-interpret the evidence,” says Charles Deeming, principal lecturer at the University of Lincoln, who’s researched pterosaur eggs and was not involved in this study.
But Deeming and others agree that the block of fossilized eggs is an amazing find that will deepen our understanding of some of the most peculiar animals to ever inhabit our planet. “I’ve been 35 years in paleontology and seen a lot of great fossils. But this is certainly one of the three or five most interesting and probably most important specimens I’ve ever seen,” says pterosaur expert David Unwin, who was not involved in the research. “It’s absolutely astounding.”
Pterosaurs were flying reptiles that lived — and went extinct — alongside dinosaurs. There were several pterosaur species of various sizes: some were as big as crop dusters, while others were as small as sparrows. The pterosaur fossils in question, found in northwestern China, belong to a species called Hamipterus tianshanensis: they had a wingspan of up to 11 feet, head crests, and long teeth to catch fish. Because pterosaurs had thin bones, optimized for flight, not many fossils remain. Findings eggs is even harder: only a handful of pterosaur eggs have been discovered — and only three of these eggs contain fossilized embryos, Unwin says.
That’s why that 35-square-foot block of sandstone filled with relatively well-preserved eggs and embryos, as well as pterosaur bones, is so extraordinary. “This was really an amazing find,” Kellner tells The Verge. “I never thought of finding something like that.” The cluster of eggs and bones formed when powerful storms swept through the area, flooding rivers. The flood snatched the eggs from their nests, and carried them to a spot where they piled up, eventually fossilizing, Kellner says. The sandstone block has eight layers, each corresponding to a different storm event that hit the area through time. Each layer has pterosaur bones, and four also have eggs.
Kellner and his colleagues scanned the eggs and found that the shells were soft and parchment-like — similar to the eggs of today’s lizards and some turtles. These animals bury their eggs instead of incubating them, like birds do, which suggests that pterosaurs likely did the same. Scientists think that each female pterosaur probably laid only two eggs at a time, so Kellner says that all the eggs in the sandstone block were probably laid by tens of pterosaurs at a spot nearby. If that’s true, it means pterosaurs laid their eggs together, similar to penguins. Because the sandstone block also has several layers containing eggs, that suggests that the flying reptiles returned to the same spot to lay their eggs over time, Kellner says.
Even more surprising clues about pterosaurs come from the embryos found inside the eggs: some of these embryos were in the late stage of development, the researchers say. In these embryos, one wing bone called the humerus wasn’t completely formed, while a leg bone called the femur was. That suggests that newborn pterosaurs could walk around but couldn’t fly, Kellner says. And if that’s case, their parents probably had to look after them.
Some researchers disagree with this interpretation, saying that the embryos were likely weeks or even months from hatching. Unwin says that three other late-stage embryos found elsewhere in China and Argentina had well-developed skeletons, including well-formed wing bones. “They look like hatchlings except they’re inside an egg,” Unwin says. “In terms of proportions, they look very much like they’re ready to go.” Instead, the embryos described in today’s study were likely halfway through development, Unwin says, meaning that they still had time to form their wings. If that’s the case, then the interpretation that young pterosaurs couldn’t fly and parents had to take care of them doesn’t stand. But even if hatchlings couldn’t fly, that doesn’t necessarily mean that the parents had to look after them, Unwin says. Modern reptiles that bury their eggs, like turtles, generally don’t come back to their eggs when they hatch. The hatchlings just take care of themselves.
“It is important to be cautious and not to infer too many aspects of the life history of Hamipterus from what remains a limited data set,” Deeming writes in an article published alongside the Science paper.
The only way to answer some of these questions is to find even more eggs and embryos — and that may soon happen. Kellner says he and his colleagues in China intend to go back to the site where the eggs were excavated, and look for more fossils. “The exciting thing is that there are more discoveries to come,” Unwin says. “We have a fantastic find, but we have the opportunity to build on that and build a more complete and a more complex picture.”
Ultimately, revealing how pterosaurs lived, behaved, and reproduced adds to our understanding of what life is capable of on our planet — at least, in the past. “Pterosaurs are completely unlike any living animal, so they show us a form of life or a way of life that doesn’t exist anymore,” Unwin says. “We’re fascinated by the idea of aliens from another world but in a sense, pterosaurs and dinosaurs are aliens of the world.”
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