In a world first, paleontologists working in northwestern China have discovered a cache of hundreds of ancient eggs laid by pterosaurs, the flying reptiles that lived alongside the dinosaurs. Some of the eggs contain the most detailed pterosaur embryos ever found.

Although scientists have studied pterosaurs for more than two centuries, no eggs were discovered until the early 2000s, and fewer than a dozen turned up in the intervening years. The new haul, discovered by Chinese Academy of Sciences paleontologist Xiaolin Wang, includes at least 215—and perhaps as many as 300— stunningly preserved pterosaur eggs.

His team also found 16 embryos within the eggs, and they suspect that more remain locked away in the stone. Wang and his colleagues announced the finds today in Science.

“We get a lot of hyperbole in paleontology, but it’s pretty phenomenal,” says David Hone, a researcher at Queen Mary, University of London who wasn’t involved with the study. “The science is at the absolute start, but the mere raw material is game-changing, potentially.”

View Images Two of the newfound pterosaur eggs. Paleontologists say that they have found hundreds of eggs so far, including at least 215 within a single sandstone block. More are probably hidden within the block's interior. Photograph by Wang et al., Science (2017)

THE PERFECT STORMS

The newfound eggs belong to Hamipterus tianshanensis, a previously known species of pterosaur that lived in northwestern China more than a hundred million years ago. With a maximum wingspan of 10 feet and a probable taste for fish, these animals may have resembled today’s herons, living near waters that crisscrossed inland terrain.

“The site is in the Gobi desert, and there are strong winds, a lot of sand, with few plants and animals,” says study coauthor Shunxing Jiang of the Chinese Academy of Sciences. “However, when Hamipterus lived, the environment [was] much better—we call it Pterosaur Eden.” (Find out how scientists are fighting fossil poachers in the Gobi.)

Hamipterus not only fed in this long-lost paradise, it also bred there, likely burying clutches of eggs in vegetation or on shorelines. The eggs fossilized in lake sediments disturbed by fast-moving water, a sign that storms may have flooded a nesting site and sent the eggs bobbing into a large lake, where soupy mud entombed them. The eggs didn’t wash in all at once: They’re spread out among four distinct sediment layers, suggesting that multiple floods deposited them over time.

Wang’s team suggests that an ancient nesting site may have flooded repeatedly. This would imply that, like modern birds and turtles, Hamipterus used the same nesting sites over and over. What’s more, the sheer number of eggs suggests that Hamipterus bred in large groups like some living birds.

LEAVING THE NEST

As the waters raged on that ancient Chinese lake, many of the pterosaur eggs split open, letting in sediments that ultimately preserved their oblong shapes. And in at least 16 of these eggs, the sediments also cradled the delicate skeletons of developing pterosaur embryos, including one bone that the team thinks belonged to a hatchling.

View Images The Hamipterus fossil site, located in the Xinjiang region in northwestern China, has yielded hundreds of pterosaur bones. Photograph by Alexander Kellner, Museu Nacional, UFRJ

“We could look at the bones and see what features characterize an embryo, a hatchling, and a young individual when he’s matured,” says coauthor Juliana Sayão, a bone-structure expert at the Federal University of Rio de Janeiro. “This is a one-of-a-kind record for pterosaurs—for the first time, we have the whole spectrum.”

By comparing isolated bones from pterosaurs of different ages, researchers can roughly piece together how Hamipterus developed. They found that the late-stage embryos didn’t yet have teeth, and their forelimbs were less developed than their hind limbs. The seemingly weak arms came as a surprise, because many paleontologists thought that pterosaurs were fliers nearly straight out of the egg. The Hampiterus fossils, however, imply a slower-growing pterosaur that scampered around on all fours as an infant.

“I think they have a good argument, [and] it’s an interesting result,” says University of Southern California paleontologist Mike Habib, an expert on reconstructing how pterosaurs moved. He hopes that future work on these fossils will use mechanical analysis to test how well the small reptiles might have actually flown. (See a pterosaur that may be one of the largest animals ever to fly.)

View Images The site's rocks contain a jumble of preserved Hamipterus eggs, as well as disarticulated bones from adults of the same species. Paleontologists aren't sure yet why they have found only remains of Hamipterus in these deposits. Photograph by Alexander Kellner, Museu Nacional, UFRJ

Further studies should help flesh out even more of the details about how these winged beasts reproduced. The shells resemble living turtles’ leathery eggs, which means that Hamipterus probably buried its eggs to protect them—but where or how is unknown. What’s more, we don’t yet know how many eggs a single female Hamipterus laid or the size of its breeding groups.

Given the incomplete fossil record, it’s also possible the proposed Hamipterus growth sequence will need to be adjusted. Perhaps the largest embryos the team found weren’t quite ready to hatch, which would throw off the developmental time line. More fossils would help, and Wang’s team remains on the hunt in northwestern China.