Artist's concept of the Hampiterus pterosaur with eggs and embryos. Credit: Zhao Chuang

A remarkable fossil slab containing hundreds of pterosaur eggs and some embryos has been discovered in China1. The find looks set to transform palaeontologists’ understanding of these enigmatic creatures.

The early life of pterosaurs — the first vertebrates to evolve powered flight — has been a mystery. It was only in 2004 that scientists even confirmed that they laid eggs, and until now, only a handful of eggs had been found. The newly discovered trove, belonging to a species called Hamipterus tianshanensis that lived around 120 million years ago, offers clues into the development and anatomy of freshly hatched pterosaurs. It also provides the first solid evidence that, like many dinosaur species, these animals nested in groups2,3.

The fossils, reported in the 1 December issue of Science1, were discovered in the Turpan-Hami Basin in Xinjiang, northwestern China. From 2006 to 2016, Wang Xiaolin of the Chinese Academy of Sciences' Institute of Vertebrate Paleontology and Paleoanthropology in Beijing and his colleagues excavated a 3-square-metre sandstone block to reveal at least 215 squashed and cracked eggs among jumbled pterosaur bones. They think that up to 300 eggs could be present, some buried below the upper layers of fossils. The team used computed tomography scanning to peer inside 42 eggs, and found 16 that contained the remains of embryos at various stages of development, with partial skulls and limb bones.

Other experts say that the find will be significant for understanding pterosaur reproduction. It’s “a crucial advance”, writes Charles Deeming, who studies bird and reptile reproduction at the University of Lincoln, UK, in a commentary also published in Science4. Mark Witton, who researches the reptiles at the University of Portsmouth, UK, says that Hamipterus now “has the potential to be one of the most completely known pterosaurs of all”.

Group nesting

Although the eggs are not in their original nest positions — they were probably washed together by a storm event — the authors argue that the series of embryos and juveniles at different developmental stages strongly suggests that they nested as a group. “This is something we’ve long suspected might happen, but it’s neat to see it confirmed with fossils,” says Witton.

Further examination of the microscopic structure of the embryonic bones also revealed a surprise, says Wang. Until now, the consensus has been that hatchling pterosaurs could fly almost from birth. The team found well-developed femur (thigh) bones, which are important for walking — but the animals’ forelimbs, which are necessary for flight, were underdeveloped. “We conclude that [baby] pterosaurs, at least Hamipterus, can walk on the ground, but not fly in the sky,” says Wang.

The pterosaur egg fossils, found at a site in northwestern China. Credit: Alexander Kellner (Museu Nacional/UFRJ)

But Witton isn't convinced. He thinks that most pterosaurs probably had well-developed wings upon hatching, but that some features were made of cartilage, which is less likely to fossilize. “These animals would weigh just a few grams when hatched, and almost certainly didn’t need crisply sculpted, well-ossified wing bones to fly,” he says. “Cartilage would be strong enough.”

His own team’s work, presented in September at the Symposium on Vertebrate Palaeontology and Comparative Anatomy in Birmingham, UK, has found that hatchling fossils of two other pterosaur species do look flight-ready, with bones that appear robust and have high bending strength5.

Deeming also cautions against inferring too much from what remains a limited data set — perhaps the Hamipterus embryos studied by Wang weren’t close to hatching after all, he suggests. Palaeontologist and co-author of the Science paper1 Alexander Kellner, of Brazil’s National Museum at the Federal University of Rio de Janeiro, hopes that other eggs already being unearthed from the Xinjiang site will fill some gaps. “We hope to find embryos in different stages,” he says, “to have a complete embryological sequence.”