Imagine, if you will, this prehistoric scene: a single pterosaur – one of those majestic flying reptiles of the "Age of Dinosaurs" – sails overhead, scanning the ground below her. She's searching for her own nest, but it's a challenge to spot because there are many nests in this colony, with many busy pterosaur parents catering to the tiny newborns that hop around at their feet, not yet ready to fly but more than ready for dinnertime.

This is the picture painted by an amazing discovery in China: a fossil site preserving more than 200 pterosaur eggs! 

Hundreds of pterosaur bones and eggs are preserved in this site, many of them in excellent condition. It's a treasure trove for palaeontologists. Image: Alexander Kellner (Museu Nacional/UFRJ)

Pterosaurs aren't common in the fossil record. Their fragile bones don't fossilise well, and their thin-shelled eggs are far rarer still. In fact, until the first pterosaur eggs were discovered in 2004, palaeontologists weren't sure these animals laid eggs at all. And since then, only about half a dozen examples have been recorded of three-dimensionally preserved pterosaur eggs.

For the past decade, a team led by Xiaolin Wang of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing has been working on a fossil site in Xinjiang, in northwestern China. This locality, dating to over 100 million years ago, has turned out to be a goldmine for pterosaurs. The crew has found the remains of more than 40 individuals of a species named Hamipterus tianshanensis, with wingspans ranging from 1.5 to 3.5 metres (5-11ft).

This lower jaw of Hamipterus shows off the characteristic large teeth of the species. Image: Alexander Kellner (Museu Nacional/UFRJ)

And in the same fossil layers as the pterosaur bones are eggs. Lots of eggs. One patch of sandstone, measuring only about three square metres (10 sq ft) contains 215 eggs, with possibly many more still hiding below the surface.

When Wang and an international team of researchers CT-scanned the eggs, they found that more than a dozen of them held tiny bones inside: the remains of pterosaur embryos!

This 3D reconstruction shows the position of embryonic bones within one egg. As the eggs fossilised, they filled in with sediment, which preserved the shapes of the eggs, but also damaged the bones within, so none of the embryos is complete. Image: Wang et al. Science (2017)

This unique collection of fossils, including pterosaurs of various ages alongside so many eggs, leads the researchers to suspect this is an ancient colonial nesting site. "Hamipterus likely made its nesting grounds on the shores of freshwater lakes or rivers and buried its eggs in sand along the shore," Wang and his colleagues stated in an earlier study.

For a modern comparison, you might look at birds like albatrosses, which gather in large (and noisy!) colonies for hatching and rearing their young. But these pterosaurs' eggshells are more like those of certain living lizards, which suggests the flying reptiles were burying their eggs instead of sitting on them like modern birds do.

These pterosaur eggs have very thin shells, and are structured much more like the eggs of certain modern-day lizards and snakes than those of birds. Image: Wang et al. Science (2017)

But those albatrosses – like most birds – care diligently for their young. Did the pterosaurs do the same? The scientists think they did.

In the new study, published in the journal Science, Wang and his colleagues took a close look at those tiny pterosaur embryos, which preserved bones of the wings, legs and skull (one complete lower jaw bone was only about 1.6cm long!) to get an idea of what these reptiles looked like when they first emerged from the egg.

The embryos that seemed closest to hatching already had well developed leg bones, but the wing bones hadn't yet formed the attachment sites needed for the animals' full suite of powerful flight muscles, and the jaws were still toothless. It appears that newborns wouldn't have been quite ready for independence.

"Newborns were likely to move around but were not able to fly … and probably needed some parental care," the researchers state in their paper.

To confirm this, palaeontologists will need yet more embryos and hatchlings to examine, but it's a tantalising hint that at least some pterosaurs – just like crocodilians and dinosaurs (including birds) – were attentive parents, caring for their young in those important formative days. 

Palaeontologists Xiaolin Wang and Alexander Kellner on site in the Turpan-Hami Basin of Xinjiang. Image: Alexander Kellner (Museu Nacional/UFRJ)

Nesting on the ground in large colonies, however, would have come with its dangers for the pterosaurs. Despite all of these fossilised eggs, there's no evidence of nests at the site, and the fossils are preserved in multiple layers of ancient lake sediments. These eggs were most likely laid near freshwater, and then a series of events – possibly heavy storms – washed them into the water.

Tragic as this would have been for the animals themselves, it's also what allowed these bones and eggs to become preserved so well, and in such incredible, informative abundance.