Baby pterosaurs were cute, defenceless and unable to fly

Zhao Chuang

By Aylin Woodward

The largest ever collection of pterosaur eggs and embryos has been found in north-west China. It includes 215 eggs, some with intact embryos. The “Pterosaur Park” is evidence that these pterosaur babies were born flightless and needed looking after, and that their parents nested in huge shared colonies.
The first flying vertebrates and the biggest animals to ever get off the ground, pterosaurs evolved some 220 million years ago from a group of reptiles that gave rise to crocodiles, dinosaurs, and later birds. The species studied, Hamipterus tianshanensis, lived in the early Cretaceous nearly 120 million years ago. They likely ate fish and other small animals.

To date scientists have only found a handful of pterosaur bone beds. Their eggs and embryos are even more rare, so we know little about how they lived and reproduced.

In 2004, the first fossilised pterosaur embryo was found in north-east China. But it was flattened “like Cretaceous roadkill”, says Alexander Kellner at the Federal University of Rio de Janeiro in Brazil, making it hard to get useful data. Ten years later, five more eggs were unearthed in north-west China, and another in Argentina.

Eggs galore

So when Kellner’s colleague Xiaolin Wang at the Chinese Academy of Sciences in Beijing called him in to say he had found over 200, he reacted with disbelief. “It was so amazing, I said, ‘Come on Xiaolin, you made it up’,” he says.
The H. tianshanensis eggs are each the size of a small chicken egg, but with a soft shell like a snake’s egg instead of a hard, brittle one. They were found in a sandstone block just 3.28 square metres in size.
Kellner and his colleagues used computed tomography (CT) to scan what’s inside. They found that sixteen of the 215 eggs held embryos.

Alexander Kellner/Museu Nacional (UFRJ)

The embryos’ wings and legs showed differing amounts of bone development. Some embryos had fully-formed femurs but not wings, meaning they were likely flightless at birth. The researchers think this means hatchlings needed parental care.

These results are in direct contrast to what scientists previously thought—that pterosaurs came out of their eggs ready to fly. Kellner says when H. tianshanensis hatched, it could walk around but not fly.
This is reasonable, but many baby birds don’t need fully ossified limbs to fly, says Kevin Padian at the University of California, Berkeley.

However, the embryos also had underdeveloped pectoral muscles, which pterosaurs needed to get off the ground. This is further evidence that hatchlings were flightless, says Kellner.
It may be that H. tianshanensis hatchlings were flightless, but other pterosaurs were more developed when they hatched and thus could fly.

Pterosaurs that nest together

Since the eggs contain embryos in a variety of growth stages, the eggs cannot all have come from the same mother. This indicates that H. tianshanensis nested in colonies, says Kellner, like modern rookeries.
The eggs are also scattered in four geological layers in the sandstone. Each level with eggs could represent a different storm and subsequent flood, which floated the eggs away from their nests to a nearby lake, where they were buried. The existence of multiple layers implies that pterosaurs used the same nesting ground year after year, Kellner says.
Nesting colonies are possible, says David Unwin at the University of Leicester in the UK. However, he says pterosaurs buried their eggs, so they may simply have laid them in the same place because it was the optimal burial environment, much as certain beaches are optimal for sea turtles.

Journal reference: Science, DOI: 10.1126/science.aan2329