Fossilised pterosaur eggs and chicks

30 November 2017
Posted by Chris Smith.

Hundreds of fossilised dinosaur eggs, some with preserved chicks still inside, have been uncovered by scientists in China.

The clutch of between 200-300 preserved eggs, each about 5cm long, were laid by a winged dinosaur species known as a pterosaur during the early Cretaceous period between 100-146 million years ago. Incredibly, 42 of the specimens that Xiaolin Wang and her colleagues at Beijing's Institute of Vertebrate Palaeontology and Palaeoanthropology have studied still contain evidence of developing pterosaur chicks within.

To date, literally only a handful of examples of pterosaur eggs and embryos have ever been discovered, 1 from Argentina and 5 from a site in China, meaning that most of our knowledge of the development and behaviour of these early bird-ancestors was based on informed guesswork.

Now, thanks to this new find and with the help of CT scans and painstaking anatomical analysis, scientists can paint a much clearer picture of the behaviour of these animals, which belonged to a species called Hamipterus tianshanensis, which were medium-sized and with a wingspan of about 3.5 metres.

Chicks of varying sizes and stages of development are visible within the preserved eggs, alongside the remains of recent hatchlings. Unlike modern reptiles, such as crocodiles and lizards, none of the pterosaur young have teeth, suggesting that these developed much later after hatching. Their lower limb bones are also relatively much sturdier than those of the wings and chest, which appear still to be made of a soft cartilage material in the developing young. 

Scientists had regarded these animals as precocious, meaning that they would have hatched out in a flight-ready state. Instead, the CT scan findings suggest that the young pterosaurs would probably have wandered and hopped about on their well-developed hind limbs, but flight would have come later once their upper limbs developed sufficiently.

This finding, and the absence of teeth in the young, would have meant that the animals were likely to have been dependent on a parent to protect and nurture them initially. The sheer numbers and different ages of the eggs present in the assemblage argues, the researchers say, that these animals were gregarious and lived together in large groups.

The eggs were uncovered at Turpan-Hami Basin, Xinjiang provice in northwestern China. They're lodged haphazardly within a slab of sandstone about 3.5 metres square and are sitting up to 8 layers deep.

To account for this, the researchers speculate that a flash flood washed the eggs into a lake from large numbers of nests that were sitting along a muddy shore line. There, the eggs would have massed together and floated for a while before sinking into the mud and becoming covered.

The discovery, Wang and her team argue in their paper in Science this week, suggests that pterosaurs, like Hamipterus tianshanensis, were a colonial, gregarious species that lived together in large numbers...

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