Three incredibly well-preserved dinosaur eggs found in China reveal that the oviraptorids inside them hatched in a very similar way to modern-day birds.

The fossilised remains were studied by a team of German researchers who pelted neutron beams at the dinosaur embryos.

It revealed the two-legged dinosaurs all hatched at different times, placing them close to birds on the evolutionary timeline.

Oviraptorids lived in Central Asia during the Upper Cretaceous Period – around 88 million to 66 million years ago — and grew to roughly the same size as humans.

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Pictured: the three oviraptorid eggs studied by scientists at the University of Bonn and the TU Munich

Artist's reconstruction of a clutch of oviraptorid eggs surrounded by the protective parents

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Scientists have assumed that they should be placed somewhere between modern crocodiles and birds in terms of their reproductive biology.

But they had been uncertainty surrounding whether oviraptorids chicks emerged from their eggs at the same time as their siblings – like today’s crocodiles – or hatched at different times – like modern-day birds.

Based on the internal thickness of the egg shells and bone structure of the three 67 million year-old egg fossils, the research team concluded they were more similar to birds in this regard.

‘This is different with the fossils we've examined – we found a pair of eggs and another egg together embedded in a block of rock,’ said Dr Tzu-Ruei Yang, co-author of the study, published in Integrative Organismal Biology.

Researchers concluded that the the 7-inch (18cm) eggs, which were taken from the Ganzhou Basin in China, were laid almost at the same time by a female oviraptorid.

By observing the length of the bones, the team estimated whether the three dinosaurs were due to hatch simultaneously or at different times based on the developmental stages of the embryos.

'The embryo with comparatively longer bones is more developed,' said Yang.

Another indication of embryo development is the extent to which bones are connected to each other, with a more strongly connected skeleton suggesting more advanced development.

Photogrammetry of 'egg 3, which was photographed and reconstructed from different perspectives.

Neutron tomogram as cross-section through 'egg 3', which was one of two eggs laid later than the other

Based on observations of the length and position of the embryo bones, researchers concluded that one egg must have been laid earlier than the other two in the same 'clutch' – a group of eggs in a nest.

However, the embryos of the pair were also at different developmental stages too.

Developing embryos also would have absorbed calcium from the shell as it grew, to strengthen its developing skeleton.

Therefore, the thinner the shell, the more advanced the embryo’s development – and the older the egg.

Researchers summarised that the embryo of this one egg was less developed than that of the two other 'paired' eggs.

This showed that eggs developed and hatched at different times despite being laid around the same time.

From this, the scientists concluded that the reproductive biology of oviraptorids were more similar to that of modern birds than crocodiles.

Paleontologists at the University of Bonn in Germany initially tried to determine the position of bones inside the eggs with the institute's own X-ray microcomputer tomograph.

This technology shoots x-rays at a physical object to create a detailed 3D model – but it was not possible to distinguish the bones from the surrounding rock using x-ray tomography alone.

The team therefore took the dinosaur eggs to the research neutron source of the Technical University of Munich at the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, Germany.

The high penetration depth of the neutrons made it possible to visualise the detailed internal structures of the eggs.

The study has brought the researchers one step closer to piecing together the life of the long extinct oviraptorids, who roamed Central Asia, including Mongolia and China, on two legs.