The fossilised skulls of dinosaur embryos that died within their eggs about 200m years ago, have been digitally reconstructed by scientists, shedding new light on the animals’ development, and how close they were to hatching.

The rare clutch of seven eggs, some of which contain embryos, was discovered in South Africa in 1976, with the developing young found to be a species of dinosaur called Massospondylus carinatus.

The plant-eaters were ancestors of sauropod dinosaurs like diplodocus and, as fully-grown adults, would have walked on two legs, measured about five metres from nose to tail, and had long necks with small heads.

Now researchers say they have carried out high-resolution CT scans to digitally reconstruct the tiny skulls of the embryos, shedding fresh light on their development.

“One of the biggest problems when looking at embryos is that a lot of the tissues that you would normally use to define these developmental stages in embryos, they are not gong to fossilise,” said Dr Kimi Chapelle, co-author of the research from the University of Witwatersrand in South Africa.

Previous attempts at working out how mature dinosaur embryos are, such looking at the level of contact between different parts of the skull, have run into difficulties, not least because of challenges in assessing such features. However the scans, said Chapelle, offer a different approach, allowing researchers to look at the degree of bone formation and explore parts of the specimen otherwise hidden from view.

The new research, published in the journal Scientific Reports, reveals embryo remains in three of the eggs, the tiny, 2cm long, skulls showing two sets of teeth.

But while one set showed serrations and were similar to adult teeth, as has previously been seen in other dinosaur embryos, the other set was different, composed of simple, conical shaped teeth. “We have never seen that before,” said Chapelle, adding that, like many reptiles today, the dinosaurs would probably have either lost or reabsorbed these teeth before developing the teeth they would hatch with.

The team also drew on previously collected data from embryos of three animals alive today – the African spurred tortoise, the chicken and the nile crocodile – and tracked the location and extent of bone tissue formation within their embryos over their incubation. They then applied their findings to another living animal, the central bearded dragon.

The results reveal that while the animals have different incubation times, the sequence and relative timing of bone tissue formation with the skulls is similar.

Since all four animals, and dinosaurs, belong to the same group of land vertebrates – the saurians – Chapelle said the development of the dinosaur embryos probably followed a similar path.

“Having that same relative timing allows us to then apply the method to dinosaurs, no matter what [their] incubation period is – we can still tell how developed they are,” she said.

While previous work suggested the dinosaur embryos were on the verge of hatching at death the team say their comparison, together with the findings from the teeth, suggest the embryos were just 60% of the way through their incubation period – making them some of the least developed dinosaur embryos currently known.

Michael Benton, professor of vertebrate palaeontology at the University of Bristol, who was not involved in the research, said the study was a fantastic piece of work showing the ability of modern technology to reveal astonishing details, even of embryo dinosaurs.

He also praised the team’s method of comparing the growth of the skulls.

“This gives a reliable method of ageing an embryo of a dinosaur for the first time, and then particular events in development can be identified – such as the early appearance of teeth and then their re-absorption, long before the little creature hatches,” he said.