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Fossil found in South Africa in 2008 could only be analyzed now thanks to X-rays more intense than the Sun — and revealed a 250 million-year-old embryo curled inside a soft-shelled egg, proving that our oldest ancestors reproduced like reptiles
For over a century, paleontologists have debated how the ancestors of mammals reproduced. Now, a 250 million-year-old fossil found in the Karoo region of South Africa has delivered the answer. Inside the rock, researchers found an embryo curled inside an egg — the first direct evidence that mammalian ancestors laid eggs.
The study was published in the scientific journal PLOS One. And the discovery changes what was known about the history of mammalian reproduction — including that of humans.
The animal is a Lystrosaurus, a herbivore that lived during the Early Triassic, shortly after the largest mass extinction in Earth’s history.
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A 250 million-year-old embryo hidden inside the rock
The fossil was found in 2008 by paleontologist John Nyaphuli in the Karoo Basin, one of the richest fossil regions in the world.
At the time, scientists suspected there was something inside the rock. But the available technology was not sufficient to confirm it.
Only in 2026, with the use of high-resolution computed tomography and synchrotron analysis — a particle accelerator that generates X-rays more intense than the Sun — did the team manage to see the embryo.
The examination was conducted at the European Synchrotron Radiation Facility in Grenoble, France.
What the rays revealed surprised even the most experienced researchers.
The embryo was curled inside a soft and leathery egg. Its jaws were not yet fully fused — a characteristic that only appears in embryos inside eggs, like those of modern birds and turtles.
The animal that survived the greatest catastrophe in history
The Lystrosaurus was not just any reptile. It was a synapsid, the group of animals that eventually gave rise to all mammals.
It lived about 250 million years ago, shortly after the end of the Permian Period, when the Great Extinction eliminated approximately 90% of all life on the planet.
Extreme global warming, drought, acid rains, and poisoned oceans devastated life on Earth.
Even so, the Lystrosaurus not only survived — it dominated the post-extinction ecosystems.
And the egg may explain why.
According to researchers, the eggs of the Lystrosaurus were relatively large in proportion to its body. They contained more yolk, allowing the embryo to develop independently, without needing parental nourishment after hatching.
This strategy of rapid and independent reproduction may have been crucial for colonizing a devastated world.
Our ancestors reproduced like reptiles — and the proof is in the egg
“This is the first time we can confidently state that mammalian ancestors like the Lystrosaurus laid eggs,” said Julien Benoit, associate professor at the Institute of Evolutionary Studies at the University of Witwatersrand in South Africa.
According to him, the discovery “represents a true milestone in the field.”
Steve Brusatte, author of the book “The Rise and Reign of the Mammals,” was more direct.
“This is concrete proof that some of our closest mammalian ancestors still laid eggs and reproduced like reptiles, and did not give birth to live young or feed them with milk.”
The discovery confirms that viviparity — giving birth to live young — and lactation came later in the evolution of mammals.
Researchers estimate that lactation evolved between the beginning and the end of the Triassic, between 252 and 201 million years ago.
The platypus: the last mammal that still lays eggs
Today, only two groups of mammals still lay eggs: the platypus and echidnas, both from Australia.
They belong to the group of monotremes, considered the most primitive mammals that exist.
Like the 250 million-year-old Lystrosaurus, the platypus produces soft-shelled eggs. But there is a fundamental difference: the platypus already has mammary glands and feeds its young with milk after hatching.
The Lystrosaurus, as the fossils indicate, did not do this. Its young depended exclusively on the yolk of the egg for development.
This shows that the evolution of mammals was a gradual process:
- First came the soft-shelled eggs, like those of the Lystrosaurus (250 million years ago)
- Then lactation emerged, already in the Triassic period
- Hard shells only evolved at least 50 million years later
- Live birth came last, when most mammals abandoned eggs
The technology that took 18 years to reach the fossil
One of the most fascinating aspects of the discovery is the role of technology.
The fossil was found in 2008. The bones of the embryo were so small and delicate that no equipment at the time could visualize them inside the rock without destroying the sample.
It took 18 years for the resolution of European synchrotrons to be sufficient to confirm what paleontologists suspected.
The European Synchrotron Radiation Facility in Grenoble uses an X-ray beam millions of times more intense than a hospital X-ray.
Thanks to this, researchers were able to see each bone of the embryo without opening the fossil.
What a 250 million-year-old extinction teaches about today’s climate
Professor Benoit emphasized that the discovery goes beyond paleontology.
“Understanding how past organisms survived global catastrophes helps scientists better predict how species may respond to current environmental stress.”
The strategy of the Lystrosaurus — rapid reproduction, early independence of the young, low parental dependence — was what allowed it to dominate a destroyed planet.
Ironically, modern mammals evolved to the opposite: long gestation, few young, high parental dependence.
If a global catastrophe were to happen today, species that reproduce quickly and independently — insects, reptiles, fish — would have an advantage. Mammals, with their slow reproduction, would be among the most vulnerable.
However, the researchers themselves warn that soft shells rarely fossilize. The preservation of this embryo is exceptional, and debates about the exact position of the Lystrosaurus in the evolutionary tree of mammals remain open.
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