Scientists have uncovered a 250-million-year-old fossilized egg, predating the age of dinosaurs, and have solved a major mystery about mammalian evolution. Researchers say the specimen, which contains a curled-up embryo from the plant-eating Lystrosaurus, is the first egg ever discovered from a mammalian ancestor and proves that the mammalian ancestor laid eggs.
The eggs could help paleontologists better understand how these animals survived the Permian-Triassic extinction (also known as the Great Extinction), which occurred about 252 million years ago. During this event, Earth faced brutal heat, drought, volcanic eruptions, ocean acidification, and 90% of Earth’s species died out.
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The researchers published their findings on April 9th in the academic journal PLOS One.
Which came first, the therapsids or the eggs?
Fossilized eggs were first discovered in 2008 during a field survey near Chaliep, South Africa. The specimen had only small bone fragments near the tubercle, but contained a nearly complete, tightly curled embryo. Researchers identified the animal as a Lystrosaurus, an early ancestor of mammals in the group known as therapsids. Therapsids are mammal-like reptiles that lived approximately 272 to 250 million years ago and are descendants of modern mammals.
“Adults [Lystrosaurus] “It was pig-like in appearance, with bare skin, a turtle-like beak, and two protruding fangs pointing downwards,” the researchers wrote in The Conversation.
Scientists have known about this mammal’s ancestor for years, but they didn’t know if the animal laid eggs.
Because the fossil had no outer shell, only the embryo, researchers were initially unable to determine whether the embryo had already been born or was still inside the egg when it died.
Professor Jennifer Botha, from the Institute of Evolution at the University of the Witwatersrand in South Africa, said in a statement: “We had always suspected that it had died inside the egg, but we just didn’t have the technology to confirm that at the time.”
In the new study, researchers used powerful CT scans at the European Synchrotron Radiation Facility in France to study the fossils without damaging them. The scan revealed small structures inside the fossil, including an unfused lower jaw that was still split in half. This meant the embryo was not developed enough to feed on its own, indicating it had not yet hatched. Researchers suspect the shell was leathery and melted.
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“In more than 150 years of South African paleontology, no fossils have been definitively identified as therans eggs,” Botha said in a statement. “This is the first time we can say with confidence that the ancestors of mammals like Lystrosaurus laid eggs, a real milestone in the field.”
Place all eggs in one surviving basket
Researchers discovered that Lystrosaurus laid unusually large eggs for its body size. In living animals, larger eggs contain more yolk, which can encourage more complete development before hatching. The researchers suggested that this finding indicates that Lystrosaurus had relatively mature and mobile offspring soon after birth. This would have allowed these animals to better feed themselves and avoid danger, allowing them to survive the catastrophe, the researchers said.
The researchers suggested that the larger eggs and their leathery texture also helped these animals survive in other ways.
“The larger the egg, the smaller its surface area (relatively), so Lystrosaurus eggs would lose less water through their leathery shells than eggs of other species at the time,” the researchers wrote in The Conversation. “Given the arid environment during and immediately after the extinction, this was a huge advantage, especially since hard-shelled eggs are unlikely to evolve for at least another 50 million years.”
Although many lineages disappeared in the Permian-Triassic extinction event, Lystrosaurus not only survived, but went on to become one of the dominant land animals.
“Growing fast, breeding young, and multiplying were the secrets of Lystrosaurus’ survival,” the researchers added in an article in The Conversation.
The discovery could help scientists better understand how animals can survive in a changing climate.
“In the modern context, this study is highly impactful because it provides a deep-temporal perspective on resilience and adaptability in the face of rapid climate change and ecological crisis,” Benoit said in a statement. “Understanding how past organisms survived global cataclysms can help scientists more accurately predict how present-day species will respond to ongoing environmental stresses, making this discovery not only a breakthrough in paleontology but also highly relevant to current biodiversity and climate challenges.”
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