Discovery of a Lystrosaurus embryo inside a fossilized egg from South Africa provides the first direct evidence that early mammalian ancestors reproduced by laying eggs, explaining how they survived the end-Permian extinction.
A remarkable fossil discovery from South Africa provides an answer to a long-standing evolutionary question: did the early ancestors of mammals lay eggs? In a new study published in PLOS ONE, an international team has identified a 250-million-year-old egg containing the embryo of a Lystrosaurus, an ancient herbivorous animal from the synapsid group, considered close to the ancestors of mammals. The researchers say this is the first confirmed case of an egg belonging to an ancestor of mammals.
Lystrosaurus was one of the few animals that not only survived the end-Permian extinction event, about 252 million years ago, but also became very common afterwards. It was the most severe extinction in Earth's history, a period characterized by extreme heat, environmental instability, and prolonged droughts. The new discovery helps to understand how this species managed to persist and even thrive in such harsh conditions.
The study was led by Prof. Julien Benoit, Prof. Jennifer Botha from the University of the Witwatersrand in South Africa, and Dr. Vincent Fernandez from the European Synchrotron in France. The fossil itself was discovered in 2008, but only now, with the help of advanced X-ray CT scans at the ESRF synchrotron, have the researchers been able to prove that the embryo did indeed die inside the egg and not after hatching.
One of the important pieces of evidence was the condition of the embryo's lower jaw. The researchers explain that in a young animal, the jaw consists of two halves that need to fuse before it can eat on its own. The scans revealed that this fusion was not yet complete, so it is clear that this is an individual that has not yet reached the stage where it could survive outside the egg.
According to the researchers, Lystrosaurus eggs probably had soft shells. This is also why such eggs have almost never been found in fossils: Unlike dinosaur eggs with hard, mineral shells, soft-shelled eggs are much less likely to be preserved over millions of years. Therefore, the discovery of such an egg is considered extremely rare.
Large eggs together for the body of Lystrosaurus
The study also suggests that Lystrosaurus laid eggs that were relatively large for its body size. In modern life, large eggs contain more yolk, allowing the embryo to develop largely without the need for parental care after hatching. The researchers conclude that Lystrosaurus did not feed its offspring milk, unlike modern mammals. In addition, large eggs are more resistant to desiccation—a particularly important advantage in the dry, harsh world that followed the end-Permian extinction.
The team estimates that young Lystrosaurus individuals were relatively well-developed at hatching, meaning they could move, feed on their own, and avoid predators soon after hatching. Combined with their rapid sexual maturity, this is a life strategy of rapid growth and early reproduction—traits that may have given the species a huge advantage at a time when entire ecosystems were collapsing.
The researchers emphasize that the discovery has significance beyond just the question of reproduction. It provides a deep look back in time at resilience and adaptation in the face of a global climate and ecological crisis. They say that understanding the ways in which ancient organisms survived global upheavals can also help understand possible responses of contemporary species to accelerating environmental pressures.
Furthermore, it is also a milestone in South African paleontology. The researchers note that in more than 150 years of paleontological research in the country, not a single non-mammalian synapsid egg has been identified with certainty. It is now possible to say with confidence that the ancestors of early mammals, including Lystrosaurus, did indeed lay eggs.
The article, titled “The first non-mammalian synapsid embryo from the Triassic of South Africa,” was published on April 9, 2026, in PLOS ONE. DOI: 10.1371/journal.pone.0345016
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