Photo: Sophie Vrard/Benoit et al., PLOS One, 2026
Researchers studied the remains of a 250-million-year-old creature.
More than 180 years ago, scientists proposed that the ancestors of modern mammals laid eggs, but there was no direct evidence. Now, paleontologists have discovered fossils confirming that this was indeed the case, according to Science Alert.
Around 250 million years ago, during one of the largest mass extinctions in Earth’s history, a pig-like creature called Lystrosaurus lived on the planet. It is considered an ancestor of modern mammals. Scientists believe it survived this harsh period thanks to egg-laying. The discovery not only helps settle a long-standing debate but also suggests that laying eggs was a key survival strategy.
During the study, researchers examined fossils of three newborn lystrosaurs, including one that died inside an egg. Paleontologist John Nyafuli found the fossil in 2008 in the semi-arid Karoo region of South Africa. However, only recently did technology allow scientists to analyze it in detail using tomography at the European Synchrotron Radiation Facility in France.
According to co-author Jennifer Botha, researchers already suspected it was a perfectly curled baby lystrosaur before imaging confirmed it. She noted that they believed it had died inside the egg, but at the time lacked the tools to prove it.
Although no eggshell was preserved, the fossil’s shape matches that of an egg, and the curled posture outlines an oval form. Scans showed the lower jaw had not yet fused, similar to embryos of modern birds and turtles, indicating it died before hatching. Its bones and cartilage were also too weak to support its body.
Scientists suggest lystrosaur eggs were likely soft and leathery rather than hard like those of dinosaurs. Previous research indicated these animals were highly adaptable; some may have even used hibernation-like strategies in cold climates. The new findings highlight egg-laying as a crucial survival mechanism.
The researchers also believe lystrosaurs laid relatively large eggs. Larger eggs would have been less prone to drying out in harsh conditions and would produce more developed offspring capable of feeding themselves, avoiding predators, and reaching maturity faster.
The egg size further suggests that lystrosaurs did not produce milk, as their young developed using nutrients from the yolk.
Beyond survival, the findings also support existing theories about the origin of lactation. Scientists suggest it may have initially evolved not to feed offspring, but as a skin secretion used to moisten eggs, supply nutrients, protect against infections, or enable hormonal signaling through the egg membrane.
Previously, researchers also suggested that an ancient primate known as Graecopithecus freybergi may have developed one of the key traits of humans—the ability to walk on two legs.
