Scientists have identified a groundbreaking fossil discovery in South Africa that contains a 250-million-year-old embryo preserved inside an egg, providing the first definitive evidence that early mammal ancestors laid eggs.
The specimen was recovered from the Early Triassic deposits of the Karoo Basin and belongs to Lystrosaurus, a herbivorous synapsid that lived during the Permian and Triassic periods and is considered an important ancestor in mammalian evolution.
Using advanced high-resolution CT scanning and synchrotron imaging, researchers confirmed that one of the fossilized specimens contains an unborn embryo still positioned in a tightly curled posture. The analysis also revealed the absence of fully developed feeding structures, indicating the individual had not yet hatched.
The study resolves a long-standing scientific debate over whether early therapsids—distant relatives of modern mammals—reproduced by laying eggs. According to researchers, this discovery confirms that egg-laying was part of their reproductive biology.
Scientists also suggest that these ancient eggs were likely soft-shelled, which may explain why no clear fossil record of them had been found until now, as such structures are less likely to fossilize.
The Lystrosaurus species itself was a medium-sized, tusked herbivore measuring up to 2.4 meters in length and is widely known from fossils found across Africa, Asia, India, and Antarctica, supporting evidence of the ancient supercontinent Pangaea.
Researchers describe the find as a major milestone in understanding how reproductive strategies evolved in the early ancestors of mammals.
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