250-Million-Year-Old Embryonated Dicynodont Egg Found in South Africa

Using high-resolution CT and synchrotron scanning, paleontologists have confirmed that the fossilized Early Triassic specimen from South Africa’s Karoo Basin contains an unborn dicynodont. Lystrosaurussolving a long-standing mystery about whether the earliest ancestors of mammals laid eggs. The researchers suggest that the dicynodont eggs were likely soft-shelled, explaining why they remained elusive for so long.

The embryonated egg of the dicynodont synapsid from the Lower Triassic Lystrosaurus. Image credit: Benoit and others., doi: 10.1371/journal.pone.0345016.

Lystrosaurus represents a major group of primarily herbivorous vertebrates that were common during the Permian and Triassic periods.

This ancient creature measured between 1.8 and 2.4 m (6 to 8 feet) in length, had no teeth, but carried a pair of tusks in its upper jaw.

Lystrosaurus Fossils are known from China, Europe, India, South Africa and Antarctica and this geographical distribution was some of the first evidence used to support the existence of the large supercontinent Pangea.

“For more than 150 years of South African paleontology, no fossil has ever been conclusively identified as a therapsid egg,” said Professor Julien Benoit of the University of the Witwatersrand.

“This is the first time that we can say, with certainty, that the ancestors of mammals like Lystrosaurus laid eggs, making it a real milestone in the field.

Using advanced imaging, Professor Benoit and colleagues examined three perinatal Lystrosaurus specimens found in the Karoo Basin, South Africa.

One of the specimens displayed a tightly hunched posture, suggesting that it was still inside an egg and completely devoid of tusks.

“Understanding reproduction in the ancestors of mammals has been a long-standing enigma and this fossil is a key piece of that puzzle,” said Dr Vincent Fernandez, a researcher at the ESRF – the European synchrotron.

“It was essential that we scan the fossil perfectly to capture the level of detail needed to resolve such tiny and delicate bones.”

“When I saw the incomplete mandibular symphysis, I was really excited,” Professor Benoit said.

“The mandible, the lower jaw, is made up of two halves that must fuse together before the animal can feed.”

“The fact that this fusion has not yet occurred shows that the individual would have been unable to feed itself.”

Artist’s impression of Lystrosaurus. Image credit: Victor O. Leshyk, www.victorleshyk.com / University of Birmingham.

According to the team, Lystrosaurus laid relatively large eggs for its body size.

“In modern animals, larger eggs generally contain more yolk, providing all the nutrients an embryo needs to develop independently, without parental nourishment after hatching,” the researchers explained.

“This strongly suggests that Lystrosaurus did not produce milk for its young, unlike modern mammals.

“Larger eggs also offer another crucial advantage: they are more resistant to drying out.”

“In the harsh, drought-prone environment that followed the extinction, this would have been an essential survival trait.”

“The results further suggest that Lystrosaurus the hatchlings were probably precocial, born at an advanced stage of development.

“These young animals would have been able to feed on their own, escape predators and quickly reach reproductive maturity.”

This discovery not only provides the first direct evidence of egg-laying in mammalian ancestors, but also offers a powerful explanation for how Lystrosaurus came to dominate post-extinction ecosystems.

“This research is important because it provides the first direct evidence that the ancestors of mammals, such as Lystrosaurushave laid eggs, thus resolving a long-standing question about the origins of mammalian reproduction,” said Professor Benoit.

“Beyond this fundamental insight, it reveals how reproductive strategies can shape survival in extreme environments: by producing large, yolk-rich eggs and precocial young, Lystrosaurus was able to thrive in the harsh and unpredictable conditions that followed the end-Permian mass extinction.

The study appears in the journal PLoS ONE.

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J.Benoit and others. 2026. The first non-mammalian synapsid embryo from the Triassic of South Africa. PLoS One 21 (4): e0345016; doi: 10.1371/journal.pone.0345016