Paleontologists have utilized high-resolution CT and synchrotron scanning technology to confirm that early Triassic fossil specimens from South Africa’s Karoo Basin feature immature dicynodonts. The mystery surrounding whether the early mammalian ancestor, Lystrosaurus, laid eggs has finally been solved. Researchers propose that dicynodont eggs were likely soft-shelled, which explains their long-elusive nature.

Lystrosaurus is a significant group of primarily herbivorous vertebrates that thrived during the Permian and Triassic periods.

This ancient creature measured between 1.8 and 2.4 meters (6 to 8 feet) in length and possessed no teeth, except for a pair of fangs in its upper jaw.

Fossils of Lystrosaurus can be found in locations such as China, Europe, India, South Africa, and Antarctica, providing evidence for the existence of the ancient supercontinent Pangea.

Professor Julien Benoit from the University of Witwatersrand stated, “In over 150 years of South African paleontology, no fossils have been definitively classified as therapsid eggs.”

“The discovery that the ancestors of mammals, like Lystrosaurus, laid eggs is a significant milestone in paleontology,” he added.

Professor Benoit and colleagues utilized advanced image processing to investigate three regions in a Lystrosaurus specimen unearthed from the Karoo Basin of South Africa.

One of the specimens exhibited a tightly curled posture, indicating it was still encased in its egg and lacked its fangs.

“Understanding the reproductive methods of our mammalian ancestors has been a long-standing question, and this fossil offers crucial insights,” said Dr. Vincent Fernandez, a researcher at the European Synchrotron (ESRF).

“Precision in scanning the fossil was crucial to attain the level of detail needed for such small, delicate structures.”

“Upon seeing the incomplete mandibular symphysis, I was filled with excitement,” Professor Benoit remarked.

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

“The absence of this fusion indicates the individual could not nourish itself.”

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

According to the research team, Lystrosaurus eggs were relatively large in relation to their body size.

“In many modern animals, larger eggs typically contain more yolk, which provides essential nutrients for the embryo’s independent development after hatching, without the need for parental feeding,” the researchers noted.

This strongly indicates that, unlike modern mammals, Lystrosaurus did not produce milk for their young.

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

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

The findings suggest that Lystrosaurus hatchlings were likely precocial, emerging at a relatively advanced developmental stage.

“These young individuals could have fed themselves, evaded predators, and rapidly reached reproductive maturity,” the researchers concluded.

This groundbreaking discovery not only serves as the first direct evidence of egg-laying in a mammalian ancestor but also elucidates how Lystrosaurus successfully dominated the ecosystem following the extinction event.

“Our research illustrates that the ancestors of mammals, like Lystrosaurus, produced eggs, offering clarity on the origins of mammalian reproduction,” said Professor Benoit.

“Beyond this significant insight, it reveals how reproductive strategies can influence survival in extreme conditions. By laying large, nutrient-rich eggs, Lystrosaurus thrived in the harsh, unpredictable surroundings after the end-Permian mass extinction.”

The study is published in PLoS ONE.

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J. Benoit et al. 2026. The first non-mammalian monoapsid embryo discovered in the Triassic of South Africa. PLoS One 21 (4): e0345016; doi: 10.1371/journal.pone.0345016