Using high-resolution CT and synchrotron scanning, paleontologists confirmed that the fossilized specimen from the Early Triassic of the South African Karoo Basin contains an unborn dicynodont Lystrosaurus, resolving a long-standing mystery about whether early mammal ancestors laid eggs. The researchers suggest the dicynodont eggs were likely soft-shelled, explaining why they have remained elusive for so long.
The embryonated egg of the Early Triassic dicynodont synapsid Lystrosaurus. Image credit: Benoit et al., 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 was between 1.8 and 2.4 m (6-8 feet) in length, had no teeth, but bore a pair of tusks in the upper jaw.
Lystrosaurus fossils are known from China, Europe, India, South Africa and Antarctica and this geographic distribution was one of the early pieces of evidence used in support of the large supercontinent Pangea.
“For over 150 years of South African paleontology, no fossil had ever been conclusively identified as a therapsid egg,” said University of the Witwatersrand’s Professor Julien Benoit.
“This is the first time we can say, with confidence, that mammal ancestors like Lystrosaurus laid eggs, making it a true milestone in the field.”
Using advanced imaging, Professor Benoit and colleagues examined three perinate Lystrosaurus specimens found in the Karoo Basin, South Africa.
One of the specimens displayed a tightly curled posture, suggesting it was still inside an egg, and completely lacked tusks.
“Understanding reproduction in mammal ancestors has been a long-lasting enigma and this fossil provides a key piece to this puzzle,” said Dr. Vincent Fernandez, a researcher at the ESRF – the European Synchrotron.
“It was essential that we scanned the fossil just right to capture the level of detail needed to resolve such tiny, delicate bones.”
“When I saw the incomplete mandibular symphysis, I was genuinely excited,” Professor Benoit said.
“The mandible, the lower jaw, is made up of two halves that must fuse before the animal can feed.”
“The fact that this fusion had not yet occurred shows that the individual would have been incapable of feeding itself.”
An 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 typically contain more yolk, providing all the nutrients an embryo needs to develop independently, without parental feeding after hatching,” the researchers said.
“This strongly suggests that Lystrosaurus did not produce milk for its young, unlike modern mammals.”
“Large eggs also offer another crucial advantage: they are more resistant to drying out.”
“In the harsh, drought-prone environment following the extinction, this would have been a critical survival trait.”
“The findings further suggest that Lystrosaurus hatchlings were likely precocial, born at an advanced stage of development.”
“These young animals would have been capable of feeding themselves, escaping predators, and reaching reproductive maturity quickly.”
The discovery not only provides the first direct evidence of egg-laying in mammal 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 mammal ancestors, such as Lystrosaurus, laid eggs, resolving a long-standing question about the origins of mammalian reproduction,” Professor Benoit said.
“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, unpredictable conditions following the end-Permian mass extinction.”
The study appears in the journal PLoS ONE.
_____
J. Benoit et al. 2026. The first non-mammalian synapsid embryo from the Triassic of South Africa. PLoS One 21 (4): e0345016; doi: 10.1371/journal.pone.0345016
Source: Read Full Article
