Every mammal gives birth to live young, except for a handful of egg-laying monotremes like the platypus. But did the earliest ancestors of mammals also reproduce through eggs? It’s a question that’s stumped evolutionary biologists for decades, but researchers finally have a definitive answer. Published on April 9 in the journal PLOS One, their findings rely on a 250-million-year-old fossilized egg, sophisticated technological advances, and a lot of patience.
Paleontologists discovered the specimen in question almost 17 years ago during an excavation in South Africa’s Karoo Basin. The arid region located over 200 miles northeast of Cape Town is particularly well known for its vast troves of ancient fossils.
“My preparator and exceptional fossil finder, John Nyaphuli, identified a small nodule that at first revealed only tiny flecks of bone. As he carefully prepared the specimen, it became clear that it was a perfectly curled-up Lystrosaurus hatchling,” University of the Witwatersrand paleobiologist Jennifer Botha said in a statement.
Lystrosaurus was a pivotal species in the evolutionary journey of mammals. The herbivores arrived on the planet during the aftermath of the End-Permian Mass Extinction about 252 million years ago. Likely caused by volcanic eruptions in present-day Siberia, the End-Permian cataclysm eventually wiped out around 57 percent of all biological life, including 70 percent of terrestrial vertebrates. Lystrosaurus managed to thrive despite the era’s volatile climate, warm temperatures, and frequent droughts. Although Botha and her colleagues suspected their discovery showcased the remains of a hatchling inside its shell, the required imaging technology to assess their theory did not exist in 2008.
Within a few years, however, the development of advanced synchrotron X-ray CT scanning allowed a path forward. Botha brought the fossil to the European Synchrotron Radiation Facility in France, where collaborators could finally examine it under the proper conditions. Only then could they identify a key piece of evidence—an incomplete mandibular symphysis. This section of lower jaw is crucial for an animal to eat, but only after its two halves fuse during gestation.
“I was genuinely excited,” recalled University of Witwatersrand paleobiologist Julien Benoit. “The fact that this fusion had not yet occurred shows that the individual would have been incapable of feeding itself.”
This means their Lystrosaurus wasn’t fully developed when it died, and its positioning could only mean one thing: it was still inside an egg. More specifically, the team believes Lystrosaurus laid soft-shelled eggs, which explains why fossilized evidence is so difficult to find.
Although small, the egg is large compared to the mammal ancestor’s body size. Today, larger eggs usually contain more yolk, which include all the nutrients needed for an embryo to develop without a parent feeding it. The bigger eggs are also much more resistant to drying—a vital strength during the harsh climate following the extinction event. Taken altogether, it appears that Lystrosaurus was already highly developed when it hatched. This made them able to evade predators, take care of themselves, and quickly begin reproducing.
Beyond filling in a major gap in mammalian evolution, Lystrosaurus can help biologists understand how species might continue to adapt to an increasingly chaotic ecosystem.
“This work is highly impactful because it offers a deep-time perspective on resilience and adaptability in the face of rapid climate change and ecological crisis,” said Benoit, adding, “This discovery [is] not just a breakthrough in paleontology, but also highly relevant to current biodiversity and climate challenges.”
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