250M-Year-Old Embryo Confirms Therapsids Laid Eggs
Get the Health newsletter
Daily health & science — research, biotech, public health, the studies worth knowing. Free.
- Researchers used the European Synchrotron Radiation Facility in Grenoble, France to image a curled-up embryo found near Oviston, South Africa in 2008, revealing unfused lower jaw bones — a trait only seen in modern egg-laying animals whose beaks must be strong at hatching.
- Lystrosaurus, a pig-sized herbivore with a turtle-like beak and two tusks, famously survived the 'Great Dying' 252 million years ago that killed 90% of all life on Earth, and the newly confirmed egg provides a possible explanation for how.
- The fossil's relatively large egg for the animal's body size indicates Lystrosaurus nourished embryos with yolk rather than milk (unlike modern platypus and echidnas) and that hatchlings were precocial — able to feed themselves and flee predators at birth.
- Lystrosaurus laid leathery-shelled eggs at least 50 million years before hard-shelled eggs evolved, and the eggs' large surface-area-to-volume ratio reduced water loss in the dry post-extinction environment.
- The fossil, held at the National Museum in Bloemfontein, had been a 20-year mystery since its 2008 discovery by paleontologist John Nyaphuli, and is now confirmed as the oldest direct evidence of egg-laying in the mammal lineage.
- Julien Benoit and colleagues published the finding in PLOS One (2026), noting it reshapes understanding of mammalian reproductive evolution and the origins of lactation.
Why it matters: The first confirmed therapsid egg pushes the origin of egg-laying in the mammal lineage back to at least 250 million years ago, showing that the ancestors of modern mammals — including humans — reproduced via leathery eggs before live birth evolved. The discovery also offers a concrete mechanism for Lystrosaurus's survival through the worst mass extinction in Earth's history: large, desiccation-resistant eggs that produced self-sufficient hatchlings ready to reproduce quickly in a devastated ecosystem.
