Did Our Ancient Ancestors Lay Eggs?

Jun 24
6 min read

New study of a 252-million-year-old Lystrosaurus embryo provides the first compelling fossil evidence that early mammal ancestors laid eggs

Imagine a tiny creature, curled up inside a leathery egg, in a hot and dusty world still reeling from the worst mass extinction Earth has ever seen. It never hatched. But 252 million years later, it would provide an answer to a question I was posed in school.

In school, I learned about three main modes of animal reproduction based on where and how an embryo develops, namely viviparous (animals are born live without an egg), ovoviviparous (eggs develop inside the mother and are then laid ‘live’), and oviparous (eggs are laid and develop outside the body). My class and I then learned that most mammals are viviparous, save for the platypus and echidna, those weirdos lay eggs. One of the questions we were asked was why do they lay eggs?

It has long been suspected that the ancient ancestors of mammals were a lot more like monotremes (platypuses and echidnas) in that they, too, laid eggs. But the evidence supporting this theory, well, it’s been somewhat lacking.

But why should you care if our ancient ancestors laid eggs or not? Well, for one, one of the leading theories on why mammals produce milk is based on the assumption that ancient mammals also laid eggs (I will explain in a minute), then there is the question of whether monotremes lay eggs because this is an ancestral trait or because they are just evolutionary weirdos.

This is THE ‘chicken or the egg’ question that scientists have been unable to conclusively answer despite centuries of research. But it seems the answer may finally have been found, in the semi-desert landscape of the Karoo, a tiny fossil creature lay for millions of years. It never had a chance to see the light of day, but that’s exactly what makes it so valuable.

No Egg, No Proof

Lystrosaurus fossil in the Staatliches Museum für Naturkunde Stuttgart. Credit: Wikimedia Commons

One of the key moments of vertebrate evolution is the development of the amniotic egg, aka the type of egg that can survive on land. In synapsids, the ancient clade that would eventually give rise to Mammals, it has long been theorized that their original form of reproduction was based on egg-laying.

However, if these early synapsids did lay eggs, they were probably much like reptile eggs, soft and flexible. Nothing like the hard-shelled chicken egg you may have had for breakfast.

But there’s a problem with these soft-shelled early eggs: they don’t preserve well in the fossil record, and so far, the fossil record only has a few examples of possible evidence of amniotic eggs. These include a possible sauropsid (related to modern birds and reptiles) amniotic egg from the early Permian of South America, though no shell survives.

The other, which preserves both a shell and an embryo, belongs to a group of dinosaurs from the Early Jurassic. However, neither of these examples is related to mammals. The only possible synapsid amniotic egg is highly disputed; in fact, it is no longer actually thought to be an egg at all.

In the Karoo, a vast semi-desert region that covers much of South Africa, rocky outcrops have yielded fossils galore, from dinosaur eggs to baby synapsids, and yet to this day, after searching for nearly 180 years, not a single convincing synapsid egg has been recovered.

This puzzled scientists even back in 1964 when paleontologist James Kitching (1922–2003) began wondering whether these ancient mammal relatives ever laid eggs.

But perhaps his doubts would have been laid to rest had he been around to marvel at the discovery of a rather unique little fossil belonging to a young Lystrosaurus, discovered a few years ago on Rheeboksfontein 5 farm in the Free State.

The Lystrosaurus of the Karoo

Photograph of NMQR 3636 (a) and a 3D digital reconstruction NMQR 3636 (b). Credit: Benoit et al. 2026

The Karoo outcrops are among the richest and largest fossil collecting grounds for an extinct group of vertebrates known as therapsids. The layer upon layer of sedimentary rock in the Karoo Supergroup has, in fact, formed a near continuous 120 million-year-long record of fossil evolution and climate change.

Among the many fossils found here are Lystrosaurus, a well-known plant-eating dicynodont that rose to prominence in the wake of the End-Permian Mass Extinction, around 252 million years ago. In this harsh, dry, and hot post-extinction world where many other species died out, Lystrosaurus not only survived but thrived, but why?

To find out, one needs to study their fossils, and to do that, one needs to go out and find them.

In 2008, Prof. Jennifer Botha was conducting a field excursion, her preparator and fossil finder, John Nyaphuil, identified a small nodule which initially yielded nothing more than a few flecks of bone. However, as the specimen was being prepared, a carefully curated Lystrosaurus hatchling was revealed. Though its position hinted that it was still within an egg when it died, the technology at the time was not sufficient to confirm this assumption.

Only almost 17 years later would advanced synchrotron X-ray CT and bright X-ray technology reveal the incredible details trapped within the ancient fossil.

The Chicken or the Egg

Comparison of the cranial features of the three specimen. Credit: Benoit et al. 2026

Okay, Lystrosaurus have nothing to do with chickens, but the title was so on the nose.

The researchers analyzed three young Lystrosaurus specimens with one another. All three were from the Karoo, and all had died at around the same age. However, NMQR 3636 was the youngest of them all. Unlike the eldest of the bunch, BP/1/9332, which had a somewhat sprawled posture, indicating that it had likely moved around at least a bit just before it died, NMQR 3636 was in a curled posture with a trace of an egg-shaped outline surrounding it.

It being the youngest was further supported by a lack of tusk buds, which had begun developing in the other two specimens, and the incomplete ossification (hardening) of its bones indicated it was still early in the embryonic development stage.

In fact, its weak pelvis suggested it would not even have been able to support its own weight. Most compellingly, it had an unfused jaw; in modern turtles and birds, the jaw fuses just before hatching to facilitate feeding. In egg-laying mammals, this isn’t necessary, as they feed on milk instead of solids, meaning they don’t require a fully fused jaw upon hatching. Based on these criteria, this baby Lystrosaurus was likely a fossil embryo, and the oldest proof that synapsids laid eggs.

Based on the analyses, these eggs would have been relatively large, which would have helped prevent them from drying out, but also, when compared to other terrestrial egg-laying animals, meant their young were probably born able to survive on their own. These factors would have been a major advantage in the tough, food-scarce world after the Great Dying. This may help explain why Lystrosaurus were able to not just survive but thrive in this environment.

Implications for Milk Production

Lystrosaurus fossil in the Zurich Natural History Museum. Credit: Wikimedia Commons

Remember how I said milk production was somehow related to egg laying. Well, there is a theory that milk did not initially develop as a food source but rather as a means to keep soft-shelled eggs moist, nourished, or to protect them from infections. If early synapsids did not lay eggs, this theory goes out the window. However, early synapsids, based on our Karoo fossil, did lay eggs. But did they produce milk?

The evidence suggests, no, not yet. In egg-laying mammals, the eggs are typically small, with only very little yolk, as the mother’s milk will compensate for that once hatched. A similar trend is seen in Kayentatherium (also a synapsid), which evolved after the Lystrosaurus in the Early Jurassic. They too had small eggs, and it is believed they produced milk (partially supported by evidence of hair, which evolved alongside mammary glands responsible for milk production).

Lystrosaurus, by contrast, had large eggs, capable of holding a lot of yolk, meaning they likely did not lactate. This finding helps map whether synapsids laid eggs, when they began to do so, and when lactation evolved. It also means our funny platypus friends are not evolutionary anomalies but rather possess an ancestral trait: the ability to lay eggs, as early mammal-like creatures did.

So, did our ancient ancestors lay eggs? It seems they did. And we know this thanks to a tiny fossil curled up for millions of years, never having gotten the chance to see the light of day. Evolution is slow, complex, and weird, but tiny fossils like these can provide us with a lot of answers. Thank you, my tiny friend, for now we know a little bit more about ourselves.

And I learned some weird things about milk, but milk is inherently weird, isn’t it, just me? Okay then.

Are you surprised our synapsid ancestors laid eggs, and did you ever wonder why the platypus lays eggs when almost every other mammal doesn’t?

References

Benoit, J., Fernandez, V., & Botha, J. (2026). The first non-mammalian synapsid embryo from the Triassic of South Africa. PLOS One, 21(4), e0345016. https://doi.org/10.1371/journal.pone.0345016

Botha, J., & Smith, R. M. H. (2007). Lystrosaurus species composition across the Permo-Triassic boundary in the Karoo Basin of South Africa. Lethaia, 40(2), 125–137. https://doi.org/10.1111/j.1502-3931.2007.00011.x

‌Karoo Palaeontology. (n.d.). Iziko Museums. https://www.iziko.org.za/collection/karoo-palaeontology/

The. (2026, April 15). 2026–04 — Ancient survivor reveals its secret — Wits University. Wits.ac.za. https://www.wits.ac.za/news/latest-news/research-news/2026/2026-04/ancient-survivor-reveals-its-secret.html