Embryo Fossil Found in South Africa Is World’s Oldest Proof That Mammal Ancestors Laid Eggs

Between 280 and 200 million years ago, a group of animals evolved which would eventually give rise to mammals, including humans: the therapsids. They were first described more than 150 years ago, based on fossils from South Africa. Since then, many more fossils have been discovered.

James Kitching, one of the most talented South African fossil hunters of the 20th century, excavated many thousands of therapsids from the rocks of the Karoo (a semi-arid region of the country’s interior). He also found fossilised dinosaur eggs, but neither he nor any palaeontologist after him ever found therapsid eggs.

They should exist, because some mammals (platypus and echidnas) do lay eggs. But Kitching began to doubt that therapsids laid eggs: perhaps, he thought, they were, like most of their mammalian descendants, already viviparous (giving live birth)?

We are scientists who study extinct animals and the environments they lived in millions of years ago to understand more about the evolution of life. In our new paper we describe, for the first time, the embryo-containing fossilised egg of a 250 million-year-old mammalian ancestor.

It finally shows that therapsids were indeed egg-laying (oviparous). This discovery sheds new light on the reproduction and survival strategy of that group of animals.

A 20-year-old mystery

The fossil egg and embryo we described was discovered near Oviston, in the Eastern Cape province of South Africa, by John Nyaphuli, a palaeontologist from Bloemfontein, in 2008. It’s been kept in the National Museum in Bloemfontein. We knew that it belonged to a species that lived 252 million to 250 million years ago called Lystrosaurus, but we didn’t know whether the species was an egg-layer. The adult looked like a pig, with naked skin, a beak like a turtle, and two tusks sticking out and pointing down.

The reason it took 20 years to prove that it had been in an egg is that this fossil preserves no shell. Only a curled-up embryo is visible. If there was a shell, it was likely leathery or had dissolved. Only the most advanced dinosaurs laid hard-shelled eggs.

So how could we find out whether this young creature had once been inside an egg?

The answer to this question lay in the advanced technology of the European Synchrotron Radiation Facility at Grenoble, France. There, we used a powerful X-ray source to image the inside of the bones of the embryo. Under this treatment, the fossil unveiled all its long-kept secrets – most crucially, its stage of development.

We discovered that the lower jaws of its beak were not completely fused. This developmental trait is only found in modern turtles and birds in which jaw bones fuse long before they are born so that their beak is strong enough for the hatchling to catch and crush its food.

This meant that our curled up Lystrosaurus embryo had died in ovo (in an egg), tightly nestled in its soft, leathery eggshell. This was the evidence palaeontologists had been looking for.

Thanks to the synchrotron-assisted examination of its lower jaw, we could finally demonstrate that this embryo was indeed that of an unhatched Lystrosaurus baby.

Famous survivor

What does it unravel about the survival strategy of Lystrosaurus?

Lystrosaurus is a herbivorous (plant-eating) therapsid famous for surviving the “Great Dying”, which was a major mass extinction of species 252 million years ago. During this event, 90% of all living things on Earth died. Life almost ceased to exist, which makes this the second most important event in the history of life on Earth after the origin of life itself.

How Lystrosaurus survived this is still an intriguing mystery, but the egg gives a possible clue. The fossil we describe shows that the animal laid arguably large eggs for its body size. Large eggs are produced by species that feed their embryos with yolk rather than milk. The young develop to an advanced stage in the egg and then they hatch. In contrast, monotremes (the platypus and echidnas), which feed milk to their young, lay small eggs because the baby is fed after hatching. The large size of its egg implies that Lystrosaurus did not feed milk to its young.

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More relevant to its survival strategy, this further indicates two things. Firstly, it means that the egg was less prone to desiccation (drying out). The larger the egg, the smaller its surface area (comparatively speaking), so Lystrosaurus eggs would lose less water through their leathery shell than those of other species of that time. Given the dry environment during and in the immediate aftermath of the extinction, this was a significant advantage, especially since hard-shelled eggs would not evolve for another 50 million years, at least.

Secondly, a large egg implies that Lystrosaurus was likely precocial, meaning that the babies likely hatched at an advanced stage of their development. Lystrosaurus hatchlings were big enough to feed by themselves and run away from predators, and would reach maturity faster so they could reproduce early.

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Growing up fast, reproducing young and proliferating were the secrets of Lystrosaurus survival.

Our ability to identify the fossil egg adds to our understanding of the origin of mammalian reproductive biology and lactation, and the survival strategy of Lystrosaurus in the most devastating biological crisis. This is significant to better grasp how modern species might cope with the current sixth mass extinction of species.

Julien Benoit receives funding from the DSTI-NRF African Origins Platform and GENUS Centre of Excellence in Palaeosciences.

Vincent Fernandez works for the ESRF synchrotron and was awarded beamtime at the ESRF for this experiment.

Jennifer Botha does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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