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 fossilized dinosaur eggs, but neither he nor any paleontologist 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 fossilized 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 paleontologist 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.