Scientists have proved for the first time that mammalian ancestors laid eggs

It is about an important stage in the evolution of the animals from which humans later evolved.

Details

Fossilia was discovered in South Africa in 2008, but its significance has only now become clear thanks to modern scanning techniques.

Scientists have studied the embryo of the ancient animal Lystrosaurus, which lived about 250 million years ago. The egg itself was not preserved, so for a long time it was unclear how exactly the cub developed.

The key to the clue came from anatomy. The embryo had an unformed lower jaw - a characteristic sign of the early stage of development in animals that are still inside the egg.

This made it possible to conclude that the cub died before hatching, and therefore developed in the egg.

Thus, the researchers have for the first time direct evidence of egg-laying in mammalian ancestors.

Why it matters

The discovery clarifies how modes of reproduction evolved in animals, including mammals.

In addition, Lystrosaurus lived during the largest mass extinction and managed to survive. Scientists believe that breeding characteristics may have played a role in this - for example, allowing for faster population recovery.

Background

Therapsids are an ancient group of animals from which mammals later evolved. Until now, there has been no direct evidence as to whether they laid eggs or were already giving birth to live cubs.

Source

The study is based on the analysis of a Lystrosaurus embryo using synchrotron scanning. The work was published in the journal PLOS One (2026).