The Planet's Oldest Roommates

In nature, most of the time, species keep themselves to themselves. While certain creatures do interact or even rely on each other to exist, it is usual for different organisms to occupy different ecological niches and not encroach on another's territory. This rather segregated lifestyle is even seen with individual species. Male lions for example are legendary for defending vast areas of land. Herd-based animals will remain within their family groups rather than hop between communities.

A 3D model showing the positions of the skeletons of
Broomistega and Thrinaxodon within the 250 million
year old fossil burrow

Yet this can all change under extreme circumstances. In Australia, predator will run alongside prey when escaping wildfires. Indeed, it is almost as if the spirit of altruism and cooperation descends upon the animal world during disasters.

Now an example of this behavior has been found in the fossil record, hundreds of millions of years ago. The Permian Great Dying was the greatest mass extinction in the history of the planet, decimating around 80% of all life on Earth 250 million years ago.

Volcanic eruptions on a global scale, wildfires, earthquakes and climate change made the planet a harsh place to live on. Yet this fossil discovery from the Karoo Basin in South Africa has shown how animals may have dealt with the inhospitable conditions facing them. An international team of palaeontologists analysed a 250 million year old fossil burrow at the European Synchrotron Radiation Facility (ESRF). Their results showed something amazing: at the end of the burrow lay the skeleton of a mammal ancestor, a therapsid reptile known as Thrinaxodon.

However, it was not alone. Next to it was the remains of an amphibian, Broomistega. The sediments from that Karoo basin show that they were derived from a flash flood, the event which led to the preservation of the burrow and its inhabitants. Scans of the amphibian revealed that some of its ribs were broken, which most likely led to it seeking shelter.

An artist's impression of Broomistega creeping into the
burrow of an aestivating Thrinaxodon

The therapsid, who initially made the burrow, was engaged in a short period of dormancy known as aestivation, in this case most likely a response to the harsh conditions of the Permian Earth. What is interesting is why it did not chase off the amphibian? As aestivation occurs in a burrow to avoid disturbance by other creatures.

'Burrow-sharing by different species exists in the modern world, but it corresponds to a specific pattern,' said Dr Vincent Fernandez from Wits University, South Africa. 'For example, a small visitor is not going to disturb the host. A large visitor can be accepted by the host if it provides some help, like predator vigilance. But neither of these patterns corresponds to what we have discovered in this fossilized burrow. Thanks to the unique possibilities for high quality imaging of fossils developed during the last decade at the ESRF, these unique specimens remain untouched, protected by their mineral matrix,' added Dr Paul Tafforeau from the ESRF.

What is equally important is understanding how prehistoric creatures interacted with each other and how these interactions built food webs, ecosystems and how these links ultimately drove evolution forward. This fossil discovery offers a unique glimpse of not just what creatures were like, but how they lived.