‘Odd couple’ fossils found in Karoo

The discovery of fossils has helped scientists piece together the history of our planet, and towards a better understanding of where we came from. We can tell quite a lot about how an organism lived and died by what remains of it under the earth, and the most interesting fossil discoveries tell the best stories.

Last month, palaeontologists from Wits University used a new X-ray scanning process to uncover fossilised remains from the Karoo – and found two unrelated species sharing the same burrow. One was an early mammal-like reptile, and the other was an amphibian, and the two had died curled up next to each other under the ground, 250-million years ago.

The odd couple, a Thrinaxodon and Broomistega, are both species previously known to science – but the unusual behaviour their fossils suggest are certainly a world first. Dr Vincent Fernandez of Wits University explains: “Burrow-sharing by different species exists in the modern world, but it corresponds to a specific pattern. 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 fossilised burrow.”

Times were tough during the Triassic period, when these two species were alive. The Earth had just experienced a massive extinction event, in which most of the land and marine animals and insects had died out, and species had to adapt fast to survive the harsh conditions. Digging burrows was one such way to do this, and the Thrinaxodon, a mammal-like furry reptile, had been sheltering inside when it received an unexpected guest. The X-ray scans suggest that the amphibian Broomistega had been injured, and was probably taking shelter in the burrow. It wasn’t chased away or attacked because Thrinaxodon was in a state of aestivation – a kind of hibernation, in response to arid conditions and lack of food resources.

“[Aestivation] was a key adaptation response together with a burrowing behaviour which enabled our distant ancestors to survive the most dramatic mass extinction event,” says Professor Bruce Rubidge, co-author of the paper and director of the Palaeosciences Centre of Excellence at Wits University. “This state of torpor explains why the amphibian was not chased out of the burrow.” A flash flood buried the two creatures in the burrow for the next 250-million years, only to be rediscovered in June 2013.

Thrinaxodon was neither a reptile nor a mammal, but an intermediate and unique animal from which mammals evolved, are thus is important for explaining the origins of humankind too. Improvements in technology are helping to uncover incredible discoveries such as this – a story that could not have been told in such detail were it not for the use of a synchronotron. Synchrotron X-rays are generated from high-speed electrons racing around the ring of a particle accelerator, and concentrated into a hair-thin beam. The X-rays are so powerful that they can even illuminate some of the chemical elements that fossils are made up of, such as calcium or phosphorus.