An amphibian and a precursor to mammals who shared a nest 250 million years ago have recently been discovered during X-ray scans of a fossilized burrow recovered in the Haroo Basin of South Africa.

The two animals were found together in the same burrow, sharing a nest as they drowned during a flash flood.

One day, around 250 million years ago, a young member of a species of amphibian called Broomistega, handicapped from a number of broken ribs. went into the burrow of an animal known as Thrinaxodon, a mammal-like reptile. The injury to the amphibian was likely sustained from one brute force event a while before the animal died, as the ribs had begun to heal.

The storm which killed the unlikely pair may have been part of the severe climatic conditions which followed the Permo-Triassic mass extinction around that time. Most of the life on earth was wiped out, and animals like the Thrinaxodon adapted to digging underground to avoid the inclement weather.

An international team of researchers from South africa, Australia and France were scanning a fossilized burrow when they discovered the remains of the animals.

An X-ray study of the burrow reveled the primitive mammal-like being first, and later, the amphibian with whom it shared a den.

"While discovering the results we were amazed by the quality of the images. But, the real excitement came when we discovered a second set of teeth completely different from that of the mammal-like reptile. It was really something else," Vincent Fernandez from Wits University in South Africa said.

Although co-habitation between different species is seen today, it is usually based on either a small guest being too little for a main host to bother with, or a larger visitor who helps protect a smaller den-owner from predators. In this case, neither explanation seems likely. Short periods of dormancy called aestivation may have been practiced by the Thrinazodon as an adaptation to climate change.

"It's a fascinating scientific question: what caused the association of these two organisms in the burrow? One of the more obvious possibilities is a predator-prey interaction, but we inspected both skeletons looking for tooth marks or other evidence implying predation, ultimately finding no support for one having attempted to feed on the other," Kristian Carlson of Wits University, co-author of the paper announcing the results, said.

These two are now the world's oddest, oldest couple.