Cape Town – Scientists from South Africa, Australia and France have discovered a world first association while scanning a 250 million year old fossilised burrow from the Karoo Basin of South Africa. The burrow revealed two unrelated vertebrate animals nestled together and fossilised after being trapped by a flash flood event. Facing harsh climatic conditions subsequent to the Permo-Triassic (P-T) mass extinction, the amphibian Broomistega and the mammal forerunner Thrinaxodon cohabited in a burrow. In the study, which was published last week in the scientific journal Plos One, the scientists describe how they believe the two small creatures came to be together.Scanning shows that the amphibian, which was suffering from broken ribs, crawled into a sleeping mammal’s shelter for protection. This research suggests that short periods of dormancy, called aestivation, in addition to burrowing behaviour, may have been a crucial adaptation that allowed mammal ancestors to survive the P-T extinction. After many impressive results obtained on fossils, synchrotron imaging has led to revived interest in the studies of the numerous fossilised burrows discovered in the Karoo Basin of South Africa and dated to 250 million years ago. The first attempt to investigate one of these burrow-casts surprisingly revealed a world-first association of two unrelated animals. The fossil was recovered from sedimentary rock strata in the Karoo Basin. It dates from 250 million years ago, at the beginning of the Triassic Period. At that time, the ecosystem was recovering from the Permo-Triassic mass extinction that wiped out most of life on Earth. In the Pangea Supercontinent context, what is now South Africa was an enclave in the southern half called Gondwana. It was the scene of pronounced climatic warming and increased seasonality marked by monsoonal rainfall. To survive this harsh environment, many animals, including mammal-like reptiles, developed a digging behaviour, attested by the numerous fossilised burrow casts discovered in the Karoo Basin. These casts have long been thought to enclose fossilised remains, triggering interest from palaeontologists. StudyEarly this year, an international group of scientists started to research the contents of these burrows using X-ray synchrotron computed microtomography. Two burrow casts were selected from the collection at Wits to be scanned using the state-of-the-art facility at the European Synchrotron Radiation Facility (ESRF). Using the unique properties of the X-ray beam which enables non-destructive probing, the scan of the first burrow started to reveal the skull of a mammal-like reptile called Thrinaxodon, an animal previously reported in another burrow. As the scan progressed, the three-dimensional reconstruction displayed results beyond expectations: the mammal-like reptile was accompanied by an amphibian Broomistega, belonging to the extinct group of Temnospondyl. “While discovering the results we were amazed by the quality of the images”, says lead author Dr. Vincent Fernandez, "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”. Besides the pristine preservation of the two skeletons, the team focused on the reasons explaining such an unusual co-habitation. Fernandez 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.” The scientists eventually concluded that the amphibian crawled into the burrow in response to its poor physical condition but was not evicted by the mammal-like reptile. Piecing all the clues together, the team finally elucidated the enigmatic association, concluding that “the mammal-like reptile, Thrinaxodon, was most probably aestivating in its burrow, a key adaptation response together with a burrowing behaviour which enabled our distant ancestors to survive the most dramatic mass extinction event. This state of torpor explains why the amphibian was not chased out of the burrow,” says Rubidge. Both animals were finally entrapped in the burrow by a sudden flood and preserved together in the sediments for 250 million years.