Unusual Duo Found in 250 Million-Year-Old Fossil

An international team of scientists discovered a world first when scanning a 250 million-year-old fossilized burrow from the Karoo Basin of South Africa: two unrelated vertebrate animals nestled together after being trapped by a flash flood event.

Facing harsh climate conditions subsequent to the Permo-Triassic (P-T) mass extinction, the amphibian Broomistega and the mammal forerunner Thrinaxodon shacked up in an apparent effort to survive by at least one of them.

Scans show that the amphibian, suffering from broken ribs, was the one to make the first, bold move, crawling into the other's shelter for protection.

The research further suggests that short periods of dormancy, called aestivation, may have been a key adaptation in surviving the P-T extinction.

Recovered from sedimentary rock strata, the fossil dates back to a time when its surrounding ecosystem was recovering from the apocalyptic mass extinction that wiped out most of the life on Earth.

At the time, South African was an enclave of the southern half of the Pangea supercontinent where pronounced climatic warming and increased seasonality marked by monsoonal rainfall was taking place.

To survive the harsh environment, many animals, including mammal-like reptiles, developed a digging behavior attested to in the casts discovered in the Karoo Basin.

Long thought to contain fossilized remains, these casts nevertheless remained difficult subjects until, earlier this year, scientists started to research the contents using X-ray synchrotron computed microtomography.

Two burrow casts were selected for scanning using the state-of-the-art facility at the European Synchrotron Radiation Facility (ESRF), which revealed the skull of the Thrinaxodon, an animal reported in another burrow.

As the scan progressed, however, the more surprising discovery of the Broomistega, which belongs to the extinct group of Temnospondyl, was uncovered.

"While discovering the results we were amazed by the quality of the images," said lead author Vincent Fernandez from Wits University, "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."

In addition to the pristine preservation of the two skeletons, the team focused on their research on the reasons behind the unusual duo.

"Burrow-sharing by different species exists in the modern world, but it corresponds to a specific pattern," Fernandez explained. "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".

For this reason, the scientists gathered all the information they could to reconstitute the events leading to the fossil aggregation, testing one scenario after another.

"It's a fascinating scientific question: what caused the association of these two organisms in the burrow?" said Kristian Carlson, also from Wits University. "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."

One thing the group did notice, however, was that that the broken ribs appeared to result from a single, massive trauma that the amphibian had survived from some time before based on evidence that the fractures were healing. Still, the researchers argue, the animal must have been handicapped.

Based on these and other facts, they finally concluded that the hurt animal crawled into the burrow due to its poor health and was not told to leave by its resident who was probably enjoying a relatively dormant state.