250 million years ago, an injured amphibian stumbled upon a hole in the ground. Seeking shelter from the heat and elements, it crept into what turned out to be the home of an ancient mammal-like reptile. Soon after, a flash flood stormed the land and the two unlikely roommates met their demises together, preserved in stone for an unsuspecting scientist to happen upon million of years later.
That scientist was Vincent Fernandez, a researcher at the University of the Witwatersrand in South Africa. Fernandez worked with a number of other researchers from around the world to determine what exactly were the contents of this peculiar piece of ancient history, a sandstone “burrow cast,” from South Africa’s Karoo Basin.
In order to avoid destroying the fossils, the scientists chose to use x-rays to scan the cast for its contents. Images of the sandstone, taken at the European Synchrotron Radiation Facility in France, showed two complete skeletons in the burrow: a cynodont—an early ancestor of mammals—Thrinaxodon liorhinus, and an aquatic amphibian, Broomistega putterilli.
Broomistega had seven broken ribs that the research team determined broke when the bone was fresh, indicating that the amphibian probably got injured outside, lived with the injury for several weeks, and then and crawled into the closest shelter it could find.
X-ray imaging shows the cynodont Thrinaxodon liorhinus (in brown) sharing its burrow with the early amphibian Broomistega putterilli (in gray)
That happened to be the burrow Thrinaxodon called home. This cynodont is a known burrower, and organisms related to it have been known to enter into a state of dormancy called aestivation—in which an organism is inactive and their metabolic rate slows down. Thrinaxodon acquired this adaptation to escape the harsh climate characteristic of southern Africa during the period after the Permian-Triassic mass extinction. Burrowed deep underground, Thrinaxodon could remain inactive with a lowered body temperature, and avoid having to search for scant resources for months at a time by being dormant.
The way in which the organisms were preserved—their skeletons intact, in natural body positions, and with evidence of soft tissue—suggests that the burrow was filled rapidly and the two were buried and died quickly.
The technology used by Fernandez’s team allowed them to see the two organisms’ anatomies separately; Thrinaxodon liorhinus (left), and Broomistega putterilli (right).
The study concludes that the two creatures were probably in a commensal relationship, in which one organism benefits while the other is unaffected. The scientists ruled out the possibility that the amphibian was washed into the hole by the flood—the opening was too small for such a random event to occur. The two weren’t in a prey-predator relationship because both specimens are whole and don’t show any signs of attack from each other. Thrinaxodon allowed Broomistega to stay as a houseguest, which provided Broomistega with protection but had little to no affect on Thrinaxodon—kind of like an ancient system of couchsurfing.
This study was published June 21, 2013 in the journal PLOS ONE.
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