Ancient Embrace: Amphibian, Mammal Ancestor Buried Together

An injured amphibian and a sleeping cynodont, a mammal-like reptile, shared a home before being trapped by a flash flood 250 million years ago, a new study says.

The unlikely cohabitants were found in a fossilized burrow in South Africa‘s Karoo Basin, an international team of scientists reported June 21 in the journal PLOS ONE. (Read about the “Stone Age Embrace”—a family buried together in the Sahara.)

reptile and mammal ancestor picture
3-D images show the mammal ancestor (larger) and the amphibian huddled together. Image courtesy ESRF/ V. Fernandez

The team used high-energy x-rays from the European Synchrotron Radiation Facility (ESRF) in France to look inside the burrow without destroying it. The scans first revealed the fossilized remains of a mammal forerunner called Thrinaxodon, a type of cynodont.

freshwater species of the week“But the real excitement came when we discovered a second set of teeth completely different from that of the mammal-like reptile,” lead author Vincent Fernandez said in a statement. Those teeth belonged to a juvenile amphibian named Broomistega.

Next, the researchers focused on an explanation for their unusual find, testing several possible scenarios. Through scientific detective work, they attempted to reconstitute the events that could have led to two unrelated vertebrate animals becoming entombed together in a single burrow.

Scientific Sleuthing

The fossilized burrow dates to the beginning of the Triassic Period, when what’s now South Africa was located on the supercontinent Gondwana. (See a prehistoric time line.)

reptile and mammal ancestor scans
A traditional laboratory microtomograph (left) is seen next to x-ray images obtained at the ESRF. Images courtesy ESRF/ V. Fernandez

At that time, the world was still recovering from a mass extinction that wiped out most life on Earth. It was a time of pronounced climatic warming and monsoons. Many animals, including the ancestors of modern mammals, learned to dig underground burrows to take shelter from the harsh environment.

The Thrinaxodon was found fossilized in a curled-up position, which is typical of other fossilized specimens of the animal. The scientists believe this means the animal not only burrowed, but also went into periods of dormancy—called aestivation—to survive the hostile conditions. This could explain why the amphibian was not chased out of the burrow. (Also see “Dinosaurs Went Underground to Wait Out Extreme Weather.”)

The scans revealed the amphibian had several broken ribs that showed signs of healing, indicating it had survived for some time after a traumatic injury. Since it was a juvenile, the animal was mostly aquatic and could not dig its own burrow.

reptile and mammal ancestor picture
A 3-D image shows a mammal-like reptile nestled with an aquatic amphibian. Image courtesy ESRF/V. Fernandez

Other theories were ruled out. For instance, it’s unlikely that the amphibian was washed into the burrow during the flood due to the small size of the burrow entrance. Another possibility is that one preyed on the other, but careful examination turned up no evidence of tooth marks on either animal’s bones.

Piecing these clues together, the researchers think the handicapped amphibian crawled into the burrow seeking protection without disturbing the dormant Thrinaxodon within.

Tell us—what other animal odd couples have you come across?


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Meet the Author
Mary Bates is a freelance science writer living in Boston. She has a PhD in psychology from Brown University where she studied bat echolocation. You can visit her website at and follow her on Twitter at @mebwriter.