The Odd Couple: Amphibian and Mammal-like Reptile Shared Burrow 250 Million Years Ago

Many species work together in order to survive. For example, ants defend the acacia tree while the plant provides shelter. Now, though, scientists have found another relationship with animals that are millions of years old. They've discovered a 250 million year old fossilized burrow that reveals how a mammal forebear and an amphibian cohabited one location.

The fossil itself was actually recovered from sedimentary rock strata in the Karoo Basin. The animals and the burrow existed at the beginning of the Triassic period, a time when the ecosystem was still recovering from the Permo-Triassic mass extinction event that wiped out most of the life on Earth. Mammals were partly able to survive this event due to their burrowing behavior, which allowed them to escape surface environmental conditions.

Actually examining this fossil was no easy task, though. There are many fossilized burrows that have been discovered in the area but until now, these fossils hadn't been X-rayed. Now, scientists have taken a closer look at the fossils. Two in particular were scanned to see if they had animal remains. The results were striking.

The scientists found the skull of a mammal-like reptile called Thrinaxodon, an animal previously reported in another burrow. They also found the reptile accompanied by an amphibian Broomistega, which belongs to the extinct group of Temnospondyl. The two skeletons were extremely well preserved and allowed the scientists to see that the amphibian was suffering from broken ribs that resulted from a single, massive trauma. Yet these ribs were healing, which showed that the creature had been living with the injury for quite some time--though most likely it was quite handicapped.

"Burrow-sharing by different species exists in the modern world, but it corresponds to a specific pattern," said Vincent Fernandez, the lead author of the paper, in a news release. "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."

It's very likely that the amphibian crawled into the burrow after being wounded. Yet that doesn't explain why the mammal-like reptile didn't evict the intruder. The scientists examined the fossils a bit more closely and then found out what may have led to this odd couple.

The reptile was probably aestivating at the time. This particular condition is a state of torpor that results from harsh conditions, such as aridity and the absence of food. Essentially, the creature can slow its metabolic processes down to the point where it can survive for long periods of time without food and water. When conditions improve, the animal can "wake up" and resume normal activities. Yet since the reptile was incapacitated, the amphibian took advantage of the burrow and used it to try to recover from its injury. That recovery process was cut short, though. It's likely that a flood trapped the two in the burrow.

The findings are hugely significant for researchers, revealing that these new scans can provide significant information. The scientists are continuing to scan these fossilized burrows, learning more about the creatures that once lived inside them.

The findings are published in the journal PLOS One.