Fossil skull of the newly described amphibian. Credit: Photo by David Lovelace.

Scientists believe that fossil amphibians found in their burrows were waiting for the next rainy season when they died.

The salamander-like creatures have been given a local Shoshone name by Indigenous students in Wyoming: Ninumbeehan dookoodukah.

Two hundred and thirty million years ago, in what’s now Wyoming, the seasons were dramatic; torrential rain would pelt the region for months on end, and when the mega-monsoon ended, the region became extremely dry.

This weather would have been challenging for amphibians that need to keep their skin moist, but one group of salamander-like creatures found a solution, as evidenced by their bizarre fossils.

In the new study, researchers described a new species of fossil amphibian, preserved in torpedo-shaped burrows where they waited out the dry season.

Study lead author Cal So, who is an incoming postdoctoral scientist at the Field Museum in Chicago, said: “Based on how the rocks in the area formed and what they’re made of, we can tell that Wyoming experienced some of the most drastic seasonal effects of the mega-monsoon that affected the whole supercontinent of Pangea.

“So how did these animals stay moist and prevent themselves from drying out during the hot and dry season that lasted several months? This is the cool thing. We find these fossils inside these cylindrical structures up to 12 inches long, which we’ve interpreted as burrows.”

An illustration from the paper, depicting Ninumbeehan digging a burrow in a riverbed for the dry season and then re-emerging when the monsoon returned. Credit: Copyright: Gabriel N Ugueto.

Serendipity Beds

So, who recently obtained their PhD from George Washington University, first encountered the strange fossil burrows when they were an undergraduate at the University of Wisconsin, working with research scientist David Lovelace of the University of Wisconsin Geology Museum.

In 2014, Lovelace was searching for fossils in Wyoming, in an area stewarded by the Bureau of Land Management in a rock layer he would eventually call the Serendipity Beds.

Lovelace said: “One of my passions is ichnology – the hidden biodiversity that can be shown through tracks of animals or traces of other living organisms.

He spotted a small cylindrical structure, and several larger ones that looked “like a Pringle can” made of rock.

Lovelace recognised the structures as in-filled burrows made by an animal long ago, but a small one stood out. (“It was tiny, it was so cute.”) He collected several of the cylinders for his research.

Back in the lab, Lovelace took a hammer to one of the preserved burrows to see if there were any fossils inside, and found a tiny, toothy skull.

He said: “I saw sharp, pointy teeth, and my first thought was that it was a baby crocodile, but when we put it all together and prepared it, we realised it was some sort of amphibian.”

Cal So and Adam Fitch using a rock saw to excavate fossil burrows. Credit: Photo by Hannah Miller.

Clues

Ultimately, the team gathered around 80 fossil burrows, most of which contained skulls and bones of the ancient amphibians. These bones contained clues to the animals’ lifestyles.

No complete skeletons have been found, but based on the partial remains, they were probably about a foot long.

They had tiny, underdeveloped arms, but the researchers think they had another way to dig their burrows.

So said: “Their skulls have kind of a scoop shape, so we think they used the head to scoop their way underground at the bottom of a riverbed and go through a period of having a lower metabolic rate so that they could survive the dry season.

“That’s similar to what some modern-day salamanders and fish do.”

Essentially, the ancient, aquatic amphibians spent the rainy part of the year swimming in rivers, but when those rivers dried up, they dug head-first into the muddy riverbed.

They spent the dry season underground, in a state somewhat similar to hibernation, until the monsoon returned a few months later and the rainwater replenished the rivers.

The fossils found by So and Lovelace just happened to be unlucky in that the rivers’ paths changed from year to year.

The spots where these animals buried themselves were no longer kept moist, so the animals never emerged and instead died in their burrows.

Ninumbeehan offers scientists a tantalising clue about what life was like in Wyoming 230 million years ago.

The newly described amphibians also could shed some light on how modern amphibians might fare in the extreme weather conditions brought on by the climate crisis.

The study is published in Proceedings of the Royal Society B.