Fossil amphibians found in caves where they waited for the next rainy season

Two hundred and thirty million years ago, in what is now Wyoming, the seasons were dramatic. Torrential rains would lash the region for months, and when the megamonsoon ended, the region became extremely dry. This weather would have been challenging for amphibians that need to keep their skin moist, but a group of salamander-like creatures found a solution, as evidenced by their bizarre fossils.

In a new study in the journal Proceedings of the Royal Society B, researchers describe a new species of fossil amphibian, preserved in torpedo-shaped burrows where they waited out the dry season.

“Based on how the rocks in the area were formed and what they’re made of, we can tell that Wyoming experienced some of the most drastic seasonal effects of the megamonsoon that affected the entire supercontinent of Pangea,” said Cal So, the study’s lead author and a future postdoc at the Field Museum in Chicago.

“So how did these animals stay moist and prevent themselves from drying out during the hot, 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 have interpreted as caves.”

Cal So, who recently earned their Ph.D. from George Washington University, first encountered the strange fossil caves as a student at the University of Wisconsin while working with researcher David Lovelace of the University of Wisconsin Geology Museum.

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

“One of my passions is ichnology—the hidden biodiversity that can be shown through the tracks of animals or the tracks of other living organisms,” says Lovelace.

He discovered a small cylindrical structure and several larger ones that looked like a Pringle can, made of stone. Lovelace recognized the structures as filled burrows made by an animal long ago, but one little one stood out.

“It was small, it was so cute,” he says. He collected several of the cylinders for his research.

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

“I saw sharp, pointy teeth, and my first thought was that it was a baby crocodile,” says Lovelace. “But when we put it all together and prepared it, we realized it was some kind of amphibian.”

Lovelace reached out to Jason Pardo, a postdoctoral researcher at the Field Museum who specializes in fossil amphibians, who made high-resolution CT scans of another of the fossil burrows and revealed a small skeleton inside.

“At this point we were like, ‘Oh my gosh, we’ve got something really cool,'” Lovelace says. “I went back to piece together the geologic history of the site, and then we just found these caves everywhere. We couldn’t not find them; the site was ridiculously loaded.”

On one of his trips home, he sent So, then a bachelor, to collect several of the caves. Ultimately, the team collected about 80 fossil burrows, most of which contained skulls and bones from the ancient amphibians. These bones contained clues to the animals’ lifestyle. No complete skeletons have been found, but based on the partial remains, they were probably about a foot long. They had small, underdeveloped arms, but researchers believe they had a different way of digging their burrows.

“Their skulls have a kind of scoop shape, so we think they used their heads to scoop their way underground at the bottom of a river bed and go through a period of lower metabolism so they could survive the dry season. This is similar to what some modern salamanders and fish do,” says So.

Essentially, the ancient aquatic amphibians spent the rainy part of the year swimming in rivers, but when those rivers dried up, they burrowed headfirst into the muddy riverbed. They spent the dry season underground, in a state somewhat akin to hibernation, until the monsoon returned a few months later and rainwater replenished the rivers.

The fossils that So and Lovelace found happened to be unlucky, as the paths of the rivers changed from year to year. The places where these animals buried themselves were no longer kept moist, so the animals never emerged and instead died in their burrows.

The ancient amphibians lived in what are now the ancestors of the Eastern Shoshone people, with whom the researchers have an ongoing collaboration.

“Our interest is in education, so we met with the Tribal Historic Preservation Officer for the Eastern Shoshone and he connected us with the schools,” says Lovelace. “It was a great multi-generational collaboration. We invited seventh grade students from Fort Washakie School, their teachers and elders out into the field with us. The elders told us about their understanding of the rocks and their history on the land, and the students had to find caves and bones.”

The middle school students are learning the Shoshone language, and they worked with the elders to create a name for the Shoshone fossil amphibian: Ninumbeehan dookoodukah.

In their paper, the researchers explained, “‘Ninumbee’ is the name of the mountain-dwelling Little People who hold an important place in Shoshone culture (among others), -he is the possessive affix indicating an association with Ninumbee, ‘dookoo’ means ‘meat’ and ‘dukah’ means ‘Eater.’ is to pay tribute to the Eastern Shoshone people, their language and the land they belong to.”

“The collaboration between our school district (Fremont County School District # 21) and Dr. Lovelace and his team illustrates reciprocity in action and the long-term transformational impacts that can occur through authentic relationship building between researchers and communities,” said Amanda LeClair-Diaz, Office of Indian Education Coordinator and a co-author of the paper.

“This process of scientists, community members, educators, middle school students and Eastern Shoshone elders coming together to learn about these fossils and choosing a Shoshone name for the fossil, Ninumbeehan dookoodukah, strengthens the intergenerational connection we as Shoshone people have to our homeland and the beings that exist in this environment.”

Ninumbeehan is giving scientists a tantalizing clue about what life was like in Wyoming 230 million years ago. “Small amphibians are really rare in the Triassic, and we don’t know why,” says Pardo. “We find some big ones, but these little ones are really quite challenging to find.”

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

“Modern amphibian diversity is under significant threat, and climate change is a big part of that,” says Pardo. “But the way Ninumbeehan could slow down its metabolism to wait out the dry weather indicates that some lineages of modern amphibians that have similar seasonal behavior might allow for greater survival than some of the models suggest. It’s a small glimpse of hope.”