The science of paleontology relies heavily on fossils. This statement is a bit inane, as paleontology is defined as the study of prehistoric life, broadly covering anything that lived before the Holocene epoch (our current age, which stretches to a point a little more than 10,000 years ago). Obviously, to glean anything about organisms that don’t currently inhabit the planet will require evidence that persists from the past. The etymology of fossil, however, doesn’t have anything to do with Latin words for “ancient” or “bone” (the mental image a fossil conjures most readily). The Latin term that gives us dino bones is fossus, the past participle of fodere, which means “to dig.” Makes sense; gotta dig to find dem bones.

Not just the domain of buried bones, this Latin root applies to another realm of the natural world. The term “fossorial” comes from fossor, which translates to “digger.” Many of Earth’s animals are fossorial, meaning they spend much of their time below the surface in chambers they have excavated or overtaken. This classification can apply to a wide array of critters. Think salamanders, clams, prairie dogs, or the bane of many lawn-owning humans, moles. According to scientist Heinrich Frank, nearly half of all mammal species alive today can be classified as semi-fossorial, which means they spend a significant portion of their time underground but need to venture into the wild to find food.

The nature of evolution is such that most organisms inherit the genetics, traits, and practices of their predecessors, incorporating changes on a slow, orderly track. Mutations and jumps occur but rather infrequently. A prairie dog, for example, might develop a radical mutation that alters its fossorial lifestyle, but most likely the generations will continue to live underground, slowly evolving in non-perceptible ways. It follows, then, that the portion of the mammalian population that counts “fossorial” as one of their traits also had ancestors that lived in burrows. This roundabout description is a way to illustrate that scientists believe many old mammals likely lived underground, just as their descendants today.

A cape ground squirrel - photo by Amada44
Molehills - photo by Rasbak

As paleontology developed through the 19th and 20th centuries, evidence of ancient animals burrowing underground remained elusive. In some sense, this fact isn’t surprising. If the moles who formed the hills pictured above move out of the area, what will happen to the tunnels they crafted? Over time, the natural machinations of the planet and the nature of the original medium (soil) will lead to the disappearance of the tunnels. Rain will cause the collapse of the structures, which will eventually fill with soil until no evidence of their existence remains. Still, the ephemeral nature of certain structures has not precluded them from preservation in the fossil record. For example, we have evidence of dinosaur nests. Proof of old burrows, however, seemed not to exist.

In the 1930s, in Brazil, researchers discovered moderately sized caves around the country. At the time, these structures seemed to belong more to the branch of archaeology than paleontology. They did not seem to be geologic formations, so scientists theorized they had been excavated by Indigenous peoples in the past, perhaps to hide from European colonists.

This notion persisted throughout the 20th century. In 2010, geologist Amilcar Adamy heard rumblings about a bizarre cave in the Brazilian state of Rondonia. He visited the site, looking to explain its formation. Talking to Discover Magazine, he said, “I’d never seen anything like it before. It really grabbed my attention. It didn’t look natural.”

The cave explored by Adamy - photo by Amilcar Adamy

Heinrich Frank began exploring other similar caves. To the scientists, it was clear that no geological process had produced these hollows. Frank told Discover Magazine, “There’s no geological process in the world that produces long tunnels with a circular or elliptical cross-section, which branch and rise and fall.”

But what had created them? No evidence of human habitation seemed to accompany these structures.

In one tunnel, Frank looked up, and the answer arrived.

The walls of a tunnel in Brazil - photo by Heinrich Frank

Etched into granite, basalt, and sandstone were long striations. And they had not been produced by human tools.

Frank and Adamy had stumbled across the claw marks of some gargantuan creatures.

Sometime in the distant past, megafauna had dug these tunnels with their own bodies!

A paleoburrow snapped by Heinrich Frank
A close-up of claw marks - photo by Michael Fox/The World

These diggers were not your backyard moles. Researchers believe the largest tunnels were chiseled by a genus of extinct giant ground sloths: Lestodon. Fossils of these sloths have been unearthed all over South America. They likely stretched 15 feet in length and weighed more than four tons.

Despite the fearsome size and apparent clawing power of this creature, they were herbivores.

Lestodon armatus - photo by Ghedoghedo
The foot of a Lestodon - photo by Ghedo

Sloths weren’t the only ones digging in ancient Brazil. Species of giant armadillos apparently could also excavate smaller tunnels.

Once the researchers knew what they were looking for, the paleoburrows began to show up all over Brazil. To date, more than 1,500 massive systems have revealed themselves. One of them is 100 meters long (300 feet). Some of them branch into multiple rooms.

The paleoburrow architects - graphic by Renato Pereira Lopes et. al.

What compelled the sloths and armadillos to dig these tunnels, especially if the larger ones would take a lifetime or more to craft? After all, clawing through granite would be no quick task. Researchers are unsure but speculate they may have been generational homes for sloth families.

No matter what the reason for their creation, scientists finally had solid proof of fossorial megafauna from the past. The bones of the giant sloths of South America can officially be termed fossorial fossils.

So, the next time someone you know complains about burrowing moles in their yards, be sure to remind them it could be a lot worse. Just imagine the damage a massive-clawed sloth could do to easy-to-sculpt soil if they could carve out huge swaths of rock!

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