At the time, Adamy, a geologist with the Brazilian Geological Survey (known by its Portuguese acronym, CPRM), was working on a general survey of the state of Rondonia in the Amazon. He eventually found his way to a large hole on a forested slope a few miles north of the Bolivian border after asking around.
Adamy couldn’t thoroughly examine the cave during that first visit because he couldn’t get in touch with the landowner. However, an initial examination indicated that it was not the result of any organic geological process. He had visited other neighboring caves created by water in the same geology that underlies this specific hillside. This wide, round passage with a smooth floor was nothing like the caves that once existed.
Adamy remarked, “I’d never seen anything like it before,” and vowed to go back someday to take a closer look. It truly caught my interest. It appeared unnatural.
Some years prior, approximately 1,700 miles southeast, a different Brazilian geologist discovered an identically strange cave. A construction site in the town of Novo Hamburgo was passed by Heinrich Frank, a professor at the Federal University of Rio Grande do Sul, as he was speeding down the highway on a Friday afternoon. There he saw a strange hole in a bank where half of a hill had been eaten away by excavators.
Given the lack of such a sight in the local geology, Frank returned a few weeks later and crawled inside. There was only one shaft, roughly fifteen feet long, and when he was lying on his back, he discovered what appeared to be claw marks all over the ceiling. After failing to find any geologically plausible explanation for the cave’s existence, he came to the conclusion that it was a “paleoburrow,” which he believes was excavated by a giant ground sloth species that is now extinct.
Frank remarks, “I had no idea that paleoburrows existed.” “As a professor and geologist, I’d never even heard of them.”
Rise of the Burrow
Actually, very few burrows associated with extinct megafauna had been documented in the scientific literature until the early 2000s. That’s particularly odd since Frank became enamored with burrows and started discovering them in large quantities following his accidental discovery in Novo Hamburgo.
For instance, when Frank and his students surveyed a 45-mile stretch of newly constructed highway close to Porto Alegre, they found paleoburrows in more than 70% of the road cuts. Many of them are completely filled with sediment, but they are still easily visible, resembling dark, round knots in a bank of dirt. Some, like the one that initially caught Frank’s interest, are still open.
Frank eventually found a suitable opening and squeezed through an elliptical shaft that was about 65 feet long, four feet wide, and covered in claw marks. He estimated that the original burrow was roughly 250 feet long, not including the twists and turns that it undoubtedly once contained, by extrapolating from the original size of the hill that was cut away for the highway.
According to Frank, “there is no geological process in the world that produces long tunnels with claw marks on the walls, that branch and rise and fall, and that have an elliptical or circular cross-section.” “I’ve also seen dozens of caves with inorganic origins, and it’s very evident that digging animals had no part in their creation in these cases.”
Frank has found at least 1,500 paleoburrows in his native state of Rio Grande do Sul, in the extreme south of Brazil. He has located hundreds more and counting in Santa Catarina, which is directly to the north.
Because of how much these burrows resemble ancient animal dens, he says, “sometimes you get the feeling that there’s some creature waiting around the next curve.”
The First in the Amazon
Amilcar Adamy of the CPRM didn’t get the chance to visit that peculiar cave in Rondonia again until 2015. Granted, it’s not the coolest part—it turned out to be the first paleoburrow found in the Amazon. It also turned out to be one of the longest ever measured, with 2,000 feet or so of branching tunnels combined. An estimated 4,000 metric tons of dirt and rock were excavated from the hillside to create the burrow; the main shafts, which have since been widened by erosion, were originally more than six feet tall and three to five feet wide.
Adamy claims, “This wasn’t made by one or two people.” It was created by many, over many generations, according to Frank, who calls it an exciting discovery that is not very surprising.
He claims, “We knew that burrows this big could exist.” “This massive one in Rondonia just serves as proof that they are real.”
Frank has discovered burrows in Rio Grande do Sul that were once several hundred feet long. In another burrow located far to the north in the state of Minas Gerais in the Gandarela Mountains, more than 1,000 total feet of tunnel have been measured. Frank has heard of one burrow in Santa Catarina that is longer than 3,000 feet, but he hasn’t looked into it yet.
Frank thinks ground sloths were the ones who dug the largest burrows, which could have a diameter of up to five feet. He and his colleagues are considering the possibility of studying the fossil remains of several genera that were once found in South America and whose adaptation suggests further research, including the massive, several-ton Lestodon, Glossotherium, and Catonyx. Others think that even the largest burrows were made by extinct armadillos, like Pampatherium, Holmesina, or Propraopus, despite their smaller stature than that of sloths.
Nevertheless, Frank and his colleagues are still at a loss as to how to account for the sheer size of the burrows. The burrows are much bigger than what would be required to protect the animals that dug them from weather or predators, regardless of whether armadillos or prehistoric sloths are to blame.
The largest surviving member of the family, the giant armadillo, is found throughout most of South America and weighs between 65 and 90 pounds. Its burrows can reach a length of 20 feet and a diameter of just 16 inches.
“So what would dig one five feet wide and 250 feet long if a 90-pound animal alive today digs a 16-inch by 20-foot borrow?Frank enquires. “Nothing explains it; not humidity, not climate, not predators. Really, I have no idea.
Since animals stop digging burrows when they become extinct, dating the burrows is also at best conjecture. But they had to have been excavated between 8,000 and 10,000 years ago, when the giant ground sloths and armadillos of South America went extinct. It is not yet known when sediments washed in, but dating organic material discovered in burrow sediments would indicate when the burrow was dug. Frank adds that although it hasn’t been tested yet, speleothems, or mineral deposits, growing on burrow walls may be utilized to determine an age.
The peculiar geographic distribution of paleoburrows is another puzzling finding. Although they are widespread in the southern Brazilian states of Rio Grande do Sul and Santa Catarina, they are presently virtually unknown in Uruguay, even though some of the earliest known specimens were discovered in Argentina, which is even farther south. Similar to this, not many have been discovered in Brazil further north, and Frank is only aware of a very small number of potential burrows in other South American nations.
Because he happens to live and work in the heart of burrow country, he doesn’t believe he is biased. Frank’s colleagues have done extensive searches in other parts of Brazil, but their results have been largely negative. He has also conducted some detective work by looking through other people’s cave photos on Google. He often recognizes paleoburrows in the south of Brazil from details inadvertently captured in photographs, such as a grinning group of Brazilian boy scouts posing in front of a wall of a cave pocked with claw marks. People share photos of caves they have visited on social media in other parts of the nation and continent, but almost none of them appear to have been originally excavated by animals.
A South American Thing
You won’t find paleoburrows here, despite the fact that giant ground sloths and armadillos once roamed North America.
“It’s possible that we haven’t noticed them here,” says paleontologist Greg McDonald of the Bureau of Land Management, who specializes in studying extinct South American sloths. “Alternatively, it’s possible that they were present here but were unable to thrive for an extended period of time due to improper soil conditions.”
The lack of paleoburrows also raises unanswered questions. Dasypus bellus, the magnificent armadillo, is extinct. It was roughly twice as large as the nine-banded armadillo of today. It was common in Pleistocene North America and shared many forelimb morphologies with modern armadillos, which love to burrow. Beautiful amardillo remains are frequently found in caves, but not ones scientists have ever thought were actually dug by the animal.
McDonald acknowledges another possibility: paleoburrows may exist in North America but have received little attention, similar to what happened in South America until recently.
“There are things that are so commonplace that people just take them for granted,” he says. “And nobody will know until someone who is at least mildly curious looks more closely and asks, ‘What’s forming these?'”
There’s a long list of research projects to design for the few scientists in South America studying paleoburrows, all centered around the same fundamental questions: who, why, what, where, and when?
Frank has been studying paleoburrows for the past ten years, and describing patterns that have emerged from his observations is at the top of his priority list. While some are straightforward shafts, others are intricate feats of subterranean construction with branching tunnels that rise, fall, and twist to form a network with multiple entrances. Every now and then, a few of them reveal much larger chambers. There are comparatively few of them. And then there are the big ones.
“We must identify the patterns. We’re beginning to get a better understanding of this,” says Frank. “And from there, it will be easier for us to deduce the kinds of animals that were digging them.”