Across Brazil and parts of Argentina, scientists have discovered dozens of strange tunnels dug deep into the earth.
The largest of these tunnels are 2m high, 4m wide, and stretch for more than a kilometre, but none were dug by human hands. Instead, scientists think these tunnels are ‘palaeoburrows’ carved out by some of the largest land mammals ever to live in South America, ground sloths.
These tunnels have puzzled scientists for more than a century. It was long thought they were archaeological structures left by people, but in 2001 Sergio Vizcaíno, head of vertebrate palaeontology at the La Plata Museum in Argentina, proposed they were palaeoburrows excavated by Scelidotherium and Glossotherium - two ground sloths that were once endemic to South America and only faced extinction as recently as 10,000 years ago.
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Vizcaíno and his colleagues pointed to scratch marks on the ceilings of these palaeoburrows as evidence they were made by ground sloths. These scratch marks matched the giant animals’ claws, as did the overall dimensions of the palaeoburrows.
Glossotherium, the larger of the two ground sloths, grew to lengths of more than 3m, stood nearly 2m high, and weighed in at a hefty 1,500kg.
Most of the South American palaeoburrows are found in a particular layer of sandstone belonging to the Botucatu Formation. This package of rocks was deposited roughly 115 million years ago during the Early Cretaceous. At this time, South America was connected to Africa and covered in a vast desert that would have dwarfed today’s Sahara Desert.
The nature of this Botucatu sandstone makes it ideal for constructing giant burrows - it’s soft enough to carve, but not so soft that it slumps, deforms, or collapses.
The largest palaeoburrow so far described is located in the state of Pará in North Brazil. When all its internal branches are combined, the palaeoburrow measures 1,500m in length, which is roughly the length of 15 football pitches stacked end to end.
Vizcaíno has argued palaeoburrows of this immense size might not have been dug by an individual. He has suggested the vast network of tunnels may rather be the work of several animals operating over different periods of time.
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It’s unclear why ground sloths dug such large burrows. Some have suggested they may have been used to hide offspring from predators and/or protect themselves from colder, drier weather during glacial periods. Others have argued against these theories, claiming animals as large and as cold-adapted as ground sloths wouldn’t need to seek such refuge.
While scientists may not agree on why ground sloths dug these giant burrows, they are united when it comes to their conservation. The Armadillo’s Den, one of the best-preserved palaeoburrows in Brazil, has been repeatedly vandalised over the last few decades, with people scratching their names onto walls and ceilings already bearing claw marks and Indigenous inscriptions.
Those who study South American palaeoburrows believe they deserve more worldwide attention. Currently, most scientists working on these palaeoburrows come from nearby institutions; they want to change that by inviting researchers from further afield to visit and share their expertise. In doing so, they hope to gain a better understanding of what created these peculiar structures, how they carved them, and why.
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Top image: Getty
