MEGABURROWS
South America tunnels were probably dug by giant prehistoric sloths
Heinrich Theodor Frank, a geologist at the Federal University of Rio Grande do Sul in Brazil, was driving through the town of Novo Hamburgo a few years ago when he noticed a strange hole about a metre in diameter at a construction site where excavators had removed half a hill. He was in a hurry to get home and didn’t stop, but a few weeks later he went back and investigated. “I noticed that it was a tunnel, about 70cm [28in] high and a few meters in length. The interior was full of scratches. When I got home I looked for an explanation on the Internet, but I didn’t find anything. Since then I have heard that the tunnels are huge anthills or that they were created by Indians, Jesuits, slaves, revolutionaries and even bears. Some even talk about a great mythological serpent, which dug the tunnels.”
He eventually sent some photographs to Marcelo Rasteiro, a member of the Brazilian Society of Speleology, who responded by sending an article about palæoburrows, tunnels excavated by any type of living organism in any geological age – for example a worm in the Cambrian, a mollusk in the Mesozoic or a rat in the Pleistocene. “I didn’t know there was such a thing as palæoburrows,” said Frank. “I’m a geologist, a professor, and I’d never even heard of them.”
Since then, thousands of such tunnels have been discovered. Many are filled with sediment that accumulated after they were abandoned, but the circular or elliptical entrances are still discernable, like dark knots in an earthen bank. “There’s no geological process in the world that produces long tunnels with a circular or elliptical crosssection, which branch and rise and fall, with claw marks on the walls,” said Frank. “I’ve [also] seen dozens of caves that have inorganic origins, and in these cases, it’s very clear that digging animals had no role in their creation.”
Some of the tunnels were known about in the 1930s, but were then considered to be some kind of archæological structure – but they were certainly not created by Brazil’s indigenous population who, prior to the arrival of Europeans, didn’t know about the existence of iron and therefore had no tools to dig through the hard rocks in which these tunnels are dug.
Geologist Amilcar Adamy of the Brazilian Geological Survey has identified a large tunnel complex in the state of Rondonia. It turned out to be the first palæoburrow discovered in the Amazon; and also one of the largest ever measured, with branching tunnels altogether tallying about 2,000ft (600m) in length. The main shafts – since enlarged by erosion – were originally more than 6ft (1.8m) tall and 3-5ft (90-150cm) wide; an estimated 4,000 tons of dirt and rock were dug out of the hillside to create the burrow.
In his home state of Rio
Grande do Sul, in the far south of Brazil, Frank has documented at least 1,500 palæoburrows so far. In Santa Catarina, just to the north, he’s found hundreds more and counting. “In neighbouring countries such as Uruguay, Paraguay, Chile and Bolivia we have detected a few caves that could also be palæoburrows,” he said. “In Argentina, there are many of them, mainly in the cliffs of the Atlantic coast, in Mar del Plata.” Scientists believe they were dug between 10,000 and 8,000 years ago; speleothems (mineral deposits) growing on burrow walls could be used to calculate an age, although this has yet to be tried.
Frank believes the biggest burrows – measuring up to five feet in diameter – were dug by ground sloths. He and his colleagues consider as possibilities several genera that once lived in South America and whose fossil remains suggest adaptation for serious digging: Catonyx, Glossotherium and the massive, several-ton Lestodon, as big as an elephant. Others believe that extinct armadillos such as Pampatherium, Holmesina or Propraopus, though smaller than the sloths, were responsible for even the largest burrows. However, Frank observed: “The biggest giant armadillo had a body width of 80cm [31in], while the tunnels reach widths of 1.4m [4.6ft]. I personally believe they were excavated by land sloths, a group of mammals that became extinct in that area about 10,000 years ago. There are large tunnels up to two metres high and four metres wide that were undoubtedly excavated by sloths.”
The function of the palæoburrows is unknown. “Perhaps the climate is an explanation,” said Frank. “It was drier and hotter than today and the tunnels were isothermal, but this can hardly explain the complex system of tunnels several hundred metres long, which were most likely inhabited by groups of sloths or armadillos. The roofs and walls of many tunnels are polished, probably thanks to the friction of the animals’ fur, which moved through the tunnels for decades or even centuries. So if a 40kg [90lb] animal living today digs a 40cm by 6m [16in by 20ft] borrow, what would dig one 1.5m [5ft] wide and 76m [250ft] long? There’s no explanation – not predators, not climate, not humidity. I really don’t know.”
Another head-scratcher is the strange geographic distribution of palæoburrows. While common in the southern Brazilian states of Rio Grande do Sul and Santa Catarina, they are, so far, almost unknown just to the south in Uruguay (though some of the first ever described were found even further south in Argentina). Likewise, very few have been found farther north in Brazil, and Frank is aware of just a tiny handful of possible burrows in other South American countries. And though North America was also once home to giant ground sloths and giant armadillos, there don’t appear to be any palæoburrows here – or maybe they just haven’t been noticed. Discover magazine, 28 Mar; Popular Mechanics, 30 Mar; Times, 17 April 2017.
He believes the biggest burrows were dug by ground sloths