What could have left those giant claw marks? Image credit: Heinrich Frank 

But what could those giant creatures be? That was exactly the question geology professor Heinrich Frank asked himself while crawling through a mysterious tunnel uncovered at a construction site in Novo Hamburgo, Brazil. Since it was unlike any geological formation he’d ever seen, Frank speculated that the tunnel must have been dug out by a living thing. 

“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, with claw marks on the walls,” Frank told Discover.

Whatever the enormous creature was, it left huge claw marks across the walls and ceiling of this tunnel and others of similar size found in the area. At that point, the professor slowly backed out and told the construction workers he’d be back in a few weeks. 

Frank, who works as a professor at the Federal University of Rio Grande do Sul, kept his promise. He ended up naming the mysterious tunnels and identifying the animal most likely to have done the excavations, not only of the tunnels in Novo Hamburgo but also thousands of others in Brazil.

A cave in the state of Rondonia explored by geologist Amilcar Adamy from the Brazilian Geological Survey. It is the largest known palaeoburrow in the Amazon, and is twice the size of the second largest palaeburrow in Brazil. 

Adamy couldn’t think of any natural process that would create such a deliberate-looking structure. Photo: Amilcar Adamy Frank found his first tunnel in the early 2000s and learned that another scientist had coined the name “palaeoburrows” for them. 

When he couldn’t come up with an explanation for their existence, this made him look for further tunnels to eventually figure out the truth. As of today, Frank and other researchers have found over 1,500 tunnels in the state of Rio Grande do Sul alone, as well as hundreds more in Santa Catarina. Many of these tunnels stretch for hundreds of feet and have numerous branches, and the largest one is a whopping 2,000 feet long, six feet tall and up to five feet wide! 

“In these burrows, sometimes you get the feeling that there’s some creature waiting around the next curve – that’s how much it feels like a prehistoric animal den, ” Frank wrote. Eventually, he did find enough evidence to convince him that the paleoburrows were most probably dug by the giant ground sloth, the second-largest prehistoric land mammal next to the mammoth. 

The primary evidence is the deep claw marks found by Frank on the walls of paleoburrows, like the ones you see on the first picture above. Most scientists now agree that they could only have been made by a giant ground sloth (Megatherium), and not by smaller giant armadillos as some others have suggested. Image credit: Rodolfo Nogueira / Source
Megatherium americanum is the scientific name for an extinct species of giant ground sloth. The name means ‘great beast from America’. But despite these creatures stretching up to 4.6 metres (15 feet) and weighing roughly 2,590 kg (5,709 pounds), a single ground sloth would have spent much of its lifespan dedicated entirely to constructing tunnels as large and extensive as these palaeoburrows are. 

So why bother? Frank and his team are unsure whether the extensive caverns were used to escape the climate, predators, or humidity, but then even those explanations seem unlikely. After all, a much smaller burrow would have suited those purposes just fine, wouldn’t it? 

Could it be that several individuals inherited the burrows over generations, and kept adding to the structure to make it so enormous. Again, that’s something the researchers will need to confirm through further observations. Charles also collected Megatherium fossils during his voyage on the Beagle in 1832. The pieces were used complete the Megatherium skeleton still on display today at the Natural History Museum in London. Photo: Ballista Discovered in 1787 by Manuel Torres in Argentina, the first M. americanum fossils were shipped to the Museo Nacional de Ciencias in Madrid, where the original skeleton is still on display. 

In 1796, comparative anatomist Georges Cuvier determined the relationships and appearance of Megatherium, establishing it was a sloth. At first, he believed that the animal used its enormous claws to climb trees, like modern sloths, but he later changed his hypothesis to support a subterranean lifestyle, with the claws used to dig tunnels. 

Megatherium americanum was up to 10 times the size of living sloths reaching weights of up to four tonnes (similar to a present day bull elephant). Model of a Megatherium at Crystal Palace, London. Image credits: Jim Linwood / failing_angel
Despite its enormous claws, M. americanum was a vegetarian. This has been confirmed through chemical analysis of the animal’s teeth which shed light on what it ate during life. And it could have also meant that it was an easier target for – humans. 

Yes, we do know they overlapped with humans in time as Megatherium fossils have been found with cut marks on them. This suggests that these giant sloths were on the menu thousands of years ago, which could have contributed to their extinction. Looking into a large paleoburrow in Brazil. Image credit: Heinrich Frank
So far, Frank and other researchers haven’t found fossils inside the caves, nor organic material or mineral deposits that could help more precisely date the exact creation of the burrows. But the quest continues.
Source