A few years earlier, a Brazilian geologist found a peculiar cave. Heinrich Frank, a professor at the Federal University of Rio Grande do Sul, was zipping down the highway on a Friday afternoon when he passed a construction site in the town of Novo Hamburgo. There, in a bank where excavators had eaten away half of a hill, he saw a peculiar hole.
Local geology doesn’t yield such a sight, so Frank went back a few weeks later and crawled inside. It was a single shaft, about 15 feet long; at its end, while on his back, he found what looked like claw marks all over the ceiling.
Unable to identify any natural geological explanation for the cave’s existence, he eventually concluded that it was a “paleoburrow,” dug, he believes, by an extinct species of giant ground sloth.
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. Others believe that extinct armadillos such as Pampatherium, Holmesina or Propraopus, though smaller than the sloths, were responsible for even the largest burrows.
Dating the burrows also remains guesswork at best—animals don’t dig holes after they go extinct. However, they had to have been dug at least 8,000 to 10,000 years ago, when South America’s giant ground sloths and armadillos vanished. Dating organic material found in burrow sediments – which has yet to be done – would reveal when sediments washed in, but not necessarily when the burrow was dug. Frank says that speleothems, or mineral deposits, growing on burrow walls could be used to calculate an age, although that hasn’t been tried yet either.