'Freakish' discovery of 9 million-year-old capybara tooth supports idea that Atacama wasn't always a desert

Researchers have discovered a roughly 9 million year old tooth belonging to a large, water-loving relative of modern day capybaras in the Atacama Desert of northern Chile. The fossilized molar adds to previous evidence suggesting that it's unlikely the Atacama, one of the driest places on Earth, was continuously dry for the last 40 million years.

The Atacama Desert, possibly the oldest desert on Earth, takes up the northern third of Chile. It's bounded by the Chilean Coast Range to the west, which blocks rain from the Pacific Ocean, and the enormous Andes to the east, which obstruct rain from the Atlantic Ocean. The driest part of the desert's hyperarid region gets less than 0.08 inches (2 mm) of rainfall a month.

Recent research estimates the Atacama's hyperarid core started forming between 47.8 million and 33.9 million years ago, in the Mid- to Late-Eocene, 20 million years before the Andes formed. It suggests the mountain ranges weren't the cause for the desert's aridification, and may have only intensified it.

Capybaras (Hydrochoerus hydrochaeris) ‪—‬ the world's largest rodent ‪—‬ exclusively live close to stable freshwater sources and feed on plants that also require water to grow, so the discovery of an extinct capybara relative (Hydrochoerus Cardiatherium) tooth in the Atacama Desert is very strange.

"Finding a capybara tooth in a marine deposit in what today is one of the driest regions on Earth is just freakish," Priscilla Martinez, a geoscientist at the University of Arizona and co-author of a new paper describing the discovery, said in a statement.

Capybaras are native to South America, although today, they are not found in Chile, where the tooth was unearthed, according to the International Union for Conservation of Nature (IUCN).

The tooth isn't the only clue that the Atacama was a wetter place in the late Miocene. Researchers found it in a layer holding the remains of a shallow marine environment that existed between 7 million and 10 million years ago, according to the statement. And they've also found microscopic pieces of plants, known as phytoliths, revealing that palm trees, grasses, gingers and other flowering plants grew there in what may have been a subtropical forest.

These findings and others are part of a growing body of evidence that the Atacama Desert was once wet enough for crocodiles and freshwater fish to thrive and palms to grow. The new study, published Sept. 5 in the journal Scientific Reports, shows that capybara relatives lived there as well during the late Miocene, which, because they are semiaquatic animals, is further evidence to support the hypothesis of a sporadically moist Atacama.

The researchers aren't sure how exactly capybaras moved into the Atacama Desert. Although there were capybaras in South America at the time, the Andes created an 18,000-foot-high (5,500 meters) barrier between their current habitats and the site where the tooth was found. The researchers theorized that, instead of traversing the mountains, the ancient capybaras migrated along the western coast of South America, following wetlands, rivers and other water sources.

According to Samantha Hopkins, an Earth sciences professor at the University of Oregon who was not involved in the study, it's unlikely the tooth found its way to the site without capybaras living there.

"If you wanted to find a way that this capybara could be here while the Atacama was still as dry as it is today, you'd have a hard row to hoe," she told Live Science in an email, because the size of the tooth makes it hard to transport it long distances. The desert wind couldn't have blown it such a long way, and while large carcasses can float down waterways for miles, "if you're going to invoke a large enough waterway to do this, you've demonstrated that the Atacama wasn't as dry as it is today anyway."