A Single Capybara Tooth Turned Up in a 9-Million-Year-Old Marine Bed in Chile's Atacama Desert, the Driest Place on Earth, Alongside Freshwater Fish, Crocodiles and Palm Phytoliths. The Textbook Says the Atacama Has Been Dry for 40 Million Years. 'Finding a Capybara Tooth in a Marine Deposit in What Today Is One of the Driest Regions on Earth Is Just Freakish.'

Capybaras like it wet. The largest living rodents, oversized cousins of the guinea pig, are what scientists call obligate semiaquatic herbivores: they need standing water, wetland plants and river corridors to move along, and their range today hugs the low, soggy Amazon basin east of the Andes. So a team of geologists and paleontologists was baffled to identify a fossilized capybara tooth from a layer of marine sediment in the Atacama Desert of northern Chile, deposited almost 9 million years ago in a region textbooks describe as the oldest continuously dry place on Earth.

"Finding a capybara tooth in a marine deposit in what today is one of the driest regions on Earth is just freakish," said Priscilla Martinez, a doctoral student in the University of Arizona Department of Geosciences and the study's lead author. The tooth came from a site called El Morro in the Bahía Inglesa Formation, a fossil-rich sequence laid down in a shallow sea between 10 million and 7 million years ago. The same formation contains Cerro Ballena, "Whale Hill," the world's largest known concentration of fossil whales. No capybara lives anywhere in Chile today, and no older capybara remains have ever been found in the country.

The find, published in Scientific Reports, does not stand alone. The same beds hold fossils of freshwater fish and gavials, slender-snouted freshwater crocodiles. And co-author Caroline Strömberg of the University of Washington identified phytoliths, microscopic silica bodies that form inside plant cells, that came from palms and other flowering plants, nothing that grows in the Atacama now. "We have multiple lines of evidence indicative of really wet conditions that are typically associated with capybaras today," said Mark Clementz, head of geology and geophysics at the University of Wyoming. "Think sunny Southern California full of palm trees," Martinez said. "You don't really expect the Atacama Desert to have palms, and we're not sure if they were blooming extensively there, but they were definitely there."

That poses a geographic puzzle. By 9 million years ago the Andes already rose to about 18,000 feet, and the high plateau between the coast and the capybaras' eastern homeland was, then as now, extremely arid. "Not only are the plateau lands of the Central Andes very tall, they're also extremely arid," Martinez said. "So, if capybaras originated east of the Andes in this foreland region, how in the world could these short-legged rodents make it across?" The authors' answer is that they didn't. They propose the animals went around, counterclockwise, following low-elevation coastal wetlands from what is now Venezuela through Colombia, Ecuador and Peru and down into northern Chile.

"We don't have any other evidence of capybaras in that part of coastal Chile," said Barbara Carrapa, a University of Arizona geosciences professor and Martinez's adviser. "They're not supposed to be there. So, we immediately asked ourselves, 'Why were they there? Where did they come from?'"

The tooth's survival was partly luck. It sat in a bed about three feet thick, and many fossils pulled from the site over the years were sold to collectors and lost. "Many of the fossils originally discovered at that location are lost in time," Martinez said, "but certain merchants were interested in understanding them a little bit better, and so they donated them to local scientists." One of those scientists is the paper's first author, Carolina Gutstein, head of paleontology at the Chilean mining company Fosfatos de Caldera and a professor at Universidad Santo Tomás in Santiago, who has long ties to local fossil hunters.

The larger claim is about the desert itself. The Atacama's hyperaridity is usually dated to roughly 40 million years ago, a paradigm Clementz said "no one really argued with, even in the face of these other fossil finds suggesting that things were different in the past. Finding this capybara, which I think everyone can agree is not a desert animal by any means, at that location is going to challenge many of the ideas about the Atacama's history." Martinez said the stakes reach beyond one rodent: deserts are acutely sensitive to shifts in climate and monsoon strength, and reconstructing how the Atacama dried out, long before humans, gives a baseline for predicting how it and other deserts will respond now.