A capybara tooth from coastal Chile puts standing water in the Atacama 9 million years ago

What makes the tooth a climate claim is what a capybara needs to stay alive. Rodent specialists call them obligate semiaquatic herbivores that indicate persistent riparian ecosystems ^[6]. They are the largest living rodents, native to South America, and today they live east of the Andes in low-elevation wetlands, their core range roughly outlining the Amazon basin ^[7]. Their way of life ties them to standing surface water, wetland vegetation and movement along creeks and rivers ^[8].

The tooth itself came out of the wrong rock for that habitat. The beds at El Morro were deposited in a shallow marine environment between 10 million and 7 million years ago ^[4], so where the animal was buried is not where it lived. Fresh water comes in with the rest of the assemblage. "We have multiple lines of evidence indicative of really wet conditions that are typically associated with capybaras today," said Mark Clementz of the University of Wyoming, a co-author. "The same deposits contain fossils of freshwater fishes and gavials (freshwater crocodiles)" ^[14]. The team also recovered phytoliths, the silica bodies that form inside plant cells as roots take up dissolved silica from groundwater, and co-author Caroline Strömberg of the University of Washington matched them to palms and other flowering plants, including dicots, nothing that grows in the Atacama now ^[15][16]. "Think sunny Southern California full of palm trees," Martinez said ^[17].

The 40-million-year figure and the fossil do not sit at the same scale. If the region's dryness began 40 million years ago and the tooth dates to about 9 million, then 31 million years of that record run before the wet interval and are untouched by it ^[18]. The tooth's age is bracketed rather than measured directly, inside a three-million-year depositional window ^[19].

The animals also had to get there. Geologists estimate the Andes rose to about 18,000 feet, roughly 5,500 metres, before capybaras appeared, walling the Atacama off from the eastern lowlands that hold the animal's range today ^[11][20]. "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?" ^[12] The authors propose they went around, counterclockwise through low-elevation wetlands from today's Venezuela into Colombia, Ecuador and Peru, then into northern Chile ^[13].

The record rests on one tooth. "We don't have any other evidence of capybaras in that part of coastal Chile," said co-author Barbara Carrapa, Martinez's doctoral adviser at the University of Arizona ^[10]. The published account does not say how long the wet conditions lasted or how far inland they extended ^[21]. I would put weight on the wet coastal setting at El Morro, which three independent fossil lines support, and much less on the 40-million-year continuity figure being wrong, because that case needs deposits of the same age from inland sites and the paper describes a coastal one ^[4][2].