Ancient Capybara Fossil Reveals the Atacama Desert Was Once a Lush Wetland

Ask any zoogoer and they will tell you that capybaras are adorable. Ask a scientist who specializes in these oversized, guinea-pig-like rodents, and they will offer a more technical assessment: capybaras are obligate semiaquatic herbivores, animals whose very existence signals the presence of persistent riparian ecosystems. In plain English, capybaras like it wet. That simple biological fact is now at the center of a discovery that has stunned paleontologists and threatens to overturn one of the most entrenched assumptions in Earth science. Researchers have identified a fossilized capybara tooth from the Atacama Desert of northern Chile, a landscape so parched that some weather stations there have never recorded meaningful rainfall. The animal that left the tooth behind lived nearly 9 million years ago, in a place where no capybaras exist today and where, according to the standard textbook narrative, arid conditions have reigned for an estimated 40 million years.

The fossil, described in a study now in press at Nature’s Scientific Reports, provides the first and oldest record of a capybara in coastal Chile. But its significance extends far beyond adding a rodent to the region’s faunal inventory. If a semiaquatic giant rodent was thriving in the Atacama during the late Miocene, the desert cannot have been the unbroken hyperarid expanse that geologists have long imagined. The finding directly challenges the long-standing characterization of the Atacama as one of the oldest continuously dry regions on the planet, a status it has held in textbooks and review papers for decades. For the research team, led by doctoral student Priscilla Martinez of the University of Arizona Department of Geosciences, the implications were immediately clear and deeply unsettling to conventional wisdom.

The tooth itself emerged from a layer of sediments deposited in a shallow marine environment between roughly 10 and 7 million years ago. The site, known as El Morro, belongs to the fossil-rich Bahía Inglesa Formation, a rock unit that also contains Cerro Ballena, or Whale Hill, famously recognized as the world’s largest concentration of whale fossils. Marine deposits full of whales are one thing; a capybara tooth in the same beds is quite another. Finding a capybara tooth in a marine deposit in what today is one of the driest regions on Earth is, as Martinez put it, just freakish. The oddity of the association underscores how radically the local environment has changed since the late Miocene, when the same coastal setting that entombed whales also supported a large freshwater-dependent mammal.

Capybaras, the largest living rodents, and their close cousins the guinea pigs are native to South America. Today, capybaras thrive east of the Andes in low-elevation wetlands, with their core range roughly tracing the Amazon River basin. Their semiaquatic lifestyle tightly binds them to persistent surface water, wetland vegetation, and the ability to move along riparian corridors beside creeks and rivers. Co-author Barbara Carrapa, a professor of geosciences at the University of Arizona and Martinez’s doctoral advisor, emphasized that there is no other evidence of capybaras in that part of coastal Chile. They are not supposed to be there, she said, which immediately raised two questions for the team: why were they there, and where did they come from?

Geography makes those questions especially thorny. Geologists estimate that long before capybaras appeared in the region roughly 9 million years ago, the Andean mountain chain had already risen into a formidable barrier, in places jutting up to 18,000 feet. That towering wall effectively separates the Atacama Desert from the eastern lowlands that stretch across parts of Venezuela and Colombia down to northeastern Argentina and Uruguay, where the animals live today. Climate compounds the problem. As Martinez noted, the plateau lands of the Central Andes are not only very tall but also extremely arid. If capybaras originated east of the Andes in the foreland region, it is hard to imagine how these short-legged rodents could possibly have crossed such an inhospitable, high-altitude barrier on foot.

The most likely answer, the researchers conclude, is that they did not cross it at all. Instead, the team proposes a dispersal scenario in which capybaras expanded out of their original range by taking advantage of low-elevation wetlands that connected coastal areas in a counterclockwise arc around the northern portion of South America. From today’s Venezuela into Colombia, Ecuador, and Peru, and finally into northern Chile, the animals could have moved along a chain of verdant, water-rich habitats without ever confronting the high Andes. The scenario implies that a continuous or near-continuous corridor of wetland ecosystems hugged the Pacific coast during the late Miocene, a picture starkly at odds with the notion of an ancient, unbroken desert.

Crucially, the capybara tooth is not an isolated anomaly. Co-author Mark Clementz, a paleobiology expert and head of the Department of Geology and Geophysics at the University of Wyoming, pointed out that the same deposits contain multiple lines of evidence indicative of the kind of really wet conditions typically associated with capybaras today. Alongside the rodent remains, the sediments yielded fossils of freshwater fishes and gavials, the fish-eating freshwater crocodilians that require rivers or permanent water bodies to survive. The presence of crocodiles and freshwater fish in the same beds that preserved the capybara paints a coherent ecological picture of rivers, marshes, and wooded waterways rather than barren sand.

Botanical evidence reinforces that reconstruction. The team also unearthed phytoliths, microscopically small accumulations of silica that form inside plant cells and tissues as plants absorb dissolved silica from groundwater through their roots. Because different plant lineages produce characteristically shaped phytoliths, specialists such as co-author Caroline Strömberg, a professor at the University of Washington, can analyze these durable structures to determine what kinds of vegetation once grew at a site. In this case, the phytolith record pointed to palms and other flowering plants, including dicots, none of which would be expected in the Atacama Desert of today. Martinez offered a vivid comparison: think of sunny Southern California full of palm trees. She cautioned that researchers are not sure whether the palms bloomed extensively there, but they were definitely present, a detail that alone rewrites assumptions about the region’s past.

Together, the mammal, reptile, fish, and plant fossils support the idea that there was enough of an ecological community to sustain capybara populations over the long term, as Clementz observed. No other remains of capybaras this old have been found in Chile to date, which makes the single tooth, preserved within a three-foot-thick bed in the middle of a desert, something of a miracle of survival, according to Martinez. The assembly of strange creatures in an unlikely place did not go unnoticed by the local population. Many residents dug for fossils and sold them to collectors, and many of the fossils originally discovered at the location are now lost to science. Fortunately, certain merchants were interested in understanding the specimens better and donated them to local scientists, creating an unusual and invaluable partnership between fossil traders and researchers.

One of the beneficiaries of that collaboration was Carolina Gutstein, the study’s first author, who heads paleontology at the Chilean mining company Fosfatos de Caldera and is a professor at the Universidad Santo Tomás in Santiago. Gutstein has a long history of working with local fossil experts in the area, and those relationships proved essential in tracing and interpreting the material. Clementz added that the findings demonstrate the value of challenging assumptions, in this case the assumption that the Atacama has been an ancient desert for millions upon millions of years. No one really argued with that paradigm, he noted, even in the face of other fossil finds suggesting that conditions were once different. Finding a capybara, an animal that by any measure is not a desert creature, at that location is going to force a serious rethinking of the Atacama’s history.

The implications reach well beyond capybaras and one South American desert. Deserts are extremely sensitive to changes in climate, including year-to-year shifts in monsoon intensity, and understanding how they formed and transformed in the deep past offers a natural experiment that no laboratory can replicate. As Martinez explained, the better scientists understand deserts, the better they can predict how these environments will fare over the long term. And if researchers can reconstruct past climate changes that had nothing to do with humans, they gain a baseline against which to measure how climate may evolve in a world where human influence is now part of the equation. A single fossilized tooth, plucked from a whale-strewn marine bed in the driest place on Earth, has thus opened a window onto a greener, wetter Atacama and a reminder that even the most confident scientific paradigms can be overturned by one small, improbable piece of evidence.

Subject of Research: Late Miocene capybara fossil evidence for a wetter ancient Atacama Desert and transcontinental rodent dispersal

Article Title: Capybaras in the desert? Fossil casts doubt on Atacama as world's oldest continuously dry region

Article References: Capybaras in the desert? Fossil casts doubt on Atacama as world's oldest continuously dry region. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: capybara, Atacama Desert, paleontology, Miocene, Chile, fossil tooth, wetlands, Andes, phytoliths, climate history, Scientific Reports, dispersal