Giant turtle fossil found in Acre reveals clues about the Amazon millions of years ago

In its first expedition, project funded by the Amazon+10 Initiative makes important discovery for Brazilian paleo

A rare and surprisingly well-preserved fossil of a giant turtle that lived between 10.8 and 8.5 million years ago, during the Miocene, was found in the Boca dos Patos region, in Assis Brasil, Acre.

The discovery was made by a research group led by professors and paleontologists Carlos D’Apolito Júnior, from the Federal University of Acre (UFAC), and Annie Schmaltz Hsiou, from the University of São Paulo (USP), coordinators of the project "New frontiers in the fossil record of Southwestern Amazonia", funded by the Scientific Expeditions call of the Amazon+10 Initiative — a program of the National Council of State Funding Agencies (CONFAP), with resources from the National Council for Scientific and Technological Development (CNPq), the São Paulo Research Foundation (FAPESP), and the Acre Research Foundation (FAPAC).

“We found the shell of the largest freshwater turtle that ever existed, Stupendemys geographicus,” says Annie, who has been studying the region for nearly 20 years. The shell was incomplete, from the midsection back, and had large dimensions—about 1.7 meters wide. “We estimate this particular shell would have reached nearly 3 meters in length. This shell is among the best-preserved ever found of this species — a spectacular record of this super turtle that lived during the Miocene in the Brazilian Amazon.”

The fossil diversity of southwestern Amazonia has been known for over a century and a half, including well-established fossil sites along riverbanks and cliffs in the region, especially in Acre. “The thing is, we usually find very fragmented fossils,” explains Carlos. “It's common to find bits of shells, isolated bone fragments — rarely together — and mostly from smaller animals, which are easier to remain articulated. Finding such large and well-preserved animals was a surprise. Now we can compare this to records from other places, like Venezuela, which has more complete specimens of this species. We might finally decipher if it’s the same species or a different one.”

Carlos compares the discovery to that of other large reptiles found in the region, such as Amazonian crocodiles. “It’s not exactly the same, of course, but in 1986, a complete skull of Purussaurus brasiliensis, a giant caiman and the largest ever known, was found. From that fossil, studies estimated its size and bite force. I find this comparison interesting because they are both reptiles that lived in the same region and the same time — a very different environment, warmer, with a lot more water, and massive lakes and rivers.”

This Miocene geological period (from 23 to 5 million years ago) is known for its rich aquatic and terrestrial fauna and was the cradle of many Amazonian lineages. “So, the more we dig into this geological time frame, the more we’ll understand how Amazonian biodiversity formed and how climate changes led to extinctions and transformations,” Annie explains.

Boca dos Patos Expedition

The fieldwork required major logistical effort. The team of 16 people — 10 researchers, five boatmen, and a cook — took an entire day to travel up and down the river to reach the excavation site. “The dry river was our main challenge. We had to stop often to push the boat until we found deeper spots to navigate,” recalls Carlos. The expedition had a strict six-day limit because water levels could drop further and make it even harder to return. “We were extremely lucky — we found the fossil on the very first day!” he celebrates.

The fossil extraction took four days, including excavation, preparing a protective plaster jacket for the shell, removal, and river transport to Assis Brasil — about a 7-hour boat trip — before heading to UFAC in Rio Branco, the state capital. “If we hadn’t found it in time, the fossil would likely have been lost. When the riverbed floods again, the water washes everything away. Nothing would have remained — or at least not in the preserved state in which we found the shell,” says Carlos.

The shell was transported on an improvised wooden base built by the boatmen, most of whom are riverside dwellers from Assis Brasil. Engaging local communities is a central part of the project, as Annie explains. “Their partnership is key to discovering new fossils and even species never before described — or only known through traditional and Indigenous knowledge. Now we can shed light on what really happened in the proto-Amazon and understand how these people relate to fossils.”

One of the project's goals is to work with local residents who know fossiliferous areas and act as sources, giving them and their communities recognition and training in preserving paleontological heritage. “We already have some knowledge of how Indigenous peoples from the Upper Juruá region view fossils,” Annie says. “We know they respect the fossils they find along the rivers. There’s a kind of reverence. Our goal is to deepen environmental education and awareness about preserving both current and ancient biodiversity. They are the guardians of these sites.”

During the expedition to Boca dos Patos, the team also visited the local Indigenous community of Aldeia dos Patos, home of the Manchineri people. “We went to ask permission and let them know what we were doing. We didn’t work within Indigenous land itself, but they came to watch our excavation,” Carlos says.

Next steps

The Stupendemys geographicus fossil was transported to UFAC’s main campus in Rio Branco, where it will undergo scientific analysis and later be added to the university’s fossil collection. One of the guidelines of the Scientific Expeditions call, which funds the New frontiers in the fossil record of Southwestern Amazonia project, is that collected materials must be cataloged and stored in Amazonian institutions to preserve this local heritage.

“UFAC’s collection has nearly 10,000 fossils, and most were discovered nearby. By going to more remote places, we increase the chance of finding materials not yet in the collection or better-preserved versions of known species, providing more taxonomic information. That way, we can better understand how the Amazon once looked and how its fauna and flora evolved and adapted to climate change,” says Carlos.

The project aims to continue exploring other rivers in Acre and southern Amazonas — a major geological unit within the Solimões Formation. “After this super turtle, we have high hopes of finding even more fascinating fossils in these less explored and remote areas,” says Annie. “This Amazon+10 Initiative call encourages and fuels our curiosity, as researchers and paleontologists, to search deeper into the Amazon for fossils.”

Annie also highlights the importance of institutional collaboration. “I have a strong partnership with UFAC researchers. All my FAPESP-funded projects, from my very first Young Investigator grant, deal with fossils from the Solimões Formation in the southwestern Brazilian Amazon — or Western Amazon,” she says. “Together, we’ve published many scientific articles and focus heavily on training new researchers, sharing resources and knowledge, and promoting student exchanges between USP and UFAC. We hope the Amazon+10 Initiative will bring even more fruitful results,” she concludes.