The beast, whose name means "stupendous turtle" in Latin, lived between seven million and 13 million years ago in the ancient wetlands that eventually turned into the Amazon rainforest in Peru, Brazil, Venezuela and Colombia.
"The two shell types indicate that two sexes of Stupendemys existed - males with horned shells, and females with hornless shells", Marcelo Sánchez, director of the Paleontological Institute and Museum of University of Zurich in Switzerland, said in a news release.
Stupendemys males boasted sturdy front-facing horns on both sides of its shell very close to the neck.
Venezuelan Palaeontologist Rodolfo Sánchez and a male carapace of the giant turtle Stupendemys geographicus, from Urumaco, Venezuela. These shells have bite marks on their surface which could possibly be the result of swamp predators like massive alligators.More news: USS Normandy seizes 150 missiles bound for Yemen
"Many questions - about its diet, if there were differences between males and females, and even if we were dealing with one or more giant turtle species - were completely unknown".
Based on the size of the shells, the researchers estimated that the Stupendemys geographicus weighed about 1,145 kilograms. He gave the name "Stupendemys", owing to their large size, with geographicus to support the cause of National Geographic in supporting the fossil turtle research.
Modern male turtles have also been known to fight with one another, although they lack the shell-mounted weapons of their ancestors.
They lived in the lakes and rivers of northern South America from as far back as 13 million years ago. It shared the setting with big crocodilians together with the 11-metre-long caiman Purussaurus and the 33-foot-long (10-meter-long) gavial relative Gryposuchus.More news: Netflix Stranger Things Teases David Harbour Big Surprises for Season 4
The turtle fossils were recovered from dig sites in Venezuela and Colombia. There are only three larger reptile species today, all of them crocodiles like the six-meter-long and one-ton inguinal crocodile.
The first Stupendemys fossils were discovered in the 1970s but many mysteries have remained about the 4-metre long animal.
Cadena added: "Its diet was diverse, including small animals - fishes, caimans, snakes - as well as molluscs and vegetation, particularly fruits and seeds".
The discoveries, including of jaws and other skeletal parts, have allowed the scientists to completely revise the species' tree of life and expand the turtle's habitat to a much wider geographic region than previously thought to cover the entire northern part of South America.More news: Succulent contract renewal offer to tie down Van Dijk
Paleontologists don't know if these turtles were biters - but with their car-sized bodies, one probably wouldn't want to find out.