Turtles the size of Smart cars once roamed the rivers in what is now Colombia, preying on creatures as large as crocodiles.
North Carolina State University paleontologists originally uncovered the fossilized remains of the 60-million-year-old creature in 2005.
Named Carbonemys cofrinii, the specimen's skull measures 24 centimeters, roughly the size of a regulation NFL football. The shell found nearby measures five feet seven inches, long.
"We had recovered smaller turtle specimens from the site. But after spending about four days working on uncovering the shell, I realized that this particular turtle was the biggest anyone had found in this area for this time period - and it gave us the first evidence of giantism in freshwater turtles," says doctoral student Edwin Cadena.
While smaller relatives of Carbonemys existed alongside dinosaurs, the giant version appeared five million years after they vanished. It co-existed with giant varieties of many different reptiles livingin this part of South America.
Researchers believe that a combination of changes in the ecosystem - including fewer predators, a larger habitat area, plentiful food supply and climate changes - worked together to allow these giant species to survive.
This particular turtle had massive, powerful jaws that would have enabled it to eat anything nearby - up to and including crocodiles.
The team hasn't found any other specimens yet, probably because a turtle of this size would need a large territory in order to get enough food to survive.
"It's like having one big snapping turtle living in the middle of a lake," says NC State paleontologist Dr Dan Ksepka.
"That turtle survives because it has eaten all of the major competitors for resources. We found many bite-marked shells at this site that show crocodilians preyed on side-necked turtles. None would have bothered an adult Carbonemys, though - in fact smaller crocs would have been easy prey for this behemoth."