Stupendemys

Stupendemys is a prehistoric genus of freshwater turtle. Its fossils have been found in northern South America, in rocks dating from the late Miocene to the very start of the Pliocene, about 6 to 5 million years ago.
"Stupendemys"'s carapace measured over in length and was also very wide. With a total carapace length of at least 11 feet (3.3 m) it was the largest turtle that ever existed, surpassing even "Archelon". 
Two species have been described to date. "Stupendemys geographicus" from Venezuela, and "Stupendemys souzai", a bit smaller and more slender, from Brazil.
Ecology.
Its weight helped "Stupendemys" stay under water for long periods, grazing on aquatic plants. It was probably a very weak swimmer, unable to move its bulk against a swift current, and probably avoided smaller streams.
Altogether, the fauna of the time had many large herbivores and few terrestrial carnivores. The habitat was mostly low-lying rainforest that was seasonally flooded, as well as floodplains and swampland. The rivers must have been wide and slow-moving.