Stupendemys is a species of turtle from the Late Miocene to the very start of the Pliocene (6-5 mya) in South America. It had a massive shell, which was also extremely wide. It was the largest turtle that ever existed, surpassing even Archelon in size. Its massive weight enabled it to stay underwater for long periods of time, feasting on aquatic plants. However, it was a weak swimmer who couldn't move its bulk against a swift current.