Sea turtles come in many different sizes. The smallest are about the size of dinner plates, while the largest, the leatherbacks, can reach sizes similar to a golf cart. But these modern-day turtles pale in comparison to ancient sea turtles. Archelon, a giant sea turtle that lived roughly 80 to 66 million years ago during the Cretaceous Period, grew to sizes roughly 13 feet in length and is one of the largest sea turtles ever to live on the planet.