The calm movements of Plesiosaurus, if they were indeed calm, were not the only calm movements in the Mesozoic oceans. Even more calm, just as they are today for the most part, were the giant sea turtles that inhabited the same seas. One of the largest and most impressive specimens has been well represented across the United States by many finds. These finds were given a name in 1872 by E. D. Cope: Protostega gigas. At approximately 3m (9.8ft) from nose to tail, Protostega is the second largest known marine turtle (Archelon being the largest). The upper Cretaceous has many frightening predators, but most of them could not have cracked the shell of an adult sized Protostega so it was most likely safe from almost all harm once it reached an adult size. It had also had an extremely well built shell, as far as early turtles go, that allowed for flexibility in addition to remaining rigid.