Not a gigantic Sea Troll but still interesting.

http://www.livescience.com/44345-missing-fossil-reveals-giant-sea-turtle.html

Missing Half of Bone Reveals Prehistoric Sea Giant

Now that paleontologists have assembled a complete humerus bone from the sea turtle Atlantocheyls mortoni, they have more information about the species and its overall size. Based on the complete limb, the researchers calculated the animal's overall size to be about 10 feet from tip to tail, making it one of the largest sea turtles ever known.