Some hundred million years ago, dinosaurs strutted across the baking Earth and declared, "Man, we are the kings of this joint." Then one of them would stray a little too close to the water's edge and get swallowed whole by the real kings of the prehistoric era: Plesiosauria. These leviathans swam the oceans from the early Jurassic period up through the end of the Cretaceous. If you want one of your own, you're going to need a time machine and a big, big tank—or you can play the latest update forJurassic Park Builder.