Pleisiosaurus was a marine reptile that lived in the open oceans at the time of the dinosaurs. Known to have swallowed rocks for ballast and for grinding food. The largest of the currently known marine reptiles. Originally this pleisiosaurus skeleton was going to be in Thunder Lizards! but I ran out of room for the aquatic reptiles chapter. I'm busy orgainizing this picture and others for a second volume of How to Draw Dinosaurs to hopefully be published in a couple of years (I have tons of other books coming your way first.)