Aquatic Dinosaurs?
If you're anything like me, you've called Elasmosaurus or Mosasaurus a dinosaur only to be immediately excoriated by the nearest paleo nerd, quick to tell you why that's dead wrong.
You see, those are merely dinosaur-like reptiles (big ol' fossil guys with names ending in "saur"). There weren't any aquatic dinosaurs . . . at least, not contemporary with the dinosaurs we're thinking of.
But you know what'll leave your paleo pedant gobsmacked? Naming an aquatic dinosaur alive right now.
You see, the fact that modern birds are living dinosaurs means that penguins are also dinosaurs. Aquatic dinosaurs, QED.