Do dinosaurs still exist underwater?
That would be some feat since dinosaurs were entirely terrestrial, not marine. If your thinking about the plesiosaurs, the likely food they ate no longer exists, the waters are too cold for them now, there has been no real sign humanity could find of them.. ever. So I would say no. Totally Extinct. By a stretch of the definition, you *can* say yes. Birds, being the distant descendants of a group of dinosaurs, have been called modern dinosaurs by a stretch of the imagination. As for “underwater”, the closest to a fully marine bird are the penguins, which are semi-aquatic, since they have to come to land to mate and raise young.
