Speaking of dinosaurs that aren't dinosaurs, and fish that aren't fish, there are the Ichthyosaurs. Marine reptiles that lived alongside the non-avian dinosaur

Speaking of dinosaurs that aren't dinosaurs, and fish that aren't fish, there are the Ichthyosaurs. Marine reptiles that lived alongside the non-avian dinosaurs, but were not dinosaurs. Being a reptile, it wasn't a fish either. They did however looked remarkably like dolphins, which is interesting because like dolphins, ichthyosaurs are tetrapods that returned to the water.