More relevantly, the fossil records for sharks are mostly their teeth and jaws, because all their other bones are cartilage and rarely fossilize.
"Sharks haven't significantly evolved in appearance in 350 million years" is therefore based on reconstructions made under the assumption that the old sharks mostly looked like current sharks, which may or may not be true.
Though we can get a surprising amount of information that way, for example one change is that their jaws used be more at the end of their snout instead of more underslung like today, like so:
You can't prove the non existence, but you can be very sure about some things. Megalodon lived near the surface, because it liked warm water(AFAIK), so it's likely that if it wouldn't be extinct there's a high chance that we would notice it, since Megalodon was kinda big.
Yeah okay, seems plausible then. It's more fun to believe otherwise though, not gonna lie. After all, there's still so much we don't know about our oceans.
They are bottom-dwelling fish, living down to 200 m (660 ft), although they can be found in much shallower water. Most species are around 30 to 40 cm (12 to 16 in) in length. They have an unusually solid skull, and many species also possess armored plates on their bodies. Another distinctive feature is the presence of a "drumming muscle" that makes sounds by beating against the swim bladder