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:

https://www.smithsonianmag.com/science-nature/new-technologies-reveal-strange-jaws-prehistoric-sharks-180977396

You'll note the Goblin Shark still has hints of that design.