We know very little about prehistoric extinct species. Large animals, especially aquatic, are almost impossible for us to find evidence of their existence. One reason is because it is rare for large animals to fossilize, the other reason is that aquatic species fossils are mostly under water! The few aquatic species from the past that we know existed is thanks to moving tectonic plates that sometimes elevate thus exposing land that was once under shallow sea. There is ANOTHER problem with sharks specifically; only their teeth fossilize!
Megalodon is the largest shark species we know of. Problem with sharks is that their skeleton is made out of cartilage-like ligaments, the only evidence we have of prehistoric sharks is their teeth. This bring up a major problem; the largest shark in the world now eats krill and have paper-sand like teeth which are almost impossible to fossilize.
So, if there were giant krill eating sharks in the past, we will likely never know what they looked like or that they existed at all.
Archeology is based on evidence, sadly sharks only evidence in the fossil records are their teeth.