Shark family trees are a thorny issue. There has been a long-running disagreement over the relationship between Carcharocles megalodon and modern Great Whites, Carcharodon carcharias, for example. That one seems to have been largely settled now, with a little distance placed between their respective branches. It had been assumed megalodon was a close relative or ancestor of Great White, but there isn't the evidence to support this in the fossil record, and it seems meg was an evolutionary dead end.
Great Whites are of the mackerel shark family, and their ancestry lies instead alongside that of the makos, the genus Isurus. The two living species of mako are sleek and powerful sharks, known for their speed and leaping ability - there are numerous examples of them jumping over or into boats, frightening fishermen. Great Whites and makos share a common ancestor sometime in the Eocene; there are a few Isurus species developing through the Oligocene and into the Miocene, but the one it's thought led to Carcharodon is Isurus hastalis. The teeth pictured above are from Isurus hastalis, from the Pliocene of South Carolina. They're not the biggest you get from this species, but are still around 2 1/2" along the diagonal, about as large as modern Great White teeth get, and considerably bigger than most. These teeth are smooth along the edge, whilst Great White teeth are serrated, better for cutting into the flesh of its largely mammalian prey. Makos today eat fish, and it's likely their ancestors did, too. Serration aside, the teeth of the two sharks are incredibly similar in root shape, thickness, blade form and positioning through the mouth. To nail the story down, a discovery a few years ago in Peru neatly plugged the gap between hastalis and carcharias. Eventually named Carcharodon hubbelli, the transitional Peruvian find was of a complete set of jaws, a rare find, which not only displayed the serration but also the positioning of teeth throughout the mouth, allowing a direct comparison.
Isurus hastalis must have been quite a sight. It was probably more similar to the White than modern makos in body form, bulkier and less streamlined. It was also a good deal bigger than both, probably five or six feet longer than today's Great Whites. It'd have been a scary proposition for fishermen. But anyway, what I really wanted to say was - 'Look at these shark teeth! Aren't they cool?'