Sharks' teeth found on a river bottom

"These are all fossilized and the river has eroded down to the layer where the teeth are. I’ve got megalodons, great whites, makos, and angustidens here. Pretty much all of the east coast states used to be under water when all the polar ice was melted. We had a lot of whales and because of that a lot of large predators. For as many teeth as I find there had to have been a lot of sharks here for a very long time. This was just two scuba tanks worth of diving. I actually found a lot more than this but most were in pretty bad shape."