Preservation of teeth depends on the environment for fossilization, but density of the material is an important factor. Over my collecting life I have noticed that somewhere around 50% of all my Great White fossils lack a root. This one is no exception. Megalodon teeth roots seem to fossilize at a much higher rate. The roots are generally thicker and possibly structurally more prone to fossilization. Victor certainly is very observant to notice the possible bourlette and if that is the case, it would be definitive. I tend to think that this is definitely a Great White, though, as the serrations in the picture look to be a little courser and irregular than the finer megalodon serrations would be.