Megalodon, or “big tooth”, was the largest shark to have ever lived. The creature was so large that it fed on whales. C. megalodon, with C. from Carcharodon or Carcharocles, depending on who you ask, as there are two schools of thought debating its classification, lived until 2.6 million years ago. So no matter what Discovery told us in the recent period, there are no giant sharks swimming around.
Establishing the age of shark fossils is more difficult than in the case of other ancient animals, as sharks are mostly cartilaginous, a material unsuitable for preservation. Researchers base their estimates on the fossilized teeth they discovered.