Simplify the following text for a Grade 5 native English-speaking person.
Original: The Megalodon , Carcharodon megalodon , was largest shark of all time . It lived from the late Oligocene to early Pleistocene epochs , 20 to 1.5 million years ago .
Simplified: It is believed that C. megalodon became extinct in the Pleistocene , probably about 1.5 million years ago .