Simplify the following text for a Grade 9 native English-speaking person.
Original: It is the youngest period of the Mesozoic Era , and at 80 million years long , the longest period of the Phanerozoic Eon .
Simplified: The Cretaceous period is the longest period in the Phanerozoic eon , lasting from 145 to 65 million years ago , a total of 80 million years . It is the last period in the Mesozoic era .