Simplify the following text for a Grade 9 native English-speaking person.
Original: These species became extinct during the Pleistocene ( 16,100 Â 100 - 50,000 years before present ) , but exact dates for their extinction have only been discovered recently .
Simplified: Many of these species became extinct during the Pleistocene ( 16,100 Â 100 - 50,000 years before present ) .