Simplify the following text for a Grade 15 native English-speaking person.
Original: For most of the Late Ordovician , life continued to flourish , but at and near the end of the period there were mass-extinction events that seriously affected planktonic forms like conodonts , graptolites , and some groups of trilobites ( Agnostida and Ptychopariida , which completely died out , and the Asaphida , which were much reduced ) .
Simplified: For most of the Ordovician , life continued to flourish , but near the end of the period the End-Ordovician extinction event seriously affected planktonic forms like conodonts , graptolites , and some groups of trilobites .