Simplify the following text for a Grade 9 native English-speaking person.
Original: They originated as marine animals , possibly in the Cambrian period , but the first confirmed chelicerate fossils , eurypterids , date from in the Late Ordovician period .
Simplified: The chelicerates originated as marine animals . The first confirmed chelicerate fossils , the eurypterids , date from 445 million years ago in the Late Ordovician period .