Simplify the following text for a Grade 10 native English-speaking person.
Original: The Iapetus Ocean was an ocean that existed in the Neoproterozoic and Paleozoic eras of the geologic timescale ( between 600 and 400 million years ago ) .
Simplified: The Iapetus Ocean was an ocean that existed in the Neoproterozoic and Paleozoic eras ( between 600 and 400 million years ago ) .