The Iapetus Ocean existed in the Neoproterozoic and Palaeozoic eras (between 600 and 400 million years ago). The ocean was in the southern hemisphere, between three paleocontinents. The ocean disappeared when these three continents joined to form one big landmass. That large continent has been given various names, such as Laurussia or the Old Red Sandstone continent. Probably the best name is Euramerica, because basically it was American and Europe stuck together.