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.
At the start of the 19th century, American paleontologist Charles Doolittle Walcott noticed differences in early Paleozoic benthic trilobites either side of the line which later was called the Iapetus Suture.
The so called "Pacific fauna" of Laurentia, as found in Scotland and western Newfoundland, was quite different from those of Baltica, often called the "Atlantic fauna". The latter is found in the southern parts of the British Isles and eastern Newfoundland. Geologists of the early 20th century thought a large trough, a so-called geosyncline, had existed between Scotland and England in the early Paleozoic, keeping both sides separated.
With the development of plate tectonics in the 1960s geologists concluded the Atlantic Ocean must have had a precursor before the time of Pangaea. That ocean had closed as three continents joined, producing the Iapetus Suture.