The Iapetus Ocean (/aɪˈæpɪtəs/; eye-AP-ih-təs)[1] existed in the late Neoproterozoic and early Paleozoic eras of the geologic timescale (between 600 and 400 million years ago).
[4] With the development of plate tectonics in the 1960s, geologists such as Arthur Holmes and John Tuzo Wilson concluded that the Atlantic Ocean must have had a precursor before the time of Pangaea.
From Cambrian times (about 550 million years ago) the western Iapetus Ocean began to grow progressively narrower due to this subduction.
[12] In the west, the Iapetus Ocean closed with the Taconic orogeny (480-430 million years ago), when the volcanic island arc collided with Laurentia.
Some authors consider the oceanic basin south of the island arc also a part of the Iapetus, this branch closed during the later Acadian orogeny, when Avalonia collided with Laurentia.
[13][B] Meanwhile, the eastern parts had closed too: the Tornquist Sea between Avalonia and Baltica already during the late Ordovician,[14] the main branch between Baltica-Avalonia and Laurentia during the Grampian and Scandian phases of the Caledonian orogeny (440–420 million years ago).