At the start of the 20th century, American paleontologist Charles Walcott noticed differences in early Paleozoic benthic trilobites of Laurentia, as found in Scotland and western Newfoundland and those of Baltica, as found in the southern parts of the British Isles and eastern Newfoundland. Geologists of the early 20th century presumed that a large trough, a so-called geosyncline, had existed between Scotland and England in the early Paleozoic, keepingboth sides separated. 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. Wilson also noticed that the Atlantic had opened at roughly the same place where its precursor ocean had closed. This led him to his Wilson cycle hypothesis.
In many spots in Scandinavia basaltic dikes are found with ages between 670 and 650 million years. These are interpreted as evidence that by that time, rifting had started that would form the Iapetus Ocean. In Newfoundland and Labrador, the Long Range dikes are also thought to have formed during the formation of the Iapetus Ocean. It has been proposed that both the Fen Complex in Norway and the Alnö Complex in Sweden formed as consequence to mild extensional tectonics in the ancient continent of Baltica that followed the opening of the Iapetus Ocean. The southern Iapetus Ocean opened between Laurentia and southwestern Gondwana about 550 Ma in the Ediacaran–Cambrian transition. At the time it did so the Adamastor Ocean further east closed. The opening of the Iapetus Ocean probably postdates the opening of the Puncoviscana Ocean with the Iapetus Ocean being separated from the Puncoviscana Ocean by the ribbon-shaped Arequipa-Antofallaterrane. However, the formation of both oceans seems unrelated.
Paleozoic
Southwest of the Iapetus, a volcanic island arc evolved from the early Cambrian onward. This volcanic arc was formed above a subduction zone where the oceanic lithosphere of the Iapetus Ocean subducted southward under other oceanic lithosphere. From Cambrian times the western Iapetus Ocean began to grow progressively narrower due to this subduction. The same happened further north and east, where Avalonia and Baltica began to move towards Laurentia from the Ordovician onward. Trilobite faunas of the continental shelves of Baltica and Laurentia are still very different in the Ordovician, but Silurian faunas show progressive mixing of species from both sides, because the continents moved closer together. In the west, the Iapetus Ocean closed with the Taconic orogeny, when the volcanic island arccollided 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. It has been suggested that the southern Iapetus Ocean closed during a continental collision between Laurentia and Western Gondwana. If factual the Taconic orogen would be the northward continuation of the Famatinian orogen exposed in Argentina. Meanwhile, the eastern parts had closed too: the Tornquist Sea between Avalonia and Baltica already during the late Ordovician, the main branch between Baltica-Avalonia and Laurentia during the Grampian and Scandian phases of the Caledonian orogeny. At the end of the Silurian period the Iapetus Ocean had completely disappeared and the combined mass of the three continents formed the "new" continent of Laurasia, which would itself be the northern component of the singular supercontinent of Pangaea.
