At the beginning of the Paleozoic Era, about 540 million years ago, most of the continental mass on Earth was clustered around the south pole as the paleocontinent Gondwana. The exception was formed by a number of smaller continents, such as
Laurentia and
Baltica. The Paleozoic ocean between Gondwana, Laurentia and Baltica is called the
Iapetus Ocean. The northern edge of Gondwana had been dominated by the
Cadomian orogeny during the
Ediacaran period. This orogeny formed a
cordillera-type
volcanic arc where
oceanic crust subducted below Gondwana. When a
mid-oceanic ridge subducted at an oblique angle,
extensional basins developed along the northern margin of Gondwana. During the late
Cambrian to
Early Ordovician these extensional basins had evolved a
rift running along the northern edge of Gondwana. The rift in its turn evolved into a mid-oceanic ridge that separated small continental fragments such as Avalonia and
Carolina from the main Gondwanan land mass, leading to the formation of the Rheic Ocean in the Early Ordovician. As Avalonia-Carolina drifted north from Gondwana, the Rheic Ocean grew and reached its maximum width () in the Silurian. In this process, the Iapetus Ocean closed as Avalonia-Carolina collided with Laurentia and the
Appalachian orogeny formed. The closure of the Rheic began in the Early Devonian and was completed in the
Mississippian when Gondwana and Laurentia collided to form Pangaea. This closure resulted in the largest collisional orogen of the Palaeozoic: the
Variscan and
Alleghanian orogens between Gondwana's West African margin and southern Baltica and eastern Laurentia and the
Ouachita orogeny between the Amazonian margin of Gondwana and southern Laurentia. == Effects on life ==