Ordovician and Silurian global geography

The main palaeocontinents during the early Ordovician were Gondwana, Laurentia, Baltica, and Siberia, and a brief survey is made of their limits in the Ordovician and Silurian. In particular Gondwana, by far the largest continent, is analysed as including a core of South America, Antarctica, Africa, Australia and peninsular India, and also the marginal terranes, all of which show faunal links with, but which may not have been attached, to the core, of Avalonia, Ibero-Armorica and other European fragments, Turkey, Arabia, and terranes from the Far East including South China, Sibumasu and parts of Australasia. Changes within the benthic faunas reveal that Baltica, whilst isolated in the early Ordovician, became united with Avalonia by the end Ordovician and the two with Laurentia by the mid-Silurian to form the new supercontinent of Laurussia. Island arcs had distinctive faunas during the Ordovician, in particular those in the Iapetus between Laurentia, Baltica and Gondwana and the huge Kipchak Arc which ran from Baltica to Siberia. As the period progressed, the Iapetus arcs became subducted beneath or accreted to their neighbouring cratons, and the Kipchak Arc gradually collapsed to form the core of the new Kazakhstania terrane. Gondwana drifted over the South Pole and this movement is reflected in the cratonic benthic faunas, particularly brachiopods and trilobites, which in the early Ordovician had formed a cline between the high-latitude faunas of North Africa and the equatorial faunas of the Far East and Australia, but by the Devonian lived in much warmer seas in southern Europe and cooler waters in Antarctica. The faunas also reflect the global palaeoclimates, which were warm in the early to mid Ordovician and mid to late Silurian but which were much colder in the half million year period of the late Ordovician and early Silurian, particularly in the latest Ordovician Hirnantian ice age with its attendant widespread Hirnantia brachiopod Fauna. The relative closeness of the chief palaeocontinents by the early Silurian enabled brachiopod and trilobite larvae to cross the narrower oceans, enabling a relatively cosmopolitan benthic fauna to be established over much of the globe, apart from the cooler-water higher latitude Clarkeia Fauna to the south and the Tuvaella Fauna to the north.