420 Ma (late Silurian and earliest Devonian)

This includes the Wenlock (base 429 Ma), Ludlow (base 424 Ma), the Pridoli (base 420 Ma), the Lochkovian (base 418 Ma) and the Pragian (base 414 Ma, top 410 Ma). In contrast to the period preceding it, although a broad band of almost cosmopolitan benthic faunas existed at low and intermediate palaeolatitudes, there were also distinctive high latitude faunas in both hemispheres. Although not figured here, the brachiopod-based Clarkeia Fauna was confined to southern hemisphere higher latitudes in southern Gondwana and the Tuvaella Fauna to northern hemisphere high latitude localities in northern (today’s southern) Siberia and North China (Cocks & Scotese 1991, Rong et al. 1995). The global climate ameliorated slowly during the early Silurian, with the last known glacial sediments before the Carboniferous being found in the Wenlock of Brazil (Caputo 1998). The Precordillera had docked with Gondwana (Argentina) before this time (Benedetto 1998).

Avalonia and Baltica collided at about 440 Ma, but from 425 Ma to 420 Ma these combined landmasses collided with Laurentia to produce the Scandian Orogeny.  Shortly after collision, these amalgamated landmasses (Laurussia) appear from palaeomagnetic data to have drifted rapidly southward whilst undergoing counter-clockwise rotation (compare Figs 8 and 9).  This rapid departure to southerly latitudes following collision is remarkable but can be explained as artificial and caused by tilting of the Earth’s rotation axis (true polar wander - Van der Voo 1994, Torsvik et al. 1996).

Most previous reconstructions show Armorica as detached from Gondwana, but we do not agree, and with our revised Gondwana mean pole (Table 1), it is appropriate to keep the two together and this is supported by the similar faunas on both areas. On our map (Fig. 8) are plotted data for Lower Devonian (Lochkovian and Pragian) fish. Despite much literature to the contrary, Blieck & Janvier (1999) make a convincing case that these fish lived in essentially marine conditions, although often in reduced salinity, rather than in fresh water. Young (1990a) described the pattern of provinciality, with the very endemic amphiaspid faunas of Siberia, the osteostracan cephalaspid province of Laurussia and the wattagoonaspid-phyllolepid province of Gondwana and (by then detached) Armorica and Perunica. Young grouped both South and North China within a single galeaspid-yuanolepid province, but Blieck & Janvier (1999, p. 93) believe that the latter province was endemic only to the South China plate in the early Devonian, and we follow them here. However, in contrast to the fish, the Lochkovian and Pragian ostracodes (whose spat usually found it notoriously difficult to cross oceans) were the same in Laurussia, Perunica and Armorica; indicating that the Rheic Ocean between Gondwana and Laurussia was unlikely to have been of substantial width.