Silurian

Abstract The Early-Middle Llandoverian was similar to the end of the Late Ordovician in the general features of palaeogeography and biotas. In the southern hemisphere, Gondwana was still integral, except for a number of small separate plates along its margins, and it was situated mostly in the middle and high latitudes south to 30°S. Present-day northern Africa and southern South America were situated in the polar circle. The late Late Ordovician is noted for widespread glaciation, especially in Gondwana. Under the influence of the immense glaciation, global sea-level underwent a sharp drop of up to more than 100 m during the late Ashgill of Late Ordovician (Rong Jiayu et al. 1984). The environmental changes were so dramatic as to cause the decline or extinction of many marine groups. The most obvious example by which to illustrate this phenomenon is the almost complete extinction of the widespread Dalmanitina-Hirnantia fauna during the late Ashgill. Furthermore, up to the 90 per cent of the Late Ordovician trilobite genera disappeared during late Ashgillian. Nautiloids also showed a sharp decline.