PALAEOZOIC/Ordovician 179 into the Ashgillian North African Gondwanan Caradocian successions include endemic genera adapted to cool-water conditions close to the pole In deepwater sites, graptolitic successions are well-studied, particularly in the Glenkiln and Hartfell Shales of the Southern Uplands of Scotland, where Lapworth was a pioneer in establishing the succession of faunas In Wales, the Caradocian was a time of intense volcanic activity in the area around Snowdon, with several kilometres of varied pyroclastic and eruptive rocks, which are classics in the study of ‘fossil’ volcanoes Ashgillian The youngest, and shortest, subdivision of the Ordovician System (Figure 2) has a type section in the English Lake District, at Ash Gill A correlative section at Foggy Gill shows continuous passage downwards into the Caradocian The base of the Ashgillian at the base of the Pusgillian local ‘stage’ is recognized entirely on a shelly fauna of brachiopods and trilobites Conodonts and, until recently, graptolites were unrecognized from this locality, and this has made it difficult to locate the base of the Ashgillian in the graptolitic (and conodont) schemes, although a horizon within the Pleurograptus linearis biozone has often been accepted Fossil faunas are not strikingly different from those of the underlying beds and are diverse, but higher in the succession there is a decline in the Rawtheyan local ‘stage’, presaging a much more drastic reduction as the climate deteriorated at the end of the Ordovician Shallow-water tropical strata in North America correlative with the Ashgillian, the Cincinnatian limestones, are prodigiously fossiliferous The stratigraphy of Ashgillian strata in Bohemia, Scandinavia, and China is known in equal detail There is, in general, a more cosmopolitan fauna in this part of the Ordovician, but cool-water high-latitude faunas still endured close to the pole in northern Africa at the core of the Gondwana supercontinent Graptolitic deposition continued in deepwater sites distant from continental influences, as in southern Scotland and Taimyr The youngest subdivision of the Ashgillian, the Hirnantian, marked the onset of the Ordovician glaciation, which is accompanied by drastic lithological and faunal changes in most stratigraphical sections caused by a worldwide regression In many places there is an unconformity at this horizon, while in all sections spanning the interval there is a reduction in faunal diversity, often accompanied by shallow-water clastic sedimentation Life in the Ordovician Figure Late Ordovician (Ashgillian) strata underlying Conway Castle, North Wales The Ordovician is arguably the period of greatest marine diversification in the fossil record Groups of animals that appeared in the Cambrian (e.g bivalved molluscs and gastropods) became much more diverse and widespread Other groups appeared and diversified for the first time These included corals and bryozoans, which, after a rare occurrence in the Tremadocian, formed diverse bioherms and even reefs before the Caradocian Cephalopod molluscs – the nautiloids – achieved great size and variety of form, and presumably were among the most effective predators that served to structure the ecosystem The free-floating graptolites (see Fossil Invertebrates: Graptolites) became widespread in the world’s oceans and are often abundant macrofossils in off-shelf sites where few Cambrian animals can be found Trilobites (see Fossil Invertebrates: Trilobites) continued to diversify from the Cambrian and included free-swimming as well as benthic forms, which did not survive into the Silurian Filter-feeding trilobites were highly characteristic of soft substrates; for example, trinucleid trilobites are abundant in, and confined to, Ordovician strata Among predatory trilobites, the largest example known from the whole group (Isotelus) was discovered in Ordovician strata in Canada