Distribution of Ordovician-Silurian ichnofossil assemblages in Wales: implications for Phanerozoic ichnofaunas

The stratigraphic succession of west Wales may be subdivided into two dominant lithologies, namely sandstone and mudstone. The distribution of these lithologies is largely determined by tectonic activity and eustatic sea-level changes in the area. The strata contain a diverse and relatively abundant ichnofaunal assemblage consisting, at the ichnogeneric level, of fifteen forms: Chondrites, Circulichnis, Cochlichnus, Cosmorhaphe, Desmograpton, Gordia, Helminthoida, Helminthopsis, Nereites, Neonereites, Palaeophycus, Paleodictyon (Glenodictyon), Paleodictyon (Squamodictyon), Planolites, Protopaleodictyon and Spirophycus. The ichnogenera are not evenly distributed throughout the succession, the main controlling factors being toponomy, anoxia, water depth and a global extinction event. The relative importance of these controls is examined with reference to 33 previously described and taxonomically well-documented deep-water flysch ichnofossil studies.