Animal—sediment relationships in the Ordovician and Silurian of the Welsh Basin

The model of depth-related communities has been used widely to interpret palaeogeography and bathymetric changes in the Ordovician and Silurian periods. Within this context this paper examines the role of substrate in determining community distribution. In some instances there is a clear correlation between fossil assemblages and particular sediments, particularly in nearshore facies. However, because these assemblages are transported, the relationship in life between the animals and substrate is difficult to prove. The selection of substrate by modern larvae is known to be influenced by various factors of which grain size is but one.

At the deeper end of the range of benthic organisms low oxygen levels, reflected by ‘black shale’ facies, put an effective block on outward migration. The margins of basins are particularly prone to anoxia during episodes of rapid transgression when the black shale facies can cover large areas of the shelf. It is suggested that a deepening of less than 100 m can promote such changes and so radically alter community distribution. In conclusion it is suggested that the depth-related model for communities is modified in nearshore areas by the variety of facies found there, and in deep water by the position of the oxygen minimum layer, but that it might be sound in intermediate depths.