Trophic structure and spatial distribution of macrofaunas in the Hurlet and Index limestones (Carboniferous : Upper Viséan and Lower Serpukhovian) of Ayrshire, Scotland

Explorative multivariate numerical analysis of British Geological Survey historical Biostratigraphy collections from the Carboniferous Hurlet and Index limestones, Ayrshire, yields results capable of palaeoecological interpretation. The faunas are distributed along environmental gradients within carbonate facies that systematically extended out into other sedimentary settings. Clusters of genera in higher taxa plotted on ternary diagrams of trophic structure show both units commonly include epifaunal suspension feeders, although one cluster from the Hurlet Limestone includes epifaunal detritus-suspension feeders and another from the Index Limestone includes vagrant-epifaunal detritus-suspension feeders. All the clusters include the surficial and semi-infaunal tiers, non-motile and suspension feeding categories, but they show trends of increasing ecological complexity. The geographical distribution of the clusters shows the Hurlet Limestone palaeoenvironment was most diverse around Sorn, whereas that for the Index Limestone was diverse in all three main areas of outcrop. This variation is attributed to local fluctuations in depth, sea floor conditions and water quality. An ‘embayment’ in the palaeoenvironment of the Index Limestone, seen by draping the interpretations over a 3D computer model of the subsurface, was associated with the Kerse Loch Fault, where penecontemporaneous displacement and an inferred palaeotopographical fault scarp influenced marine water flow, environmental distribution and genus diversity.