Micropalaeontological investigations of Upper Ordovician carbonate mud-mounds and enclosing strata of subsurface Gotland, Sweden, demonstrate that jaw-bearing polychaetes formed the most diverse faunal element associated with these build-ups. Although not present within the mound cores (intra-mound facies), scolecodonts, or polychaete jaws, occur abundantly immediately below and particularly above the mounds; the supra-mound facies also has the most diverse fossil assemblages. By contrast to the scolecodont distribution, the most diverse conodont faunas were recorded in the intra-mound facies. This reinforces the fact that scolecodont and conodont abundance and diversity numbers are commonly inverse to one another, suggesting that these metazoans occupied different niches and responded differently to taphonomical processes. The polychaete assemblage has no less than 27 species belonging to 12 genera, of which Oenonites, Mochtyella and Pistoprion are the most abundant. The assemblage has a characteristic Baltic signature and is similar in taxonomic composition to coeval ones from other areas of the Baltoscandian palaeobasin, such as that of present-day Estonia. A principal component analysis clusters the Gotland assemblage most closely to those recorded from shallow to transitional shelf environments of Estonia, indicating that the mud-mounds were formed in such environments.

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