Late Ordovician Holorhynchus succession in the Siljan district, Sweden: facies, faunas and a latest Katian event

Abstract

The pentameride brachiopod genus Holorhynchus is a useful tool for correlation of the upper part of the Katian Stage in the Upper Ordovician. Holorhynchus occurs in the Boda Limestone, Siljan district, Sweden, but is generally only known from taxonomic descriptions. Here, we report in detail fossil occurrences relating morphology and sedimentary facies within the context of a major faunal turnover during the latest Katian. The specimens occur exclusively within the intercalated beds of pelmicritic mudstone and fossiliferous layers, which are down-slope storm deposits. The comparison of sedimentary features between the beds with and without Holorhynchus suggests that the animal inhabited fluid conditions on a muddy substrate, such as the channel environment reconstructed herein. The geological and autecological evidence suggests that the environmental conditions on the seabed were eutrophic. The opportunist Holorhynchus rapidly explored new habitats during a time of transition.