Brachiopod faunal dynamics during the Silurian Ireviken Event, Gotland, Sweden

Sammanfattning: The brachiopod fauna from parts of the Lower and Upper Visby formations on Gotland has been investigated through a combined outcrop and literature study. The strata of these formations were formed during the Ireviken Event, one of the severest identified oceanic events on Gotland, with diversity changes and extinctions among several groups of marine taxa. Seven bulk samples were collected from stratigraphical intervals below and above datum points 1 to 5 of the Ireviken Event, yielding a total of 4261 identified brachiopod specimens. This assemblage is greatly dominated by small, millimetre-sized articulate brachiopods, mainly belonging to the genus Orthida, but also to the genera Dicoelosia, Atrypa, Sphaerirhynchia, Katastrophomena, Eoplectodonta, Athyridida, Pentlandina, Leptaena and Costricklandia. Of the twelve brachiopod species identified in the Lusklint 1 samples, six have their last occurrence in the investigated interval. Five species are interpreted as Lazarus taxa and ten species are strongly Lilliput affected. Previous literature on the brachiopods from the Lower and Upper Visby formations comprise 54 species (including many large-sized species) of which 23 species have their last occurrences within those formations. The discrepancy in brachiopod size and number of identified species, between the new samples and the literature, indicates the importance of bulk samples to uncover many small-sized species. On the basis of the combined outcrop and literature data, it is suggested that further detailed studies are needed to clarify the stratigraphic ranges of brachiopod species for the Lower and Upper Visby formations, and that substantial changes took place within the brachiopod fauna during the early Ireviken Event.