The Late Silurian Lau Event and brachiopods from Gotland, Sweden

Sammanfattning: The impact of the Ludfordian Lau Primo-Secundo Event on brachiopods from Gotland has been investigated. About 12 000 individuals were identified from five samples. The results are supplemented by an extensive literature study. On Gotland, the Lau Event spans the Botvide Mbr (uppermost När Fm, upper Hemse Group) and the Eke Fm. It caused extinctions, faunal, sedimentary and isotopic changes. Conodonts responded heavily; more than 50% of the taxa became extinct or disappeared and during the later part of the event the conodont fauna became dominated by a single taxon. Brachiopod diversity was not as dramatically affected, but there was a considerable faunal re-organisation. Based on mainly brachiopod frequencies and ranges, the investigated interval have been subdivided into five parts: before and after the event, and early, intermediate and late during the event. Out of the 63 listed taxa in this study, 42 existed already before the investigated interval. However, only 27 taxa are reported from the När Fm and only 18 from localities containing strata deposited before the onset of the event. Ten of those are for the first time proven to occur in the upper När Fm. During the early part of the event at least five taxa became extinct (e.g. Jonesea grayi, Atrypa (A.) sowerbyi, cf. Kirkidium knighti) and five other first appeared (Ptychopleurella bouchardi, Atrypa (A.) alata, Endrea ekenia, Navispira pusilla, Boucotinskia sulcata). More than nine Lazarus taxa are also reported from the early part, the majority known form a wide range of facies. The intermediate part of the Lau Event seems to have allowed for the greatest diversity among brachiopods at Gotland. Out of the 42 pre-event taxa, 31 occur at the upper Lower Eke locality Lau Backar 1, together with six taxa first appearing during the event. By the end of the intermediate part at least seven taxa became extinct (e.g. ”Dinorthis rigida”, Endrea ekenia, Mesopholidostrophia laevigata, Janius barrandi). During the later part of the event no taxa are proven extinct, except for at the Eke-Burgsvik boundary, where lithology as well changed considerably. The majority of the eight disappearing species seems to have been more or less facies dependent and whether they responded to direct event conditions or facies changes is not clear. Only Atrypa (A.) alata, and possibly Nucleospira pisum, are considered extinct due to the changing oceanic conditions. At least 25 of the 42 pre-event taxa survived the event. Among taxa that first appeared during the event, at least two (Navispira pusilla and Boucotinskia sulcata) also occur in the Burgsvik and Hamra formations. After the event especially the atrypids radiated quickly. Only nine or ten taxa have more or less continuous ranges through the whole event. Nevertheless, several of these probably suffered from adverse conditions. Isorthis canaliculata, Protochonetes striatellus and Microsphaeridiorhynchus? nucula show distinct Lilliput effects, and the pentamerids may also have been affected. The herein established re-organisation pattern among brachiopods from Gotland is also found in other brachiopod faunas, on Baltica and outside. It is clearly visible in published faunas from Lithuania, Ukraine, Russia, Australia, and to some extent also from Bohemia. Lastly, the extended ranges of Ptychopleurella bouchardi, Mesopholidostrophia laevigata, Strophonella euglypha, Endrea ekenia and Nucleospira pisum, as well as the identification of cf. Kirkidium knighti at Botvide 1, reflects the importance of large collections.