Changes in beyrichiid ostracode faunas during the Late Silurian Lau Event on Gotland, Sweden

Sammanfattning: This paper deals with how palaeocope ostracodes of the family Beyrichiidae, from the Silurian strata at Gotland were affected by the Late Silurian Lau Event. The Lau Event is associated with a positive carbon isotope excursion, lithology changes and faunal changes in accumulated strata related to sea-level changes and a glaciation in the Late Ludlow. In order to find out how the beyrichiid faunas where affected by the event, fossils have been studied through samples collected from five different localities on Gotland, exposing pre-Lau Event, Lau Event, and post-Lau Event strata.A total of 633 beyrichiid specimens representing 22 different taxa were identified in the samples. Based on literature studies on the beyrichiid biostratigraphy on Gotland, an additional 15 taxa were included in this study, and an additional 27 localities. Subfamilies of Beyrichiidae treated herein are; Craspedobolbininae, Amphitoxotidinae, Treposellinae, Beyrichiinae and Hexophthalmoidinae. Except Hexophthalmoidinae, which disappears before the event, all subfamilies have been affected by the Lau Event. Several taxa were negatively affected, reported as e.g. 11 last appearance datums (LAD: s), four Lazarus taxa and three Lilliput taxa. Taxa that have been negatively affected by the Lau Event are e.g. Neobeyrichia (Neobeyrichia) lauensis and Neobeyrichia (Nodibeyrichia) scissa which occurs frequently in pre-event strata, but diminishes in frequency in middle event strata, to eventually disappear in upper event strata. The lineages of taxa such as Hemsiella anterovelata, Calcaribeyrichia bicalcarata, Hoburgiella tenerrima and Plicibeyrichia ornatissima have gaps through extensive portions of the event strata, and reappear again in post-event strata. Other taxa show a positive response to the event reflected by frequent occurrences and high abundances in relation to other taxa in the Lau Event strata. Neobeyrichia (Neobeyrichia) ctenophora occurs scarcely in pre-event strata, but is very frequent in event strata. The lineage of N. (Ne.) ctenophora continues into post-event strata where it is less frequent. Similarly, Hemsiella loensis and Sleia equestris occur frequently in event strata compared to many other taxa which are either rare, occur as Lazarus taxa or have their LAD: s in Lau Event strata. A recovery of the beyrichiid faunas is apparent in the Hamra Fm., where the fauna is characterized by a moderate diversity and with reappearing Lazarus species and Lilliput species that have recovered. Based on the faunal changes, it can be concluded that a re-organisation among beyrichiids did occur during the Lau Event.