Lanarkia

Abstract: The Late Silurian Lau Event is one of the three major events identified in the Gotland strata. It is associated with one of the largest stable carbon isotope excursion recorded in the Phanerozoic, as well as rapid lithology changes, and extinctions and faunal turnovers among various fossil groups. In this study, early vertebrates (fishes) were investigated over the Lau Event interval. The study was based on isolated scales, which are common in the Late Silurian Gotland bedrock. Twenty-six samples from the Nar Formation of the upper Hemse Group through the Hamra Formation yielded over 2500 vertebrate scales. They were studied in light microscope and identified in order to establish the vertical ranges of the taxa. Six different vertebrate groups occur in the Gotland strata; the agnathan thelodonts, anaspids, heterostracans, and osteostracans, and the gnathostome acanthodians and actinopterygians. There were pronounced faunal turnovers among the vertebrates during the event. The diverse and abundant pre-event fauna of the Nar Formation is dominated by acanthodians and includes Archegonaspis lindstroemi, Andreolepis hedei, Gomphonchus sandelensis, Nostolepis striata, Phlebolepis elegans, Thelodus carinatus, Th. parvidens, and Paralogania martinssoni. The succeeding event-fauna of the Botvide Member is marked by the extinctions of five (63 %) of these species; Pa. martinssoni disappears during the very beginning of the event, at the base of the Upper Polygnathoides siluricus conodont Subzone, the base of Botvide Member, whereas Ar. lindstroemi, An. hedei, Ph. elegans, and Th. carinatus disappear simultaneously in the Upper P. siluricus conodont Subzone slightly below the top of the Botvide Member. Paralogania? sp., Pa. ludlowiensis, Thelodus sp., Gen. indet. sp. A, Lanarkia horrida, Gen. indet. sp. B, and Thelodonti sp. A appear during the event and the latter five of these taxa occur exclusively in the event strata. The vertebrate fauna during the later phase of the event in the Icriodontid conodont Zone of the Eke Formation is low both in diversity and abundance. There is an evident decrease in scale frequency in the collections from the Botvide Member to the Upper Eke Formation event interval, also reflecting the severity of the event. Diversity and abundance increase considerably in the thelodont-dominated post-event fauna in the Ozarkodina snajdri conodont Zone of the Burgsvik and Hamra formations. It is characterised by the appearances of Poracanthodes porosus, Th. sculptilis, Septentrionia mucronata, Tahulalepis elongituberculata, Osteostraci? indet., type A, and Osteostraci? indet., type B. The changes in the vertebrate fauna can be noted also in other regions, indicating that the effects of the Lau Event on the vertebrates were restricted not only to Gotland and that vertebrates can be a useful tool for identifying the event elsewhere.