The middle Lanna/Volkhov Stage (middle Arenig) of Sweden and its conodont fauna

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Abstract
In the established European conodont zonation theParoistodus originalisZone represents an interval in the upper middle Arenig or middle Lanna/Volkhov Stage. I have so far found it at 14 Swedish localities, represented in my collections by more than 116 000 conodont elements from 79 limestone samples. Presence/absence data together with major frequency shifts among conodont taxa allow the recognition of five successive phases within theP. originalisZone and the corresponding chron. Phase 1 occurs in the basal part of the zone, where an increase in frequency ofTriangulodus brevibasisandP. originalisdistinguishes it from immediately subjacent beds. Phase 2 corresponds to the (transgressive) lower middle part of the zone with an influx ofPeriodon, andDrepanoistodus basiovalisreplacingD. forceps. Phase 3 is equivalent to the upper middle part of the zone with generally more than 30% of the zonal denominator and withMicrozarkodina parvareplacingM. flabellum. Phase 4 is a short (regressive) event where the frequency ofParoistodusis drastically lowered, and phase 5 corresponds to the upper part whereParoistodusagain increases. The zone is particularly well represented at Lanna, Province of Närke, in south-central Sweden, where it also seems possible to connect the conodont succession with the standard trilobite zonation. Thus, at Lanna as well as at Finngrundet in the Bothnian Bay,Megistaspis (M.) simonfirst appears in the basal lower middle part of theP. originalisZone.

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