Conodonts from the lower Ordovician at Hunneberg, south-central Sweden

Six sections through lower Ordovician shale and limestone at Hunneberg have been sampled for conodonts. Forty-nine limestone horizons and 34 shale surface samples yielded over 40000 conodont elements. The degree of biostratigraphic resolution permitted further subdivision of the post-Tremadoc Paroistodus proteus Zone into four successive intervals. Preliminary studies of coeval rocks in other areas of Sweden show that this subdivision applies elsewhere as well. Comparison with published sections from, for example, Newfoundland, Estonia and Kazakhstan indicates that the fourfold subdivision of the P. proteus Zone proposed here could significantly increase the correlation accuracy, even internationally, within this critical part of the Ordovician. Graptolites and trilobites found together with the conodonts at Hunneberg indicated that the new subdivision could help to correlate biostratigraphic units based on these fossil groups. Thus the two lower intervals of the P. proteus Zone correspond to the M. (E.) armata trilobite Zone, and the upper two are approximately equal to the M. (V.) planilimbata Zone. The base of the T. phyllograptoides graptolite Zone at Hunneberg lies close to the boundary between the upper middle and uppermost intervals of the P. proteus Zone, but the graptolitic chronozonal boundary may be lower. The overlying Prioniodus elegans Zone is mainly represented by graptolitic shale at Hunneberg, but conodont faunas have been found in it, and co-occurrences of conodonts and graptolites have been noted