Faunal succession in the upper Cambrian (Furongian) Leptoplastus Superzone at Slemmestad, southern Norway

Abstract

The ‘Middle’ Cambrian–Tremadocian Alum Shale Formation is generally strongly tectonised in the Oslo area (southern Norway) and, as such, no complete succession through the Furongian exists in the area. The present paper presents a restored part of the Furongian interval through the Leptoplastus Superzone that has been measured and sampled at Slemmestad harbour. The limestone layers and nodule levels within the shale are preserved in the correct stratigraphic order with only limited repetition due to thrusting. Here the Leptoplastus Superzone is about 3.1 m thick. Within Scandinavia the superzone has previously been subdivided into the Leptoplastus paucisegmentatus, L. raphidophorus, L. crassicornis, L. ovatus, L. angustatus and L. stenotus zones, of which the L. paucisegmentatus Zone has not been recorded in the Oslo area. It may be truly absent or the interval may be devoid of fossiliferous limestone. It is alternatively possible that the index fossil is absent for ecological reasons. Variable first appearances of key taxa have somewhat confused the subdivision of the Leptoplastus Superzone. The order of appearance of L. ovatus, L. crassicornis and L. angustatus thus differs between Slemmestad and Andrarum in southern Sweden, and consequently the L. ovatus Zone is herein proposed abandoned as L. ovatus co-occurs with L. crassicornis. The L. angustatus Zone has also been proposed abandoned by recent authors. Based on detailed biostratigraphical data from Slemmestad and Andrarum, a combined L. crassicornis–L. angustatus Zone is defined; the lower boundary is characterised by the first appearance of L. crassicornis. The present study furthermore shows that Protopeltura holtedahli occurs in the Parabolina spinulosa Zone and that Protopeltura broeggeri is restricted to the upper part of the new Leptoplastus crassicornis–L. angustatus Zone. These Protopeltura species do not characterise separate zones or subzones as previously proposed. A lectotype of Ctenopyge neglecta var. bornholmensis Poulsen, 1923, now Leptoplastus bornholmensis, is designated and reillustrated..