Mass concentration of Hirnantian cephalopods from the Siljan District, Sweden; taxonomy, palaeoecology and palaeobiogeographic relationships
- 1Museum für Naturkunde Berlin, Invalidenstraße 43, 10115 Berlin, Germany
- 2Museum of Evolution, Uppsala University, Norbyvägen 16, 752 36 Uppsala, Sweden
- 3Tromsø University Museum, Naharat Sciences, 9037 Tromsø, Norway
- 4Department of Earth Sciences, Palaeobiology, Villavägen 16, 752 36 Uppsala, Sweden
- *Museum für Naturkunde Berlin, Invalidenstraße 43, 10115 Berlin, Germany
Abstract. The Hirnantian Glisstjärn Formation (Normalograptus persculptus graptolite Biozone) is a succession of limestones and shales onlapping the Katian Boda Limestone in the Siljan District, Sweden. It contains a conspicuous, up to several decimeter thick bed densely packed with bipolarly oriented, orthoconic cephalopod conchs that can reach lengths of more than 120 cm. Conch fragmentation, bioereosion and the generally poor preservation of the conchs indicate time averaging and the conchs are tentatively interpreted as beached, and a result of winnowing. Ten nautiloid species were collected from the Glisstjärn Formation of which five are new: Dawsonoceras gregarium n. sp., Discoceras siljanense n. sp., Isorthoceras dalecarlense n. sp., Retizitteloceras rarum gen. et sp. n., and Transorthoceras osmundsbergense gen. et sp. n. The non-endemic taxa in most cases are known from elsewhere in Baltoscandia, except one species which is known from Siberia, and North America respectively. Proteocerid orthoceridans dominate the association, of which T. osmundsbergense is the predominant species. Oncocerids are diverse but together with tarphycerids very rare. Notable is the lack of many higher taxa, that are typical for other Late Ordovician shallow water depositional settings. Based on the taxonomical composition of the cephalopod mass occurrence it is interpreted as an indicator of eutrophication of the water masses in the area.