Mass concentration of Hirnantian cephalopods from the Siljan District, Sweden; taxonomy, palaeoecology and palaeobiogeographic relationships

Abstract

The Hirnantian Glisstjärn Formation (<i>Normalograptus persculptus graptolite</i> 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: <i>Dawsonoceras gregarium</i> n. sp., <i>Discoceras siljanense</i> n. sp., <i>Isorthoceras dalecarlense</i> n. sp., <i>Retizitteloceras rarum</i> gen. et sp. n., and <i>Transorthoceras osmundsbergense</i> 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 <i>T. osmundsbergense</i> 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. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.201000014" target="_blank">10.1002/mmng.201000014</a>

Palaeoecology and palaeogeography of Late Ordovician (Katian–Hirnantian) cephalopods of the Boda Limestone, Siljan district, Sweden

“…The most common genus in the Upper Boda Member is the annulated orthocone Palaeodawsonocerina , and in the densely packed cephalopod mass, occurrences of the Glisstjärn Formation smooth orthoconic proteocerids are overwhelmingly dominant in abundance (Kröger et al . ).…”

“…Additionally, the discosorid Strandoceras sphinx (Schmidt ) occurs in Podolia in the same horizon, a species of Nathorstoceras Kröger (); and a species assigned by Barskov () to Michelinoceras dnestrovense Barskov (); which probably is conspecific with Isorthoceras dalecarliense Kröger et al . ().…”

“…Kröger et al . () and Kröger () described 61 cephalopod species from the Katian–Hirnantian Boda mound limestones and associated facies (Table ). Of these, 55 species have occurrences that are restricted to the Boda Core Member, six are restricted to the Hirnantian parts of the sections, Palaeodawsonoceras suzukii Kröger, ; and Kiaeroceras heroyense Strand, are restricted to the Hirnantian Beds A–C of the Upper Boda Member (see Suzuki et al .…”

Silurian tarphyceridDiscoceras(Cephalopoda, Nautiloidea): systematics, embryonic development and paleoecology

“…More than 30 species of the widely distributed genus Discoceras have been described from Middle and Upper Ordovician strata. Discoceras has been reported from Australia (Teichert and Glenister, 1954); Canada (Whiteaves, 1897); Denmark (Rasmussen and Surlyk, 2012); Estonia (Balashov, 1953; Stumbur 1962); Kazakhstan (Barskov, 1972); northwest China (Lai and Wang, 1986; Lai, 1987); Norway (Strand, 1934; Sweet, 1958); Sweden (Kröger et al, 2011; Kröger, 2013); Tibet (Lai, 1982); and Wisconsin (Whitfield, 1882; Hyatt, 1894). Discoceras has also been found in erratic boulders of Baltoscandian origin (Lossen, 1860; Roemer, 1861; Remelé, 1890; Hyatt, 1894; Dzik, 1984).…”

“…It is known from the Baltica, North China, NE Gondwana, Kazakhstania, Laurentia, and Tibet paleocontinents. Discoceras inhabited carbonate platforms, in shallows as well as in their deeper parts up to a depth of ~100 m. Some species are also known from off-shore shales deposited under dysoxic/anoxic conditions (Kröger et al, 2009, 2011; Kröger and Ebbestad, 2014).…”