Isorthoceras wahlenbergi, a new Late Ordovician cephalopod from the Boda Limestone of Dalarna, Sweden

Ebbestad

2014

LET

The carbonates associated with the Late Ordovician (Katian-Hirnantian) Boda mounds of the Siljan district, Sweden, contain a rich cephalopod fauna. Cephalopods are rare in the micritic stromatactis facies of the Katian Boda Core Member, but are concentrated together with other molluscs and trilobites in synsedimentary fillings of caves and crevices of the lithified mound limestone. More than 60 cephalopod species are known from these fillings. The assemblage is dominated in richness and abundance by small orthoconic proteoceratids of the genus Isorthoceras and by curved barrandeocerids, and is considerably less endemic than the benthic fauna of the Boda mounds. Similarities are remarkable at the species level with assemblages from elsewhere in Baltica, with Avalonia and south-western Kazakhstan. A genus-level comparison reveals affinities with Laurentia. However, the genus-level similarities are interpreted as reflecting mainly similarities of the depositional environment and of physicochemical conditions. The presence of calcareous algae in the Boda Limestone and maximum hydrostatic septal strength of cephalopods indicate a depositional depth of <100 m, well within the euphotic zone. The dominance and diversity of cephalopods with vertical migrant lifestyle and the absence of actinocerids indicate a cool, nutrient-rich deeper neritic environment. The post-Katian sediments contain a drastically reduced diversity, dominated by orthocerids. The maximum hydrostatic septal strength of the Hirnantian cephalopods and sedimentological features indicate a shallowing during the Hirnantian, but a continuation of high food availability for cephalopods in the water column. □ Assemblage analysis, cephalopod diversity, Ordovician mud mounds.

Section: Geological Settingmentioning

confidence: 99%

“…Note the high morphological diversity. A, I sorthoceras wahlenbergi Niko (), PMU 26893, 26894, from K allholn. B, D awsonoceras fenestratum (Eichwald ), PMU 26702, from K allholn.…”

Section: Taxonomic Composition Of the Boda Mound Cephalopods And Assomentioning

confidence: 99%

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

Ebbestad

2014

LET

“…Fourteen specimens are assigned to this species. They are similar in all aspects to I. wahlenbergi Niko, 2008, but differ from the type of I. wahlenbergi in having a significantly lower angle of expansion.…”

Section: Comparisonmentioning

confidence: 71%

“…The species was described in detail by Niko (2008). Because I. wahlenbergi is one of the most common cephalopods in the Boda Limestone.…”

Section: Descriptionmentioning

confidence: 99%

The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden

2013

EJT

442

The late Katian, Late Ordovician Boda Limestone of Dalarna, Sweden contains a rich cephalopod assemblage. The assemblage consists of 61 species, of 31 genera, comprising almost all major Ordovician cephalopod orders. Most common and diverse are the Orthocerida. The Ascocerida are also remarkably comman and diverse. The new ascocerid species, Redpathoceras bullatum sp. nov., R. depressum sp. nov., R. magnum sp. nov., and Probillingsites scandinavicum sp. nov., give reason to revise current hypotheses on the origin an evolution of this group. An ascocerid origin from barrandeoceratids or aspidoceratids is hypothesised. The absence of actinocerids in the Boda Limestone is notable, and is interpreted as an indication of relatively cool or/and deep depositional environments. The dominance of orthocerids is provisionally interpreted as evidence for nutrient-rich waters during the time of the deposition of the Boda Limestone. Additionally, the assemblage contains the new barrandeocerids >em>Schuchertoceras fryi sp. nov., Siljanoceras varians gen. et sp. nov., Warburgoceras gen. nov. (for Cyrtoceras longitudinale Angelin in Angelin & Lindström, 1880), the new endocerid Cameroceras turrisoides sp. nov., the new oncocerid Cyrtorizoceras thorslundi sp. nov., and the new orthocerids Dawsonoceras stumburi sp. nov., Isorthoceras angelini sp. nov., I. curvilineatum sp. nov., Nathorstoceras adnatum gen. et sp. nov., N. kallholnense gen. et sp. nov., Palaeodawsonocerina? nicolletoides sp. nov., Pleurorthoceras osmundsbergense sp. nov., and Striatocycloceras isbergi sp. nov

“…The present paper expands on a similar study by Kraeger (2007) from largely coeval beds in Estonia, where the influence of Hirnantian cephalopod taxa in shaping the post-Ordovician evolutionary history of the group was highlighted. A plethora of nautiloid cephalopods are found in the Katian Boda Limestone itself, but despite a number of primary taxonomic papers many taxa remain to be described (Angelin & Lindstraem 1880;Mutvei 1957;Frye 1982Frye , 1987Niko 2008).…”

Section: Introductionmentioning

confidence: 99%

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

Ebbestad

Högström³

et al. 2011

Foss. Rec.

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. doi:<a href="http://dx.doi.org/10.1002/mmng.201000014" target="_blank">10.1002/mmng.201000014</a>