Middle Ordovician cephalopod biofacies and palaeoenvironments of Baltoscandia

Abstract

During the Middle Ordovician cephalopods became an important part of the macrofauna of the Baltoscandian carbonate platform. The earliest cephalopod abundance peak was reached during the early Darriwilian, within the Kunda Stage Yangtzeplacognathus crassus and Lenodus pseudoplanus conodont zones. In sediments of this time interval large orthoconic cephalopods often occur in masses with more than one specimen per square‐meter on bedding surfaces. The assemblages are characterized by the strong dominance of often large endocerids. In proximal depositional settings coiled tarphycerids and other cephalopod groups are an important additional component. In the most distal settings orthocerids are the most important secondary component. Correspondence Analysis of assemblages throughout Baltoscandia revealed three distinct biofacies, which here are termed Orthocerid, Proterovaginoceras and Anthoceras Biofacies, respectively. The biofacies reflect differences in depth and proximity to the shoreline and are consistent with the Baltoscandian Confacies Belts. Spatial changes in absolute abundance and taxonomic composition indicate increased original cephalopod population densities and habitat expansion within the Y. crassus and L. pseudoplanus conodont zones. A nearly coeval abundance peak in a similar facies in South China indicates supraregional causes of the mass occurrence, probably reflecting a globally increased nutrient availability in the water column during the Darriwilian.

Baltoscandian conodont biofacies fluctuations and their link to Middle Ordovician (Darriwilian) global cooling

“…Thus, the distribution of the Baltoniodus–Microzarkodina Biofacies and these two cephalopod biofacies is different from that of the confacies belts of Jaanusson (). The Orthocerid cephalopod biofacies of Kröger & Rasmussen () was recorded from distal shelf deposits of the Scandinavian Caledonides and Scania (southern Sweden), and overlaps with the Protopanderodus and Periodon conodont biofacies.…”

“…Dark grey wackestone and mudstone dominate (Nielsen ; Stouge & Nielsen ). Trilobites (Nielsen ) and orthocerid cephalopods (Kröger & Rasmussen ) are common associated faunal elements in this biofacies.…”

“…), ostracods (Jaanusson , ; Tinn et al . ), cephalopods (Kröger & Rasmussen ), chitinozoans (Vandenbroucke et al . ) and microfacies analyses (Jaanusson ; Eriksson et al .…”

“…), meaning that it is possible to test whether or not their lateral distribution was related to palaeoenvironmental factors. Accordingly, Kröger & Rasmussen () analysed Darriwilian (Dw1–Dw2; Lenodus variabilis – Lenodus pseudoplanus conodont zones) cephalopods from the so‐called ‘Orthoceratite limestone’ of Baltoscandia and established three distinct cephalopod biofacies across the continental platform. The Anthoceras Biofacies was shown to characterize the proximal shelf palaeoenvironment of Öland (southern east Sweden) and northern Estonia, the Proterovaginoceras Biofacies populated the intermediate shelf settings of the Oslo Region, Norway (Slemmestad, Krekling) and central Sweden (Hällekis), while the Orthocerid Biofacies was typical of the distal shelf settings west of Lake Mjøsa, Norway (Røste) and Scania (Killeröd) (Kröger & Rasmussen , fig.…”

Normal giants? Temporal and latitudinal shifts of Palaeozoic marine invertebrate gigantism and global change

“…) and they occur regionally in rock‐forming numbers (e.g. Ordovician: Kröger & Rasmussen ; Silurian: Bogolepova & Holland ; Gnoli ; Devonian: Wendt ; Fig. ).…”