Conodont biostratigraphy and diversity in The Lower-Middle Ordovician of Eastern Baltoscandia (St. Petersburg region, Russia) and Kazakhstan

Sammanfattning: Conodonts were first discovered and described by Christian Pander in 1856 from the St. Petersburg region. In this area, however, there are only a few subsequent studies dealing with the Lower and Middle Ordovician (Hunneberg-Volkhov regional stages) conodonts and the main aim of this work is to provide the first detailed investigation of conodont biostratigraphy, taxonomy and diversity changes within this interval. The studied sequence encompasses the following eight conodont zones from the Hunneberg, Billingen and Volkhov regional stages: (1) Paltodus deltifer, (2) Paraistodus proteus, (3) Prioniodus elegans, (4) Oepikodus evae, (5) Baltoniodus triangularis, (6) Baltoniodus navis, (7) Microzarkodina parva, and (8) Baltoniodus norrlandicus. Quantitative and methodological aspects of the condensed and strongly bioturbated Lower to Middle Ordovician strata in the vicinity of St. Petersburg have been studied, including conodont sampling, errors in determinations of the first appearance of taxa, community properties, bioturbation, sample size and laboratory treatment. The Lower and Middle Ordovician conodont communities are characterised by relatively high evenness (Shannon/Weiner index J' ~ 0,6), high abundance of elements (up to 25 000 per 1 kg of sediment) and moderate species richness (16-12 species). The abundance of conodont elements is variable through the studied interval and demonstrates a high degree of lateral variability. The number of conodont species gradually decreases from the Hunneberg to the upper Volkhov. Diversity changes within the conodont communities through the investigated interval appear to reflect minor faunal response to environmental change and indicate a pelagic mode of life for the investigated taxa. Two conodont species Scolopodus striatus Pander and Drepanodus arcuatus Pander collected from the type locality (Popowka River section, Russia) were revised within the framework of this study. The relatively shallow water palaeocommunities in the Lower to Middle Ordovician of Eastern Baltoscandia are directly compared with recently discovered and contemporaneous deep-water pelagic faunas from Kazakhstan. The pandenmic species Paracordylodus gracilis Lindström has been studied from the shallow water sediments of Baltoscandia as well as from the pelagic deep-water successions of Central Kazakhstan.