The Ordovician System: From overlapping unit stratotypes to Global Boundary Stratotype Sections and Points

For nearly a century the Ordovician System was hidden as Murchison and Sedgwick tussled over the overlapping ground between their Silurian and Cambrian systems. The Ordovician is, in fact, one of the longest of the geological periods, characterised by major magmatic and plate tectonic activity; the roles of microcontinents and volcanic archipelagos were significant in shaping the Ordovician planet and the evolution of its biotas, associated with an immense biodiversification, significant fluctuations in climate and sea levels, and the first Phanerozoic mass extinction of marine invertebrates. The period was unique in being thalassocratic; epicontinental seas had a wider reach than during any other geological period. The land areas were restricted to isolated microblocks of archipelagos of various sizes with low relief, with rivers traversing gentle gradients, carrying sparse terrigenous material seaward. It is an ancient world with few parallels elsewhere in the Phanerozoic, and little in common with Holocene ecosystems and environments. The Ordovician System was introduced by Charles Lapworth as a solution to the stratotypes of overlapping units loosely defined by Adam Sedgwick for the Cambrian and by Roderick Murchison for the Silurian. Following a period of intensive research into all the key regions of the globe, unit stratotypes in the type areas of England and Wales have been replaced by seven global stages and three series based on Global Boundary Stratotype Sections and Points, enhancing the definition of these chronostratigraphic units and facilitating global correlation. As a consequence, the biological and geological events during the period can be recognised, and the magnitude and significance of originations and extinctions understood. A global synthesis of successions in Europe (Geological Society, London, Special Publications, 532) and the rest of the world (Geological Society, London, Special Publications, 533) has emphasised the importance of a universal language for Ordovician chronostratigraphy and its dividends

Ediacaran and Cambrian stratigraphy in Estonia: an updated review; pp. 152–160

Previous late Precambrian and Cambrian correlation charts of Estonia, summarizing the regional stratigraphic nomenclature of the 20th century, date back to 1997. The main aim of this review is updating these charts based on recent advances in the global Precambrian and Cambrian stratigraphy and new data from regions adjacent to Estonia. The term ‘Ediacaran’ is introduced for the latest Precambrian succession in Estonia to replace the formerly used ‘Vendian’. Correlation with the dated sections in adjacent areas suggests that only the latest 7–10 Ma of the Ediacaran is represented in the Estonian succession. The gap between the Ediacaran and Cambrian may be rather substantial. The global fourfold subdivision of the Cambrian System is introduced for Estonia. The lower boundary of Series 2 is drawn at the base of the Sõru Formation and the base of Series 3 slightly above the former lower boundary of the ‘Middle Cambrian’ in the Baltic region, marked by a gap in the Estonian succession. The base of the Furongian is located near the base of the Petseri Formation

The Late Glacial and Early Holocene environmental history of shal low lakes in Es to nia, re vealed from subfossil ostracod data

The Late Glacial and Early Holocene shallow lake history in Estonia is documented from the fresh water ostracod subfossil record. Three cores studied consist of Late Glacial and Holocoene sediments: gyttja, calcareous mud and peat, with ostracod subfossils being well preserved in the calcareous mud. 18 fresh water ostracod species were recorded in the cores: the most common species are Metacypris cordata, Limnocythere inopinata, Cyclocypris ovum, Cypridopsis vidua, and Candona candida. Changes in the ostracod succession of the lakes can be interpreted in the context of environmental changes that were not contemporaneous but were related to the evolution of particular water bodies. Darwinula stevensoni and Metacypris cordata do not appear together in the Late Glacial and Early Holocene lacustrine sediment records of Estonia. M. cordata appears as the water body evolves or by lowering of the water level. The appearance of Scottia pseudobrowniana in the sediments refers to the stage of an overgrown lacustrine system. M. cordata appeared in south eastern Estonia at the end of the Late Glacial (~12 800 cal. BP), when the calcareous mud started to form. The ea li est subfossil r cord of M. cordata from south wes ern Estonia is from the Early Holocene

A Hirnantian deep-water refuge for warm-water ostracods in Baltoscandia

The latest Ordovician is marked by a severe climate change, the Hirnantian glaciation. This climatic event affected many marine taxa including ostracods. Rich and abundant ostracod assemblages of the Baltic Palaeobasin were severly impoverished. Many of the typical pre-Hirnatian warm-water ostracod species died out, but also some distinct, cold-water species appeared. Two very different but likely coeval latest Ordovician ostracod assemblages are recorded in the Baltic countries and north-eastern Poland. The latest Ordovician Estonian Shelf (inner ramp) is characterized by the Medianella aequa association whilst sections in the Livonian Basin (middle to outer ramp) reveal the Harpabollia harparum association that is thought to represent a cold-water assemblage belonging to the Dalmanitina-Hirnantia Fauna sensu lato. A transitional assemblage composed of a “species mixture” of typical Hirnantian cold-water and some pre-Hirnantian warm-water ostracod species is described for the first time from the Kętrzyn IG1 borehole, north-eastern Poland. The assemblage is dominated by Cryptophyllus pius sp. n. The genus Cryptophyllus is rare in the two other well-known assemblages. The discovery suggests that marginal parts of the Baltic Palaeobasin could serve as a kind of refuge for the last representatives of the ostracod faunas of the inner shelf of Baltic Palaeobasin. The Hirnantian assemblage is replaced by the low-diversity recovery assemblage that is dated as late Hirnantian-Silurian in Estonia and other areas. This suggests that the position of the systemic boundary in the Kętrzyn borehole and elsewhere in north-eastern Poland should be re-evaluated

The oldest ascocerid cephalopod from the Silurian of Estonia and notes on the biogeography of the order Ascocerida (class Cephalopoda)

The early Palaeozoic order Ascocerida is a group of morphologically unique and rare cephalopods known from the Ordovician and Silurian rocks of Avalonia, Baltica, Laurentia and Perunica. The limited Estonian record of Silurian ascocerids is complemented with a specimen from the Pähkla locality (Island of Saaremaa; Paadla Regional Stage, Ludlow Series) representing the stratigraphically oldest known occurrence of ascocerids in the Silurian of Estonia. The strata that were formerly exposed in Pähkla are likely correlated to the Hemse Group of the Island of Gotland (Sweden) having a remarkable record of ascocerids. The appearance of Silurian ascocerids in Estonia is confined to a time interval when the group had the highest species diversity and the widest geographic dispersion, reaching also outside Baltica for the first time

An enigmatic early ? palaeocope ostracode from the Arenig of NW Russia

The ostracode Lavachilina evae gen. et sp. nov. is described from the Arenig of Western Russia. Its carapace possesses a unique discontinuous velum, developed at both ends as a narrow spinose ridge. The systematic position of L. evae is uncertain. Its character set shows mostly palaeocope affinities but also some features of leiocopes. L. evae is tentatively assigned to the palaeocope family Oepikellidae, possibly representing an early member of that family. Although L. evae existed contemporaneously with palaeocopes such as Laccochilina, it does not appear to be closely related to these early eurychilinoids

Ontogeny of the ostracod Conchoprimitia osekensis

The ontogeny of the species Conchoprimitia osekensis (Pribyl, 1979) from the Šárka Formation (lower and middle Darriwilian) is evaluated. Eight ontogenetic stages (A-8 to A-1) and one adult specimen are described and illustrated. This study displays that another common species of the same stratigraphical unit, Conchoprimitia? dejvicensis Přibyl, 1979, represents early instars of C. osekensis. It clearly demonstrates the importance of such ontogenetic studies for understanding real taxonomy and diversity of fossil ostracods

Microfossils in the Ordovician erratic boulders from Southwestern Finland

Chitinozoans, ostracods and acritarchs found in four glacially transported limestone boulders from the south-western coast of Finland have been studied in order to test the usefulness of these microfossil groups in age determinations. Also rare specimens of conodonts, inarticulated brachiopods and foraminifers were found. Baltic limestone (or Östersjö limestone) was the most problematic, because only fossils with calcitic or phosphatic shells are preserved. It is concluded that the boulders identified correlate with the Uhaku and Rakvere stages of the Middle Ordovician