Peep Männik

Integrated biostratigraphy of the lower Silurian of the Aizpute-41 core, Latvia

Integrated graptolite, conodont and chitinozoan biostratigraphical data is presented from the Rhuddanian through to lower Sheinwoodian of the Aizpute-41 core, Latvia. Correlation of the biozonation schemes based upon the three groups is achieved from the cyphus through to lowermost riccartonensis graptolite biozones, except for the upper Aeronian and lower Telychian, which lack both chitinozoans and graptolites, and upper lapworthi through to approximately base murchisoni graptolite Biozone, where there is interpreted to be an unconformity. Datum 2 of the Ireviken Event is correlated with a level at the base of or within the murchisoni Biozone. It is possible that the changes in conodont assemblages at Datum 2 on Gotland are the result of an unconformity here. Streptograptus? kaljoi sp. nov., from the lower spiralis graptolite Biozone, is described.

Implications of Gondwana glaciations in the Baltic late Ordovician and Silurian and a carbon isotopic test of environmental cyclicity

Four glaciations -- the extensively studied Hirnantian episode and three in the Llandovery-earliest Wenlock -- are well established on the Gondwana palaeocontinent. New data [Hamoumi, 1999] shift the beginning of the glacial epoch to the early Caradoc when Baltica was moving from middle to low latitudes of the southern Hemisphere. Despite the rather considerable distance between polar areas of Gondwana and subtropical Baltica all four glacial events are reflected via global climatic mechanisms in the East Baltic. It is generally accepted that glaciations are marked by positive excursions of {delta}18O and {delta}13C values caused by increase of the polar ice caps, bioproduction and decrease of oceanic water temperature, etc. Based on these relationships, the Gondwana glacial events are correlated with coeval isotopic shifts established in sections of Baltica. In addition, agreement of the oceanic processes and corresponding carbon isotopic trends predicted by Jeppsson [1990] to real measured values is analysed. The following positive {delta}13C excursions are recorded in the Baltic area (peak values in parentheses) : middle Caradoc (2.2 {per thousand}), early Ashgill (2.5 {per thousand}), Hirnantian (6 {per thousand}), early Aeronian (3.7 {per thousand}), early Telychian (2.7 {per thousand}), early Wenlock (5.2 {per thousand}). Most of these shifts correlate well with glacio-eustatic sea level lowstands and biodiversity changes, as shown by the most extensive Oandu crisis in the Caradoc, Hirnantian mass extinction and the Wenlock Ireviken Event. Analysis of data allows the following conclusions : (1) all four Gondwana glaciations identified by tillites, microconglomeratic clays, etc. and dated biostratigraphically are recognised in the Baltic area through clear positive carbon isotope excursions at the same levels; (2) three smaller carbon isotope excursions in the Caradoc and Ashgill together with algal abundance data suggest the presence of several colder climate episodes during the late Ordovician. This may support the idea of the earlier onset of the glacial epoch on Gondwana; however, correct biostratigraphic dating of supposed glacial sediments is required; (3) the carbon isotopic testing of the oceanic model by Jeppsson reveals too many contradictions between model predictions and measured values. This means that the environmental background of isotopic events and relationships with oceanic events should be revised ; (4) for delimitation of the climatic --oceanic episodes, a more general marker identifying environmental change via a basinal approach seems useful. For this purpose lithological, geochemical or palaeontological criteria can be used.

Integrated biostratigraphy of the lower Silurian of the Kolka-54 core, Latvia

Integrated graptolite, conodont and chitinozoan biostratigraphical data are presented from the Llandovery and Wenlock of the Kolka-54 core, Latvia. Correlations between graptolite and chitinozoan biozones are consistent with those published from other East Baltic sections and the Welsh Basin. While most correlations between graptolite and conodont biozones agree with those presented in previous studies, there are important exceptions. Significantly, we report here the discovery of Distomodus staurognathoides Biozone conodonts in the lowest Aeronian Demirastrites triangulatus graptolite Biozone. The base of the D. staurognathoides Biozone was previously considered to lie much higher in the Aeronian. Also it is shown that Walliserodus survived the late Wenlock Mulde Event, during which it was considered previously to have become extinct.

New data on Ordovician–Silurian conodonts and stratigraphy from the Severnaya Zemlya Archipelago, Russian Arctic

Thirty samples from 22 sections collected by the SWEDARCTIC international expedition to Severnaya Zemlya in 1999 contained Ordovician and Silurian conodont faunas. Several taxa, including Apsidognathus cf. milleri , Aulacognathus cf. kuehni , Nudibelodina sensitiva , Ozarkodina broenlundi and Pterospathodus eopennatus , allow precise dating of the strata in this region for the first time. The occurrence of Aphelognathus pyramidalis and Rhipidognathus aff. R. symmetricus in samples from the Strojnaya Formation fits well with the earlier dating of these strata as latest Ordovician. However, Aphelognathus sp. in sample BG-99/14-a, collected from the upper Ushakov Formation, indicates that at least in the lower reaches of the Ushakov River the top of this formation is considerably younger than considered earlier: the sampled strata are Late, not Early Ordovician in age. In the Ordovician and Silurian the present-day Severnaya Zemlya region was dominated by extensive shallow-water, mainly semi-restricted basin environments with habitat specific faunas. The occurrence of Riphidognathus aff. R. symmetricus at some levels in the Upper Ordovician suggests extreme shallowing episodes in the basin. On Severnaya Zemlya, ‘normal-marine’ faunas (including Pt. eopennatus ) invaded the distal peripheral regions of the wide shallow-water platform at times of maximum sea-level rise only. The occurrence of Oz. broenlundi and N. sensitiva indicates that in the early Silurian the Severnaya Zemlya basin was quite well connected to the basins over modern North Greenland as well as to the Baltic Palaeobasin. The lower Silurian conodont assemblages in the Vodopad to Samojlovich formations are most similar to those described from the eastern Timan–northern Ural region.

New data on Ordovician stable isotope record and conodont biostratigraphy from the Viki reference drill core, Saaremaa Island, western Estonia

The Viki drill core is one of the reference sections for the Estonian bedrock succession, characterizing strata of Mid Ordovician through early Silurian age in the eastern part of the Baltoscandian basin. In this article, we present the hitherto missing Ordovician stable carbon isotope curve and Middle Ordovician conodont biostratigraphy from the Viki core to complement the previous lithological, geochemical, geophysical and biostratigraphic studies. For the first time, the Hirnantian sulfur isotope excursion is reported from the eastern Baltic region, showing high amplitude and tight coupling with carbon cycling.

Conodonts from the Niur Formation (Silurian) of the Derenjal Mountains, Central Iran

A Llandovery to Ludlow age for the Niur Formation in the Derenjal Mountains (east-central Iran) is proposed based on new conodont data and previous work on other fossils. The uppermost part of the studied section yielded no diagnostic conodonts but may be Pridoli in age. Some intervals can be dated more precisely: Unit 11 (at least its upper part) is middle Telychian in age and corresponds to the Pterospathodus amorphognathoides lennarti Zone; the lowermost part of Unit 16 is earliest Ludlow in age and corresponds to the Kockelella crassa Zone; the uppermost Unit 16 is late Ludlow (Ludfordian) in age and corresponds to the Ozarkodina snajdri Interval Zone. The Llandovery-Wenlock boundary lies between units 12 and 13 based on sedimentological evidence. The precise location of the Wenlock- Ludlow boundary in the section is not clear but lies below Unit 16. Present-day Iran was located far away from Baltica and Laurentia, on the other side of the Rheic Ocean. This ocean does not seem to have been a major migration barrier for most organisms including the conodonts.

A new early Silurian prioniodontid conodont with three P elements from Iran and associated species

A prioniodontid conodont Arianagnathus jafariani gen. et sp. nov. from the late Llandovery part of the Niur Formation of the Derenjal Mountains, East Central Iran had an apparatus bearing 3 pairs of P elements. Pa elements of its apparatus are closest to those of Icriodella sandersi (Llandovery—Wenlock boundary interval, Wales, Great Britain) in the weak development of an icrion. Due to the small sample size not all S-elements have been identified but those present are similar to those described in the Icriodella and Icriognathus apparatuses. Based on similarities with previously described apparatus Notiodella we suggest that Arianagnathus jafariani gen. et sp. nov. probably had an apparatus of 17 elements. Arianagnathus is therefore an important additional example that has potential for aiding the future revision of the palaeobiological arrangement of elements within and the phylogeny of conodont apparatuses with 3 P elements, one of which is icrion bearing. The completely known apparatus of associated Ozarkodina derenjalensis sp. nov. shows similarity to some unnamed Ozarkodina from Wales, Great Britain. Many of the conodonts found in the Llandovery part of the studied section are cosmopolitan; the new conodont species seem to have their possible closest relatives in Avalonia.

New data on the stratigraphy of the Ordovician and Silurian of the central region of Kotelnyi Island (New Siberian Islands) and correlation with the synchronous successions of the Eastern Arctic

Much of the material of Paleozoic successions of Kotelnyi Island (New Siberian Islands) is composed of various carbonates, which accumulated in relatively shallow conditions in the Ordovician–Middle Devonian. Such deposits are widespread in synchronous sections throughout the Eastern Arctic and therefore attract attention as a potential tool for interregional correlations. In paleotectonic reconstructions, Kotelnyi Island either is included in terrain which was disconnected from the Siberian Platform in the Paleozoic or is joined to it. These different interpretations result from lack of primary evidence. This paper presents new data on stratigraphy and macroand microfauna of the Upper Ordovician and Silurian of the central regions of Kotelnyi Island. For this interval, we propose a correlation of the studied sections with those of adjacent regions. A conclusion is drawn on the basis of the similarity of lithology and fossil assemblages and also shared trends in the evolution of the sedimentary environment that the Upper Ordovician–Silurian beds of Kotelnyi Island, Taimyr, and the Siberian Platform accumulated in a single shelf basin, which apparently also extends to northeastern Chukotka.

The ordovician-silurian boundary on the western slope of the Subpolar Urals

Data obtained using different methods: paleontological, sedimentological, event stratigraphy and C-isotope chemostratigraphy of a unique succession of the Upper Ordovician and lower Silurian, located on the western slope of the Subpolar Urals, are presented in this work. The data obtained made it possible to revise some existing ideas about the texture of the Upper Ordovician succession and clarify the position of the Ordovician-Silurian boundary in the region. In addition, the Upper Ordovician Yaptiknyrd Formation was correlated with the synchronous formations in Scotland and Estonia.

Conodonts in Silurian hypersaline environments: Specialized and unexpectedly diverse

Hypersaline environments are commonly assumed to be barren of metazoans and therefore are avoided by paleontologists, yet a number of early Paleozoic jawless vertebrate groups specialized to live in such settings. Sampling bias against restricted settings resulted in substantial underestimation of their diversity. Rare studies venturing into such environments yielded multiple new species of conodonts, suggesting that the diversity and habitat range of these hyperdiverse predators of the early oceans are equally underestimated. We describe here autochthonous conodont fauna from evaporite-bearing horizons from the middle Silurian of Estonia that provide evidence for efficient osmoregulation in this group. Based on a global compilation of coeval conodont assemblages, we show that marginal-marine, periodically emergent environments were characterized by higher conodont diversity than open-marine shallow settings. This diversity is due to a high number of species occurring in these environments only. The high degree of specialization is also reflected by the highest within-habitat variability (β diversity) in marginal settings. Most conodont species had narrow environmental niches and, unlike in marine invertebrates, extreme environments were inhabited by the most specialized taxa. Such environments represent a large proportion of early Paleozoic tropical epicratonic basins. Our analysis allows quantification of the degree to which mid-Silurian conodont diversity is underestimated as a result of sampling bias against marginal-marine settings.