Tatiana Tolmacheva

Evidence for 15 m.y. of continuous deep-sea biogenic siliceous sedimentation in early Paleozoic oceans

Upper Cambrian to Lower Ordovician radiolarites (alternations of ribbon-banded radiolarian cherts and siliceous shales) from Kazakhstan are dated accurately by means of conodonts. They represent the only known continuous sequence of abyssal deposits across the Cambrian-Ordovician boundary. The radiolarites are associated with remnants of oceanic crust (ophiolites) and form the oldest-known radiolarian ooze accumulations, which presumably formed in equatorial paleolatitudes. The Cambrian cherts contain clusters of conodonts (especially paraconodonts) that are very poorly known at present, despite their potential for improved understanding of conodont community structure and paleoecology.

The Hunneberg Stage (Ordovician) in the area east of St. Petersburg, north-western Russia

ZusammenfassungDie Bio- und Lithostratigraphie der wenig bekannten früh-ordovizischen Hunneberg-Stufe (Arenig) wird aus dem Gebiet östlich von St. Petersburg (Steinbruch Putilo und Lava Fluß) beschrieben, wo die Lakity-Schichten (Leetse-Formation) aus glaukonitischen Sanden und siltigen Tonen bestehen. Die hier beschriebene Fauna umfaßt Graptolithen, Conodonten, Brachiopoden Acritarchen, Foraminiferen und verschiedene problematische Gruppen. Die Lakity-Schichten sind innerhalb der unterordovizischen Abfolge des östlichen Baltikums einzigartig im Hinblick auf die Fülle an Graptolithen, die zusammen mit Conodonten auftreten. Die Conodonten des unteren Teils der Lakity-Schichten gehören zur obersten Subzone (Oelandodus elongatus — Acodus deltatus deltatus) derParoistodus proteus Zone, die dem unteren Teil derTetragraptus phyllograptoides Zone entspricht. Dieser Umstand deutet darauf hin, daß ein Hiatus zwischen den Lakity-Schichten und der darunter liegenden Nazya-Formation vorhanden ist. Die unteren Lakity-Schichten enthalten auch die BrachiopodenLeptembolon lingulaeformis undEosiphonotreta cf.acrotretomorpha, was darauf hindeutet, daß sie mit dem Joa-Member der Leetse-Formation in Nord-Estland korreliert werden können. Im oberen Teil der Lakity-Schichten treten Conodonten derPrioniodus elegans Zone zusammen mit Graptolithen auf und weisen auf die obere Subzone derTetragraptus phyllograptoides Zone in den skandinavischen Profilen hin. Probleme bestehen noch hinsichtlich der Definition und Korrelation der Basis der Billingen-Stufe, was jedoch außerhalb der Themenstellung dieser Arbeit liegt.AbstractThe bio- and lithostratigraphy of the poorly known early Ordovician (Arenig) Hunneberg Stage is described from the area east of St. Petersburg (Putilovo Quarry and Lava river), where the Lakity Beds (Leetse Fm.) consist of glauconitic sands and silty clays. The described fauna includes graptolites, conodonts, brachiopods, acritarchs, foraminifers, and several problematic groups. The Lakity Beds are unique within the lower Ordovician sequence of the East Baltic in the abundance of graptolites, co-occurring with conodonts. The conodonts from the lower part of the Lakity Beds belong to the uppermost subzone (Oelandodus elongatus — Acodus deltatus deltatus) of theParoistodus proteus Zone, coinciding with the lower part of theTetragraptus phyllograptoides Zone. This suggests that there is a hiatus between the Lakity Beds and the underlying Nazya Formation. The lower Lakity Beds also contain the brachiopodsLeptembolon lingulaeformis andEosiphonotreta cf.acrotretomorpha, which suggest that they may be correlated with the Joa Member of the Leetse Formation in North Estonia. In the upper part of the Lakity Beds, conodonts of thePrioniodus elegans Zone co-occur with graptolites indicating the upper subzone of theTetragraptus phyllograptoides Zone in the Scandinavian sections. There are still problems of defining and correlating the base of the Billingen Regional Stage, which are outside the scope of this paper.

Faunal composition and dynamics in unconsolidated sediments: a case study from the Middle Ordovician of the East Baltic

The Volkhov Regional Stage (Middle Ordovician) in the East Baltic preserves diverse fossil assemblages dominated by epibenthic suspension feeders. Brachiopods, ostracodes, conodonts, echinoderms and bryozoans are the main components of palaeocommunities obtained from clay horizons in the Putilovo section (St Petersburg region, Russia), whereas trilobites, machaeridians, hyolithids, graptolites, benthic foraminifers and gastropods are rare or occur sporadically. Brachiopod bioclasts volumetrically dominate the debris of the studied sediments. Quantitative faunal data are used to assess species diversity patterns, as expressed by the species richness (total number of species in the standardized sample size) and by the evenness or equitability of the community. The numerical abundance of particular taxa in each standardized sample was used to evaluate the density of the fauna. The communities in the Volkhov Stage in Putilovo Quarry reveal a remarkable stability throughout the studied interval and are characterized by high density (1000–6000 specimens per 100 g), relatively moderate species richness (10–15 species) and a moderately variable equitability (0.3–0.7) for the dominant fossil groups (conodonts, ostracodes and brachiopods). Ostracodes significantly increase in numbers within the upper part of the section. This confirms a shallowing of the basin during the late Volkhov interval. Small-scale variability of the diversity estimates does not correlate with the small-scale sea-level changes reconstructed for this part of the basin. It may be connected with error in diversity measurement, or the result of undetected environmental parameters. Variability estimates for different faunal groups are poorly correlated because particular groups have different environmental tolerances.

Conodont biostratigraphy and faunal assemblages in radiolarian ribbon-banded cherts of the Burubaital Formation, West Balkhash Region, Kazakhstan

Biostratigraphical study of the early to mid-Ordovician conodont fauna from ribbon-banded radiolarian cherts of the middle Burubaital Formation in Central Kazakhstan reveals an almost complete succession of conodont biozones from the late Tremadocian to the early Darriwilian. During this interval, biosiliceous sediments were deposited in basinal environments, inhabited by lingulate brachiopods, sponges, pterobranchs and caryocaridids in conditions of high fertility and primary productivity of surface water. The community structure of taxonomically diverse conodont assemblages typifying open oceanic environments is not significantly different from that of epicratonic basins of the North Atlantic conodont province. The regional increase of oxygenated bottom waters at the base of the Oepikodus evae Biozone is possibly related to considerable changes in palaeo-oceanographical circulation patterns. The finds of three natural clusters of Prioniodus oepiki (McTavish) enable us to propose an emended diagnosis of this species.

Morphology, evolution and stratigraphic distribution in the Middle Ordovician conodont genus Microzarkodina

Microzarkodina is a genus of mainly Middle Ordovician conodonts that has its centre of distribution in Baltoscandia, and much less commonly occurs in southern China, Australia, Argentina and Laurentia. In Baltica a series of species, Microzarkodina russica n. sp., M. flabellum, M. parva, M. Bella, M. Hagetiana and M. ozarkodella, established themselves successfully. The succession of species ranges from just below the base of the Middle Ordovician (M. russica) to the upper part of the Middle Ordovician (M. ozarkodella). The species are frequently used for biostratigraphical purposes. The largely contemporaneous species Microzarkodina bella and M. hagetiana probably both evolved from M. parva and mostly occurred in separate areas. Microzarkodina ozarkodella probably evolved from M. hagetiana. This present investigation is based on a total of 94,208 elements, collected from 20 sections and one drill-core site in Sweden, one drill-core site and one outcrop in Estonia and two sections in the St Petersburg area in Russia. The Microzarkodina apparatus probably consisted of 15 or 17 elements: four P, two or four M and nine S elements. The S elements include different Sa, Sb-1, Sb-2, and Sc element types. (Less)

Taxonomy and distribution of the Ordovician conodontDrepanodus arcuatus Pander, 1856, and related species

ZusammenfassungDrepanodus arcuatusPander, 1856, ist eine der am weitesten verbreiteten Conodontenarten des Ordo-viziums, und ihre stratigraphische Reichweite umfasst nahezu das gesamte Ordovizium. Zur besseren Definition und Ab-grenzung dieser Art wurde Material aus derPrioniodus-ele-gans-Zone untersucht, das vonPander’s Typlokalitat in der Nahe von St. Petersburg stammt. Umfangreiche Sammlungen vonD. arcuatus, die aus dem ZeitabschnittPaltodus-deltifer-Zone (Tremadoc) bis in die basalePygodus-serra-Zone (Dar-riwilian) stammen, wurden untersucht, um die Variationsbrei-te innerhalb der Spezies zu dokumentieren. Das meiste Material vonD. arcuatus kommt aus dem Arenigium. Im Rahmen dieser Revision wurden zwei weitere Arten vonDrepanodus in den schwedischen Sammlungen identifiziert. Zu-mindest eine dieser Arten,Drepanodus reclinatus (LlND-STROM, 1955), besitzt eine weite Verbreitung und ist aus Nordamerika und Siidkina bekannt. Diese Spezies ist wieDrepanodus parformis n. sp. viel seltener alsD. arcuatus, beide Arten scheinen jedoch dieselbe stratigraphische Reichweite wieD. arcuatus zu besitzen. Sowohl biofazielle als auch pala-obiogeographische Aspekte der hier diskutierten Arten wer-den dokumentiert.AbstractDrepanodus arcuatusPander, 1856, is one of the geographically most widely distributed conodont species in the Ordovician, and has been reported to range practically throughout the entire Ordovician System. Topotype material from thePrioniodus elegans Zone in the St. Petersburg area of Russia has been studied in order to better define the species, and largeD. arcuatus collections from Sweden, ranging in age from the TremadocianPaltodus deltifer Zone and into the basalPygodus serra Zone in the Darriwilian, have been examined to estimate the limits of variation. Most of the investigated material is of Arenigian age. In this process, two additional speciesof Drepanodus were identified in the Swedish collections, and at least one of these,Drepanodus reclinatus (Lindström, 1955), also has a rather wide distribution in North America and southern China. This species and the new species,Drepanodus parformis n. sp., are much less common thanD. arcuatus in the great majority of the investigated samples, but they seem to have largely the same stratigraphical range asD. arcuatus. Biofacies patterns as well as palaeogeo-graphic distribution of the taxa discussed are documented.

Spatial variations in faunal composition, Middle Ordovician, Volkhov Stage, East Baltic

Abstract Small-scale lateral variations in faunal skeletal composition and taphonomic characters were analysed at five successive levels containing soft clay layers in the Middle Ordovician (Middle and Upper Volkhov Stage) of the Putilovo quarry in the East Baltic region of Russia. At the studied levels, the relative abundance of faunal elements (mainly brachiopods, ostracodes, and conodonts) generally shows high spatial homogeneity. This even faunal composition is probably due to major time averaging, leading to equalisation of the composition of the assemblages within the units. Differences in taphonomic characters between the sub-samples are also mostly minor; variation in the degree of shell breakage is most probably the result of intensive bioturbation rather than of hydraulic transportation. Changes in the relative abundance of taxa either spatially or temporally, can be considered reliable only if they exceed the small-scale spatial heterogeneity of faunal composition as well as the errors related to the sample size and laboratory treatment of the samples. The relative error in the relative abundance of conodont taxa connected with the laboratory treatment is rather high (approximately 12%).

MORPHOLOGY AND PALEOGEOGRAPHY OF THE ORDOVICIAN CONODONT PARACORDYLODUS GRACILIS LINDSTRÖM, 1955: COMPARISON OF TWO POPULATIONS

Abstract Populations of the widely distributed Early Ordovician conodont Paracordylodus gracilis Lindström, 1955, have been measured and studied in detail. Natural clusters and rich collections of isolated elements allowed calculations of size frequency distributions and construction of survivorship curves. Small morphological differences, as well as dissimilarities in population structure between collections from separate areas, were observed. This led to the recognition of Type A populations from Kazakhstan and other Arenigian warm water, low latitude regions and Type B populations from Baltica and other areas with cool water, mainly located at high latitudes in the Arenigian.

Volkhov graptolites from the lower-middle Ordovician boundary beds of the St. Petersburg region, Russia

The lower and lowermost Middle Ordovician strata of the Volkhov Stage exposed in the vicinity of St. Petersburg, Russia, yield a graptolite fauna at several stratigraphic levels, being more often confined to the mud-mound clays than to the normally stratified, Condensed carbonates. Overall taxonomic diversity of the Volkhov graptolites is relatively low and associations include no more than seven species in ail localities. TheTetragraptus, Didymograptus andXiphograptus species found are pandemic and have quite extensive stratigraphie ranges within the mid-Arenig Series. Based on the graptolite zonation, the Volkhov Stage can be correlated to thePseudophyllograptus angustifolius elongatus andDidymograptus (Expansograptus) hirundo graptolite biozones of Baltoscandia and to the upperDidymograptus simulans toDidymograptus (Expansograptus) hirundo biozones of the British graptolite succession. TheIsograptus victoriae lunatus lower zonal boundary probably falls within the graptolite-bearing interval of the lower Volkhov Stage. As the result of indirect biostratigraphic and interpretative lithological correlations, the first appearance ofUndulograptus austrodentatus at the base of the Darriwilian Stage is deemed to be close to the boundary of theMegistaspis simon andM. limbata trilobite biozones and the lowerBaltionodus norrlandicus conodont Biozone of the upper Volkhov Stage. Seven dichograptid species belonging to four genera are described, one of them,Azygograptus volkhovensis n. sp., new.KurzfassungDie unter- und tiefst-mittelordovizische Abfolge der Volkhov-Sufe bei St. Petersburg führt in mehreren stratigraphischen Abschnitten eine Graptolithen-Fauna. Diese ist häufiger an Mud-mound-Tonlagen als an normal geschichtete, kondensierte Karbonate gebunden. Die gesamte taxonomische Diversität der Volkhov-Graptolithen ist relativ gering, und die einzelnen Vergesellschaftungen umfassen an allen Fundstellen nicht mehr als sieben Arten. Die vorliegenden Arten vonTetragraptus, Didymograptus undXiphograptus sind pandemisch und stratigraphisch recht langlebig innerhalb der mittleren Arenig-Serie. Basierend auf der Graptolithen-Zonierung läßt sich die Volkhov-Stufe auf die Biozonen desPseudophyllograptus angustifolius elongatus und desDidymograptus (Expansograptus) hirundo in Baltoscandia bzw. auf diejenigen desDidymograptus simulans bisDidymograptus hirundo in Großbritannien einengen. Die untere Grenze der Zone desIsograptus victoriae lunatus fällt wahrscheinlich in das Intervall der Graptolithen-Folge in der unteren Volkhov-Stufe. Als Ergebnis der indirekten biostratigraphischen und der interpretativen lithologischen Korrelationen dürfte das Einsetzen vonUndulograptus austrodentatus an der Basis der Darriwilium-Stufe nahe der Grenze derMegistaspis simon/M. limbatus-Trilobiten-Biozonen und der unterenBaltionodus norrlandicus-Conodonten-Biozone der oberen Volkhov-Stufe liegen. Es werden sieben Dichograptiden-Arten aus vier Gattungen beschrieben, von denen eine,Azygograptus volkhovensis n. sp., neu ist.