Juvenile shells of the strophomenate brachiopods Antigonambonites (Polytoechioidea, Billingsellida), Plectella, Bilobia and Ujukella (Plectambonitoidea, Strophomenida), Leptaena (Strophomenoidea, Strophomenida) and Valdaria (Ortothetoidea, Orthotetida) from the Lower to Middle Ordovician of the East Baltic and the Silurian of Gotland preserve exceptional morphology of the umbonal region in both valves, which allows reconstruction of early stages of ontogeny and inferred larval morphology. The first formed shell in all taxa is about 350-600 mu m wide, marking a growth stage very soon after metamorphosis when secretion of calcite was initiated. Dorsal valves preserve two pairs of lateral lobes, which can be interpreted as sets of two pairs of larval setal sacs, and a posteromedian mound resembling the 'alimentary mound' of the paterinide larval shell. Ventral valves at that stage possess a well-developed pseudodeltidium and a short, wide pedicle sheath formed by accretionary growth involving the ventral mantle lobe. The delthyrium is completely sealed at the earliest observed growth stages, and there is no gap between the valves along the hinge line. The position of the pedicle opening suggests that it was not formed by a larval pedicle lobe, but represents a modification of the ventral mantle lobe. This can be compared most closely with the pedicle of chileides and dictyonellides, but not of rhynchonellates. Brachiopoda, strophomenata, Ordovician, Silurian, East Baltic, Gotland, early ontogeny.

New data on the earliest ontogeny of Mid-Ordovician Baltoscandian clitambonitoid (Apomatella, Neumania and Oslogonites) and polytoechioid (Antigonambonites and Raunites) brachiopods reveal significant differences in the life history of the taxa belonging to these two superfamilies. The Polytoechioidea and probably other members of the Billingsellida had planktotrophic larvae, in which the dorsal and ventral mantle lobes formed separately and without reversion. The ‘pedicle sheath’ in Antigonambonites is secreted by a section of modified ventral mantle and thus this ‘pedicle’ is not homologous within the pedicle of rhynchonellate brachiopods. It is likely that polytoechioids and other members of the strophomenate clade had the same type of ontogeny and mode of attachment. In contrast, the ontogeny and mode of attachment of clitambonitoids are similar to that of recent rhynchonellates: their mantle lobes and both valves formed simultaneously, and the pedicle most likely formed from the larval pedicle lobe. Evidence for the lecithotrophic nature of clitambonitoid larva is discussed. This confirms that the Clitambonitoidea, unlike the Polytoechioidea, represents an ingroup within the Rhynchonellata.

Abstract Three new orthide species: Orthidium lavensis, Orthidium gambolovensis and Ranorthis rotunda, and one new protorthide species: Skenidioides minutus, are described from the Early–Middle Ordovician (Billingen–Volkhov stages) of Estonia and north-western Russia. This is the first record of Orthidium from Baltica, whereas Skenidioides was known previously only from the Keila and Oandu stages in Estonia.

Syntrophiidine brachiopods are a rare and poorly known component of Ordovician Baltoscandian faunas. They appear in the East Baltic in the Billingenian (lower Arenig) as part of the earliest known benthic assemblages dominated by elements of the Palaeozoic Evolutionary Fauna. These faunal assemblages usually include bryozoans, ostracodes, and the earliest known porambonitoids, strophomenides and endopunctate orthides, such as Idiostrophia and Orthidium, which later became characteristic of the Whiterockian brachiopod assemblages in Laurentia, but by that time had disappeared from Baltica. The superfamily Syntrophioidea reappears in Baltoscandia in the mid Caradoc. In contrast, Porambonitoidea remained the integral part of the Baltoscandian brachiopod associations through the Ordovician. Porambonites, herein redefined on the basis of restudy of the type species P. intermedius, includes only smooth porambonitoids; taxa with the distinctive ornament of radiating rows of pits first appeared in the group in the mid Arenig. The taxa Eoporambonites gen. nov., Tetralobula peregrina sp. nov., Idiostrophia prima sp. nov. and Idiostrophia tenuicostata sp. nov. are erected.

Abstract A new orthide brachiopod Siljanostrophia jaanussoni gen. et sp. nov., is described from the Ashgill Boda Limestone of the Siljan district, Dalarna, Sweden. The spirifer-like shell has remnants of dense, hollow spines and the ventral interior closely resembles Platystrophia King, 1850, but its cardinalia shows similarity to Gnamptorhynchos Jin, 1989, which has been assigned to the Platystrophiidae. This study indicates that the affinities of Siljanostrophia and Gnamptorynchos to the Platystrophiidae require further confirmation.

Lower-Middle Ordovician (Arenig) successions in the East Baltic have been investigated for more than one hundred and fifty years. Nevertheless detailed sampling still yields new species and better ...

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.

Microbially mediated clay mud mounds are widely developed in the Lower Ordovician succession east of St. Petersburg (Russia) and are associated with a diverse and abundant fauna of brachiopods, ostracodes, echinoderms, bryozoans and conodonts. The lithology of one such mud mound in Putilovo Quarry has previously been studied, but the faunas associated with the mounds have not been investigated to date. Clay lenses in the Putilovo mud mound yield conodont assemblages belonging to the Baltoniodus triangularis and lowermost part of the Paroistodus originalis zones and these stratigraphical intervals are much thicker in the mud mound than in the coeval Lower Ordovician succession lateral to the mound. The compositions of the conodont and brachiopod assemblages are generally the same in the mud mound as in contemporaneous beds. The occurrence of relatively fewer conodont elements in the mud mound than in the surrounding successions probably indicates the higher rate of accumulation of the mud mound clays. Juvenile brachiopods are more numerous in the clays of the mud mound than outside the build-up, supporting the hypothesis that the mounds included ecologically stressed environments.

Lower-Middle Ordovician (Arenig) successions in the East Baltic have been investigated for more than one hundred and fifty years. Nevertheless detailed sampling still yields new species and better knowledge of the environment in which these organisms lived. The successions are well suited for bed by bed sampling because of the lack of tectonic disturbance and because the sequences are well documented. This study analyses collections of Billingen-Volkhov age mainly from the St. Petersburg region, but also from Estonia. A great deal of the material was obtained from the marly to clayey, soft sediment that intercalates the compact packstones and wackestones in the succession. Twenty-nine of these clay horizons were used for diversity estimates on the fauna through the succession. The most thoroughly investigated groups for this investigation were rhynchonelliformean brachiopods, conodonts and ostracodes. The results indicate that variances in diversity and abundance levels for these groups were not correlated, either to each other or to the small-scale sea level fluctuations that have been suggested for the region. However, diversity dynamics of brachiopods and ostracodes confirm the large-scale upward shallowing of the basin into the Upper Volkhov. Comparison with fossils from the limestones did not reveal any differences in faunal composition between the two preservation modes. The detailed sampling, coupled with sampling of the recently described mud mounds that occur in several outcrops, yielded large numbers of specimens. This enabled revision of earlier poorly known rhynchonelliformean genera such as Ujukella Andreev, as well as better known genera such as Porambonites Pander. In total the examined faunas include 31 genera assigned to 53 species of rhynchonelliformean brachiopods. Of these Leoniorthis and Eoporambonites are defined as new genera, and the following new species are described: Neumania paucicostata, Ranorthis rotunda, Orthidium gambolovensis, Orthidium lavensis, Skenidioides minutus, Tetralobula peregrina, Idiostrophia prima and Idiostrophia tenuicostata.

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%).

New material of the poorly known plectambonitoid brachiopod Ujukella fastigata (Rubel) is described from the Lower Ordovician (upper Volkhov) of Ingria, Russia. The closely related plectambonitoid brachiopods, U.? geometrica (Kutorga), from the Lower Ordovician of Ingria, Russia, U. sp. from the Lower Ordovician of Oland, Sweden, and U. alexandrae Andreev (= U. minima Andreev) from the Middle Ordovician of southern Siberia, Russia are redescribed in order to draw comparison with U. fastigata. These taxa are referred to the Family Leptestiidae, based on a new cladistic analysis.

Occurrences of orthid brachiopods, usually identified as Platystrophia, have repeatedly been reported from the Upper Ordovician (Caradoc, Ashgill) and lower Silurian (Wenlock) of Sweden (Lindstrom 1880, 1888; Wiman 1907; Troedsson 1921; Jaanusson 1960, 1963, 1982). However, detailed studies of their morphology and taxonomy are lacking. It has already been demonstrated in the East Baltic and St. Petersburg Region that platystrophiid brachiopods are useful for detailed biostratigraphy. In particular, Paskevicius (2000) subdivided the Ordovician deposits of Lithuania (LCB) into nineteen biostratigraphical units based on brachiopods, whereas six of them were based on a taxon of Platystrophia (s.l.) as the index species. The biostratigraphical significance of platystrophiids for the North Estonian Confacies Belt (Estonia, St. Petersburg region, north-western Moscow Basin) is also well established (Alichova 1953, 1960). This report presents preliminary results of the study of Swedish Platystrophia-like brachiopods from the Kullsberg and Boda limestones, housed in the Palaeontological Museum of Evolution, Uppsala University, and in the Department of Palaeozoology, Swedish Museum of Natural History, Stockholm. These are traditionally referred to the genus Platystrophia (s.l.). Differences in the morphology of the dorsal cardinalia of Swedish platystrophiids allow recognition of two different morphological groups. Shells from the Kullsberg Limestone (Caradoc) show a dorsal cardinalia, which is similar to most of the contemporaneous species from other parts of Baltoscandia and eastern Avalonia. The species Platystrophia lynx lynx Eichwald (1830) can be definitely identified. Probably it is the westernmost occurrence in Baltoscandia of the species otherwise widespread in the East Baltic. However, the Ashgill Boda Limestone of the Siljan district (Province of Dalarna, Sweden) contains also shells with a different morphology of dorsal cardinalia somewhat comparable to the cardinalia of Gnamptorhynchos globatum (Twenhofel 1928) from the Ashgill of Anticosti Island, Canada (Jin 1989; Jin & Zhan 2000) and of an unnamed species from the Llandovery of UK. The Silurian species, known as Platystrophia jaaniensis Rubel (1963) from Gotland, also differs markedly from the typical Platystrophia that occur in the Caradoc of Sweden and the East Baltic in characters of cardinalia and dorsal muscle field. Taxonomically the shells from the Boda Limestone were recently assigned to a new genus (Zuykov & Egerquist, in press.) whose affinities to the family Platystrophiidae, however, need further confirmation. Platystrophia-like brachiopods belong to the most easily recognizable and widespread brachiopods in the Middle and Upper Ordovician of Baltoscandia. A sequence of short-lived species of these brachiopods can be potentially used as a basis for the biostratigraphical zonal scheme for the shallow-water facies in the Baltic basin, as an alternative to the microfaunal, trilobite or other classical biostratigraphical subdivisions. Zuykov, M.A. and Egerquist, E.