Eva Egerquist

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.

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%).

Revision of the plectambonitoid brachiopod Ujukella Andreev and related genera

Abstract 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 Leptesti-idae, based on a new cladistic analysis.

Pedicle preservation in a Silurian rhynchonelliformean brachiopod from Herefordshire, England: soft-tissue or an artefact of interpretation?

The interpretation of pedicle soft tissue preservation in a unique brachiopod specimen of Wenlock (Silurian) age from Herefordshire, western England, is re-assessed. Bethia serraticulma, assigned originally to the Orthida, is more probably a member of the Strophomenida (Plectambonitoidea). The supposed pedicle structure is more plausibly a weakly mineralised pedicle sheath, which is a common morphological and functional development in the early ontogeny of a number of Palaeozoic brachiopod lineages.

Early-Middle Ordovician (Billingen-Volkhov stages) Orthide and Protorthide brachiopods from the East Baltic

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.

A new Platystrophia-like brachiopod genus from the Ordovician Boda Limestone of Dalarna, Sweden

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.