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

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%).