The sculpture and histology of osteostracan head shields and trunk squamations from the Silurian of Estonia are described, illustrated and used for the identification of disarticulated microremains collected from outcrop sections and numerous drill cores in Estonia and Latvia over the last 40 years. The scattered osteostracan material contains thousands of specimens of scales and shield fragments. The sculpture and histology of species of the previously identified eight genera in the region (Tremataspis, Dartmuthia, Saaremaaspis, Oeselaspis, Aestiaspis, Thyestes, Procephalaspis, Witaaspis) are redescribed, together with Ateleaspis, found in Estonia for the first time. The sculpture on the cornua of several taxa is described for the first time. The new taxa Tremataspis perforata sp. nov., Dartmuthia procera sp. nov., Eldaaspis miklii gen. et sp. nov., Tahulaspis ordinata gen. et sp. nov., Tahulaspis praevia gen. et sp. nov., Meelaidaspis gennadii gen. et sp. nov. and Ohesaareaspis ponticulata gen. et sp. nov. are established, based upon sculpture and histological characteristics of the exoskeleton. The supraoral field with denticles of that field are described for the first time in Oeselaspis pustulata, as is the supraoral plate with buccal denticles in Meelaidaspis gennadii gen. et sp. nov. Thin sections of all taxa (except of Witaaspis) have been studied. This work sheds light on East Baltic osteostracan biodiversity from the Maasi Beds of the Jaagarahu Stage, Sheinwoodian, lower Wenlock up to the Ohesaare Stage, upper Přidoli, Silurian.

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In the past decade, considerable progress has been made towards a better understanding of the development of the dermal skeleton in osteostracans. Based on the study of the fine shield structure in Thyes tes verrucosus Eichwald, 1854 from the Silurian of Saaremaa Island (Estonia), a model for the formation of its exoskeleton was proposed (Afanassieva, 2002(Afanassieva, , 2004b. New information on well preserved juvenile Super ciliaspis gabrielsei (Dineley et Loeffler, 1976) from the Lower Devonian of Canada, North West Territories, allowed the determination of ontogenetic patterns of the dermal skeleton in the tessellated taxon (Hawthorn et al., 2008). A comparison of the data on shield development in consolidated Thyestes and tessellated Super ciliaspis and also additional study of the material of Thyestes verrucosus corroborated the previous conclu sions and displayed new features of the formation of exoskeleton in this species. A different formation pattern of the exoskeleton was established in Paraungulaspis arctoa (Afanassieva et Karataj t Talimaa, 1998) from the Lower Devonian of October Revolution Island of the Severnaya Zemlya Archipelago, Russia (Afanass ieva, 2011;Afanassieva and Karataj t Talimaa, 2013). A comparison of the exoskeleton structure and growth of the hard covering in Thyestes verrucosus and Paraungulaspis arctoa corroborated the conclusion that these species belong to different phyletic lineages of osteostracans.

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