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 Thyestes verrucosus Eichwald, 1854 from the Silurian of Saaremaa Island (Estonia), a model for the formation of its exoskeleton was proposed (Afanassieva, 2002, 2004b). New information on well-preserved juvenile Superciliaspis 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 Superciliaspis and also additional study of the material of Thyestes verrucosus corroborated the previous conclusions 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 Karatajūtė-Talimaa, 1998) from the Lower Devonian of October Revolution Island of the Severnaya Zemlya Archipelago, Russia (Afanassieva, 2011; Afanassieva and Karatajūtė-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.