Devonian Radiolarian Ecozones of Northern Eurasia.

This study provides a description of the Devonian radiolarian associations of northern Eurasia and discusses their role in the evolution of Earth's biosphere. It contributes to evolutionary studies of the Earth and its organic world by reconstructing the diversity dynamics of the Devonian radiolarian biota and the chorology of its components, at the same time revealing the biostratigraphic and paleoecological potential of radiolarians. The taxonomic composition and characteristics of the dominant morphotypes in radiolarian communities are indicators of environmental conditions. Associations of radiolarians vary significantly between different, even neighboring, water areas, so they can be considered as ecozones. Data on the structure of associations of radiolarians at the level of taxa of higher rank (classes) are a kind of "certificate" or "passport" of ecozones of individual paleobasins. Radiolarian ecozones replace each other in time, which allows their use for biostratigraphy. The results of the new and revisional studies of the Devonian radiolarians from 15 regions of northern Eurasia enabled a refined and detailed radiolarian scale of the Devonian system and description of 39 Devonian radiolarian ecozones, 24 of which are described for the first time: 17 ecozones are recognized based on the literature data, while seven ecozones are based on analysis of new data. Of 39 ecozones, 33 are dated based on conodont data, and six are dated using the co-occurring foraminifers, ostracodes, and/or ammonoids. General patterns in the evolution of Devonian radiolarians of northern Eurasia are considered and discussed. Spherical porous Sphaerellaria distinctly dominated in the Devonian. A clear trend towards an eastward increase in the relative abundance of spongy and latticed morphotypes of Spumellaria is shown. Spiny Aculearia of the orders Fasciculata and Triangulata are found to have been widespread. The early evolution of the bilaterally symmetrical Albaillellata is recorded. It is shown that in the Famennian, radiolarians with a pylome, Popofskyellidae and Archocyrtiinae, were common, while the genus Caspiaza first appeared at that time. The habitats and the distribution of the discoidal and ellipsoidal radiolarian morphotypes are analyzed in space and time. The dominance of these morphotypes can be used as an indicator of shallow water settings and a marker of transgression and regression boundaries in the paleobasin. A model of cyclic deposition of Domanik-type rocks in an anoxic basin is proposed, and the ecology of radiolarians in the anoxic environments of the Middle Frasnian Domanik Paleosea is analyzed. The predominance of the spiny morphotypes over spherical radiolarians is proposed as a paleoclimatic coefficient indicating cooling of the seawater in a paleobasin. Devonian radiolarians are represented by indigenous species and migrant species: (1) the value of the settled index exceeds that of the migration index by 3–5 times, (2) radiolarians occupied new ecological niches by emigration of evolutionarily young species representing genera with one–two species in total. Four radiolarian migration episodes are established in the Famennian basins of Laurussia. The monograph is intended for widespread use in paleontology, biostratigraphy, paleoecology, paleobiogeography and for university courses. [ABSTRACT FROM AUTHOR]