Abstract Over the past 15 years, significant progress has been achieved in our understanding of Lower Palaeozoic radiolarian faunas. However, description of biogeographical patterns of Ordovician Radiolaria is hampered by the paucity of known occurrences as well as possible taphonomic influences. The distribution of all known assemblages is analysed in time and space. Lower Ordovician (especially Tremadocian) Radiolaria are known from two distinct tropical localities of Laurentia. Geographical coverage is much better for the Middle Ordovician (Darriwilian). However, data are concentrated in tropical palaeolatitudes (between 30°N and 30°S). The absence of data from mid/high-latitude localities limits any biogeographical insights. In addition to this there are taphonomic and taxonomic biases. Data are also sparse for the Upper Ordovician. However, comparison between Australian and Nevadan material of Katian age shows strong similarities, suggesting the presence of a coherent tropical radiolarian bioprovince, as in the modern ocean.

Abstract A comprehensive review of the current state of research on Kazakh Ordovician litho-, bio- and chronostratigraphy is presented. An Ordovician lithostratigraphic framework applied to eight Kazakh first-order tectonic units is outlined and its correlation with the International Chronostratigraphic Scale is given. Presently used criteria for definition of the Kazakh Ordovician regional stages are critically discussed and revaluated. The archipelago model is considered as the most appropriate for reconstruction of the relative position of inferred Kazakh volcanic island arcs and microcontinents in the Ordovician Period. A biogeographical assessment of Kazakh Ordovician benthic faunas suggests strongest affinity to the contemporaneous faunas of Tarim, South and North China and to a lesser degree to the Australian sector of Gondwana, while biogeographical connections with Siberia and Baltica are negligible. During the Sandbian to Katian, a loose cluster of Kazakh microcontinents and island arcs became a major biodiversity hotspot and species pump located at low latitudes on both sides of the equator. Radiolarian cherts from accretionary complexes preserved an almost complete record of biogenic sedimentation for at least 30 Ma from the Furongian to Darriwilian, providing a unique opportunity to study biotas and environments in Ordovician oceans.