Early Aeronian (middle Llandovery, Early Silurian) radiolarians collected from calcareous nodules of the Kallholn Formation in the Siljan district, Sweden, are described. Seven genera and 11 species, including one new family Gyrosphaeridae and two new species, Palaeoscenidium kuriharai and Gyrosphaera primigena, are described. This fauna is characterized by the dominance of large spherical radiolarians, which constitute 54% of the total number of specimens per nodule. The fauna is similar to the middle Telychian fauna insofar as it is dominated by Haplotaeniatum cathenatum. However, it can be distinguished from the late Rhuddanian fauna by the sparse occurrence of H. aperturatum. These data suggest that the period from the Aeronian to the middle Telychian was marked by the dominance of H. cathenatum.
Radiolarian spines, a morphological innovation, are believed to have evolved in the Aeronian. The time of radiolarian diversification corresponds to that of other planktonic/nektonic faunas including graptolites, chitinozoans, and fish, which suggests that marine macroplanktonic/nektonic faunas and microplankton diversified simultaneously in the Aeronian.