Abstract
A small reticulated morpho type of Amphimelissa setosa dominates the radiolarian fauna in one recent western Norwegian fjord and in Late Weichselian sediments from coastal environments around Norway. It is also found abundantly in Late Weichselian and Holocene sediments from the Iceland Plateau. These environments represent warmer, more productive conditions than prevail in the polar waters of the recent and Pleistocene Norwegian Sea, where a larger form with well rounded smooth pores is most abundant. A biological model based on the known developmental stages of the skeleton describes how the higher temperatures and productivity of the coastal environments might induce this morphotype of the species.
