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Because the western equatorial Pacific Ocean is famous for its calcareous oozes, various biostratigraphic studies on planktonic foraminifers and calcareous nannofossils have been conducted in this region. As a result, western equatorial Pacific Ocean–based studies have established that the tropical Pacific region drives the atmospheric circulation by high seawater temperature, strongly influencing the global climate. Based on this finding, a number of sites were cored in this region during International Ocean Discovery Program (IODP) Expedition 363. Shipboard results unexpectedly revealed that radiolarians are abundant and well preserved in sediment from Eauripik Rise Site U1490 between 220 and 350 m core depth below seafloor, Method A (CSF-A), within an interval of calcareous ooze composed primarily of calcareous nannofossils and foraminifers. Paleomagnetic reversal event and calcareous nannofossil and planktonic foraminiferal bioevents suggested that the interval from 220 to 350 m CSF-A corresponded to the Middle to Early Miocene. This study investigated radiolarian assemblages and the biostratigraphy of core catcher samples obtained from Hole U1490A between 250 and 350 m CSF-A. Notably, the last occurrence (LO) of Artophormis gracilis (Riedel) was confirmed at ~338 m CSF-A, demonstrating that the base of the site is older than 22.6 Ma in age. Additionally, the first occurrence (16.9 Ma) and LO (13.9 Ma) of Calocyclas costata Riedel were identified at 246.7 and 227.8 m CSF-A, respectively, and the LO of Didymocyrtis prismatica (Haeckel), indicating an age of 17.7 Ma, was recorded at 258.5 m CSF-A. We also calculate preliminary sedimentation rates based on the Early Miocene radiolarian biostratigraphy and provide taxonomic notes on radiolarian species.