A radiolitid rudist, Eoradiolites cf. gilgitensis is described from a late early to late Aptian shallowmarine limestone block in the lower part of the Yezo Group, central Hokkaido, northern Japan. The Hokkaido Eoradiolites is characterized by a compact (non-cellular) outer shell layer, as seen in E. gilgitensis (from Iran, Afghanistan, Pakistan, and northern India), E. griesbachi (from Afghanistan), and E. ngariensis (from western Tibet in China), which are all located in the Southwest Asian region of the northern Tethyan margin. This is one of the earliest records in the world of primitive radiolitids with a radiolitiform myocardinal arrangement. This finding demonstrates that radiolitids had already expanded to the western Pacific at an early evolutionary stage, and that a faunal connection between the northwestern Pacific region and Southwest Asia existed at least in the late Aptian.
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Vol. 17 • No. 4