Early to Middle Devonian Ceratoikiscidae (Radiolaria) from the Yokokurayama Group in the Kurosegawa Terrane, Southwest Japan

Abstract

This paper focuses on ceratoikiscid radiolarians from the Devonian Nakahata Formation of the Yokokurayama Group in the Kurosegawa Terrane. The fauna is assigned to the Emsian to Eifelian (late Early to early Middle Devonian). There are 13 species of radiolarians, including 4 new species, which belong to the genera Ceratoikiscum, Glanta, Protoholoeciscus, Circulaforma and Helenifore. The morphology and stratigraphic distribution of the genera suggest that Protoholoeciscus evolved from Ceratoikiscum via Glanta with the process of acquisition of shell.