Biogeographical and Biostratigraphical Significance of a New Middle Devonian Phacopid Trilobite from the Naidaijin Formation, Kurosegawa Terrane, Kyushu, Southwest Japan

Abstract. A newly discovered trilobite from the Naidaijin Formation in the Kurosegawa Terrane of southwest Japan is referred to Toxophacops (Atopophacops) fujiwara sp. nov. The trilobite occurs in strata 2 m below a level with detrital zircons that yield an age of 383.9±4.4 Ma that is likely Givetian (late Mid-Devonian). Elsewhere, Early to Mid-Devonian trilobite species of T. (Atopophacops) and T. (Toxophacops) have been recorded from the Nakazato Formation of the South Kitakami Terrane of northeast Japan, and from the Zhusilenghaierhan region of western Inner Mongolia, North China. The new trilobite species extends the geographical range of the phacopid subfamily Echinophacopinae, and supports the affinities of some Devonian Japanese trilobites with those of north China. The occurrence of Toxophacops in the Lower Member of the Naidaijin Formation, combined with other trilobite evidence, confirms a clear biostratigraphical distinction between the Lower Member (upper Lower and Middle Devonian) and Upper M...