906. JURASSIC EUCYRTIDIELLUM (RADIOLARIA) IN THE MINO TERRANE

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  • 906. 美濃帯の放散虫化石 Eucyrtidiellum
  • 美濃帯の放散虫化石Eucyrtidiellum〔英文〕
  • ミノタイ ノ ホウサンチュウ カセキ Eucyrtidiellum エイブン

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Abstract

Jurassic Eucyrtidiellum in chert, siliceous shale and manganese carbonate rock in the Mino terrane of central Japan was studied together with that of the Snowshoe Formation of Oregon with the result that eighteen species are recognized in the early Late to Upper Jurassic formations. Of all these species, Eucyrtidiellum disparile, E. unumaense, E. pustulatum, E. semifactum and E. ptyctum are most prevalent. These species are morphologically related with each other, and show an evolutionary trend composed of two consecutive steps of change, early in shell structure of abdomen, represented by transition from E. disparile to E. unumaense and later in superficial plication of abdomen, represented by transition from E. pustulatum through E. semifactum to E. ptyctum. On the basis of occurrence of E. pustulatum in the upper Bathonian part of the Snowshoe Formation, radiolarian biostratigraphical correlation for the Japanese Jurassic formations is discussed. Two new species, Eucyrtidiellum disparile and E. semifactum, are described in this paper together with other species relevant to them.

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