L ate T riassic ammonoid S irenites from the S abudani F ormation in T okushima, S outhwest J apan, and its biostratigraphic and paleobiogeographic implications

Discovery of S irenites senticosus ( D ittmar) in the upper part of the S abudani Formation of the K urosegawa B elt, K ito area, T okushima P refecture, J apan, establishes a late E arly C arnian age for this part of the stratigraphic unit. Because S . senticosus was mainly distributed in the T ethyan region, its occurrence provides evidence that L ate T riassic ammonoids of J apan had strong affinities with those of the T ethyan faunas. This finding clearly differs from the biogeographic distribution of contemporary bivalves in the region, which are referred to as the K ochigatani bivalve faunas, and show strong affinities to faunas of the B oreal region.