Origin and Biogeographic History of Scapharca broughtonii (Schrenck, 1867) (Bivalvia: Arcidae) and Its Related Species

Abstract

Scapharca broughtonii (Schrenck, 1867) and S. aff. broughtonii have been recovered for the first time from the upper Pliocene (Piacenzian) deposits on the Japan Sea side of Honshu. This is the oldest record of S. broughtonii and its related species. S. satowi is known from the same locality as S. broughtonii in the upper Pliocene Tentokuji Formation in Akita Prefecture, Japan Sea side of northern Honshu. These fossils suggest that the genetically similar ark shells speciated near the northern limit of the warm-water current in the semi-enclosed Japan Sea by the late Pliocene. In the middle Pleistocene (Chibanian), S. broughtonii first adapted to cold-temperate water, being known from the Dateyama Formation on the Japan Sea side of central Hokkaido and associated with many cold-water and a few temperate-water species.