A diverse bryozoan fauna from pleistocene marine gravels at kuromatsunai, Hokkaido, Japan: Bryozoan diversity in a Japanese pleistocene gravel

Abstract

A remarkable locality, nicknamed ‘Kokemushi Paradise’, at the town of Kuromatsunai in Hokkaido, northern Japan, exposes 6.5 m of the Nakasato Conglomerate Member of the Pleistocene Setana Formation, dated at about 1.2–1.0 Ma. The sediment consists of marine gravels containing abundant shells, pebbles and cobbles, most densely encrusted by bryozoans that are typically well preserved. Broken branches of erect bryozoans, especially Myriapora, are common in the matrix. Several bryozoan genera are recorded for the first time in the fossil record, notably the cold-water genera Septentriopora, Harmeria, Doryporella and Rodinopora. Up to 25 bryozoan species can be found on a single clast, and 120 bryozoan species have been provisionally identified, dominated by ascophoran cheilostomes. Although this species richness is high compared to other fossil bryozoan assemblages from marine gravels, viewed in the context of bryozoan diversities in modern rocky subtidal habitats in the northern Pacific region it may be unexceptional. Lack of calcareous algae and presence of stylasterid hydrocorals suggest a subtidal environment, possibly in a fast-flowing channel between the Sea of Japan and the Pacific Ocean.