Habitats of Bivalves in the Lower-Cretaceous Aptian Tanohata and Aptian to Albian Hiraiga Formations, Iwate Prefecture, Northeastern Japan

We reconstructed the depositional environments, assemblages, and habitats of characteristic bivalve species in the Cretaceous Miyako Group, Iwate Prefecture, Northeast Japan. The upper part of the Aptian Tanohata Formation and the lower part of the Aptian to Albian Hiraiga Formation are composed mainly of wave- and storm-dominated shoreface and inner-shelf fossiliferous deposits. The shoreface deposits contain abundant allochthonous bivalve shells. In-situ preserved Cucullaea acuticarinata Nagao, Pterotrigonia hokkaidoana (Yehara), Pterotrigonia yokoyamai (Yehara), and Goniomya subarchiachi Nagao are common in inner-shelf deposits. These fossils provide important knowledge concerning the evolutionary history of Lower to mid-Cretaceous shallow marine benthic communities.International Symposium, "The Origin and Evolution of Natural Diversity". 1–5 October 2007. Sapporo, Japan