Autochthonous occurrence of rudist and microencrusters from the late jurassic torinosu-type limestone in nakatsugawa in the shirokawa area, western Shikoku, southwest Japan

There are few studies on paleoenvironment and paleoecology of the early rudists of Late Jurassic to earliest Cretaceous, though Late Cretaceous rudists have attracted much attention as a major contributor to carbonate platform. Rudist and microencruster framestone was discovered from the Late Jurassic Torinosu-type limestone of the Imaidani Group in the Nakatsugawa North Section, Shirokawa area, western Shikoku, Southwest Japan. The limestone succession, 17 m thick is divided into the lower part (9 meters thick from the bottom) composed of coral framestone and pack-grainstone, and the upper part (8 meters thick) consisting of coral-rudist-microencruster framestone. Abundant rudist individuals of a primitive uncoiled form occur in 9-14 m interval from the bottom, where the framestone does not contain corals. The rudist individuals often preserve bivalved appearance with little fragmentation, and were bound by a microencruster Bacinella. Occurrence and texture of the framestone substantiate that the rudists and Bacinella construct a framework structure in a relatively high energy environment Further studies of the rudists of the Torinosu-type limestones will provide clues to the paleoenvironment and paleoecology of the rudists in their early evolutionary phase.