JSPS Research Fellow. ; Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, Japan .
Fukui Prefectural Dinosaur Museum, Fukui, Japan .
Proc Jpn Acad Ser B Phys Biol Sci. 2006 Nov;82(7):216-23. doi: 10.2183/pjab.82.216. Epub 2006 Nov 18.
In this paper, we describe an Aptian (Early Cretaceous) larger foraminiferal species Orbitolina (Mesorbitolina) parva from the limestone olistoliths in the lower part of the Yezo Group in the Yubari-Ashibetsu area, central Hokkaido and from limestone pebbles in the lowermost part of the Yezo Group in the Nakagawa area, northern Hokkaido. This is the first report of this species from the circum-North Pacific regions. Based on its occurrences, the shallow-marine carbonates, re-deposited in the lower part of the Yezo Group, are precisely assigned in age to the Late Aptian. Comparison of the lower part of the Yezo Group in central and northern Hokkaido indicates differences of the Aptian-Albian depositional history between the two areas. This study reveals that after Late Aptian, Mesogean key taxa (typical Cretaceous Tethyan biota) demised in the Northwest Pacific.
本文描述了早白垩世阿普第阶（Aptian）一种较大的有孔虫属种——Orbitolina（Mesorbitolina）parva，该种发现于北海道中部由布院-Ashibetsu 地区的下野州群下部的灰岩滑塌体中，以及北海道北部中川地区的下野州群最下部的灰岩砾石中。这是该种在环北太平洋地区的首次报道。根据其出现的位置，在下野州群下部再沉积的浅海碳酸盐岩可准确地确定为晚阿普第阶。北海道中部和北部下野州群的对比表明，这两个地区的阿普第阶-阿尔比阶沉积历史存在差异。本研究表明，在晚阿普第阶之后，中大西洋生物地理区系（特提斯洋典型白垩纪生物群）的关键分类群在西北太平洋地区灭绝。
