Yasuhiro Iba

The first Pacific record of the Late Aptian warming event

Carbonate platforms consisting of hermatypic coral, rudists and larger foraminifers formed along the margin of the NE Japanese islands during the mid-Late Aptian. The geographical northern limit of the carbonate platforms extended to c. 36°N at that time, which is the highest latitude for coral and/or rudist formation in the North Pacific region. This geographical extension of the carbonate platform in the NW Pacific indicates extreme climatic warmth in the mid-latitudes and strong poleward heat transport systems. Contemporaneous warming has been detected in European and Australian regions, as represented by the migration of Tethyan fauna toward the boreal realm and the positive excursion of δ13C and negative excursion of δ18O in deep-sea carbonates. Our new constraint on the northern limit of ‘reef’ growth reinforces the view that an interval of greenhouse-like warmth punctuated the Late Aptian climate.

TWO COLEOID JAWS FROM THE UPPER CRETACEOUS OF HOKKAIDO, JAPAN

Abstract Two isolated cephalopod jaws recovered from the middle Turonian of the Obira area and the Campanian of the Nakagawa area, Hokkaido, Japan, consist of short outer, and large and posteriorly elongated inner “chitinous” lamellae, with a sharply pointed rostrum in the outer lamella. These features are common with the upper jaws of Recent cephalopods. Comparison with the upper jaws of ammonoids and Recent cephalopods indicates that the two Cretaceous upper jaws are attributed to the Coleoidea other than the Octopodida. This assignment is also suggested by the cladistic analysis of the Nakagawa specimen compared with five upper jaw characters on 22 Recent cephalopod species. The Obira specimen differs from the Nakagawa specimen in having a much smaller jaw and a larger jaw angle, but its order-level assignment could not be determined because of imperfect preservation. The Nakagawa specimen shares several common features with the upper jaws of Recent Oegopsina; thus we assigned its higher systematic position to this suborder. Based on the extremely large upper jaw (97 mm maximum length), a new genus and species (Yezoteuthis giganteus) is proposed. This new taxon would have been as large as the modern giant squid Architeuthis, which commonly exceeds more than 5 m in body length. Our study postulates that studies of jaws are important to reconstruct the phylogeny of the Coleoidea.

Orbitolina-bearing limestone pebbles from the lowermost part of the Lower Yezo Group (Lower Cretaceous) in the Nakagawa area, northern Hokkaido, Japan and its significance

り下部蝦夷層群,中部蝦夷層群,上部蝦夷層群,函淵層群に 区分されている(Okada, 1983).このうち夕張山地周辺の下 部蝦夷層群中部には,“オルビトリナ石灰岩”と呼ばれる石 灰岩体が古くから知られている(矢部, 1901).この石灰岩体 は下部蝦夷層群中の異地性岩体,すなわちオリストリスであ ると解釈されている(高嶋ほか, 1997など).“オルビトリナ 石灰岩”には大型有孔虫 Orbitolina をはじめ,厚歯二枚 貝,ネリネア,造礁サンゴ,石灰藻など白亜紀当時の熱帯~ 亜熱帯地域に特有な化石群集が含まれている(橋本, 1936; 佐 野, 1995).このことから“オルビトリナ石灰岩”の分布は, 北西太平洋における白亜紀の古気候,古生物地理を考える上 で重要であると考えられる.また,“オルビトリナ石灰岩” を含むオリストストロームは単調な岩相を示す下部蝦夷層群 において鍵層として有効である.さらに Orbitolina属は, 種のレンジが短いため地層の年代決定にも用いられている. これまで“オルビトリナ石灰岩”は,夕張山地を中心に南 は じ め に 北約 30 kmという限られた範囲に知られ,その分布の北限 は北海道中央部の比布地域であった(鈴木, 1957).筆者らは 今回,それよりも約 100 km北方に位置する北海道北部中川 地域の下部蝦夷層群最下部の礫岩層から“オルビトリナ石灰 岩”に類似する石灰岩礫を多数発見した.礫岩層とその中に 含まれる石灰岩礫の産状および石灰岩礫の微岩相の観察,石 灰岩礫に含まれる大型有孔虫 Orbitolinaの分類学的検討を 行った結果,この地域の下部蝦夷層群最下部の地質年代,お よび下位の空知層群との時間間隙について新知見を得た.ま た,下部蝦夷層群堆積時における後期 Aptianの浅海域での 石灰岩の形成とそれを起源とする堆積物の深海域への供給が 広範囲にわたる現象であったことが確かめられたので,ここ に報告する.