化石

Several specimens of Steller's sea-cow (Hydrodamals gigas) have been collected from the North Pacific region. These are, however, not enough to investigate intraspecific variation. As a result of study of the specimens newly found from the Bering Island and those kept at the Nikol'skoye Museum in the island, the author confirmed that all of the characters used in the cladistic analysis by Furusawa (1988) exhibit few variation among specimens and are appropriate for phylogenetic analysis.

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From the lower Pleistocene strata in Japan Sea borderland, the following four species of rock-boring bivalves were obtained; Hiatella arctica (Linnaeus), Zirfaea subconstricta (Yokoyama), Penitella gabbii (Tryon) and Nettastomella japonica (Yokoyama). All these species are now living around Japanese Islands. Such a high extant rate is probably due to their endolithic life style. The Nettastomella and the Penitella-Zirfaea associations are recognized by the combination of above mentioned species. The southern limit of the Nettastomella association living around northern Hokkaido was situated at central Honshu in early Pleistocene. This distributional pattern and the low diversity of associations are attributed to the colder climate in those age than in the recent.

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これまで述べてきたように, 日本における新第三紀の暖流系軟体動物群は海水準の上昇を伴う, 汎太平洋海域の海洋気候温暖期にそれぞれ対応して時空的分布を変化させてきたことが読みとれる.本論のまとめとしてChinzei (1978, 1986a)のダイヤグラムを改訂した軟体動物群の時空分布に関するダイヤグラムを提示したい(図1).本研究においては, (1)これまで十分な認識がなされていなかった前期中新世末の明世動物群および中期中新世前期の布志名動物群が識別され, その時空分布が示された, (2)従来, 実体が良く分からなかった相良動物群(中期中新世から前期鮮新世の暖流系動物群)は, 中期中新世の茎永動物群と後期中新世-前期鮮新世の逗子動物群とに置き換えられた, (3)それぞれの動物群の時空的な関係が新しい資料で改訂された, そして(4)新第三紀の古海洋気候の変遷と日本の暖流系動物群との時間的対応関係がより明確にされた.

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The Shimodani Limestone in the Oga upland in Okayama prefecture, western Japan, is composed of massive limestone with basic pyroclastic rocks at the base. This limestone is divisible into two foraminiferal zones in upward sequence : the Endothyra-Mediocris Zone and the Eostaffella-Millerella Zone. The former is characterized mainly by Endothyra exilis, E. similis, E. irinae, Endostaffella sp., Planoendothyra sp., Mediocris mediocris and M. adducta. The latter is composed of Eostaffella kanmerai, E. etoi, E. akiyoshiensis, E. mosquensis, E. ikensis, Millerella marblensis, Ozawainella japonica, Zellerinella discoidea and others. These two zones are regarded as ranging in age from the late Osagean to the early Morrowan in North America, and the early Visean to the late Serpukhovian in Russia. The Shimodani Limestone has a successive sequence without nonconformity, making a gentle basin structure, and thrust over the Upper Triassic Nariwa Group. The Shimodani Limestone is similar to the western and central parts of the Koyama Limestone in the lithofacies and the foraminiferal assemblages. Therefore, the Shimodani Limestone is considered to be the so-called klippe separated from the Koyama Limestone. This thrust fault has a large fracture zone, though the structure of the limestone is not disordered. This fact suggests that basic pyroclastic rocks of the Shimodani Limestone and coal beds of the Nariwa Group played an important role in its sliding surface.

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