日本産鯨化石の層序と生息環境  [in Japanese] Stratigraphy and inhabited environments of the Cetacea in Japan  [in Japanese]

日本産鯨化石の産出期は3期に区分される。第1期は後期漸新世の北九州芦屋層群脇田層と北海道川上層群の茂螺湾層から産出した足寄動物群である。前者は, OdontocetiのSqualodontidaeの複数標本が発見されており, 後者はOdontocetiのSqualodontidae, Squalodelphidaeの複数標本の他にMysticetiのAetiocetidaeの複数標本が産出しており, 当時の北海道地域は有孔虫の研究から勇昇流地帯であった。第2期は前期中新世末葉から中期中新世前半の時期で, 16.5から12 Maの関東以西に集中しており, 東北・北海道での発見例はまれである。この第2期の前半は門ノ沢動物群期にあたり, 鯨種はMysticetiのCetotheriidae, OdontocetiのRhabdosteidaeが5〜6体の複数で産出する。第2期の後半は塩原-耶麻動物群にあたり, 鯨種はMysticetiのBalaenopteridae, OdontocetiのDelphinidae, Squalodontidae, Ziphiidae, Physeteridae, が産出し前半とは種の構成が異なる。第2期の東北・北海道の地層には安山岩や玄武岩質の火山砕屑岩層を挟んでおり, 噛乳動物の生息環境としては恵まれた環境ではなかった。第3期は後期中新世から現世まで継続するが, 東北・北海道の竜ノ口-滝川-本別動物群に共産する鯨種はMysticetiではBalaenopteridaeが多くなり, 他にBalaenidaeを産する。OdontocetiはPhocoenidae, Physeteridae, Delphinidaeが認められる。関東以西ではBalaenopteridaeの発見例はなく, 大桑動物群に共通した鯨種はBalaenidaeとDelphinidaeである。

Whale fossils are found more frequently than those of any other oceanic mammals. This suggests that whales have proliferated because they are the best adapted mammals to aquatic life style. The fossils seem to have occurred in close relationship with paleoenvironmental change of the Japanese Islands. The oldest whale fossils in the Japanese Islands were yielded from the Ashiya fauna in Kyushu and the Ashoro fauna in Hokkaido both of the Late Oligocene. Many whale fossils were reported from the early Middle Miocene formations with the Kadonosawa fauna in Kanto and southwest Honshu, though a few occurrences have been reported from northeast Honshu and Hokkaido. This tendency continued in to the Late Miocene Shiobara-Yama fauna stage, though the species are different between the two faunas. Many whale fossils have been discovered in the Late Miocene formations of northeast Honshu and Hokkaido. Occurrence of whale fossils during the Pliocene is wide spread through out the Japanese Islands, but the species found in the Tatsunokuchi-Takikawa fauna and those found in the Omma-Manganji fauna differ. These facts suggest that each whale species has a different habitat (e.g. cold currents or warm currents), even thought whales are highly mobile species. It is likely that the difference of species between southwest Honshu and northern Japan (including Hokkaido) reflects this tendency to inhabit different areas.