北太平洋海牛類(ヒドロダマリス亜科 : Hydrodamalinae)の進化と古環境(<特集>生物イベントとしての哺乳類の海生適応)
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概要
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Since 1965, thirty-five sirenian fossils have been found in Japan. Most are classified in the subfamily Hydrodamalinae. The Hydrodamalinae includes two genera, Dusisiren (Early Miocene-Late Miocene) and Hydrodamalis (Late Miocene-Recent). Dusisiren was 4 to 5m in body length and had functional teeth, whereas Hydrodamalis attained a body length of over 7m and was edentulous. Shift of these characters in this lineage was assumed to be adaptation to cold temperatures. However, species of Dusisiren inhabited both sides of the North Pacific Ocean at the low stand of Late Miocene (ca 11 Ma) sea levels under extremely cool conditions. It is assumed that the loss of functional teeth and enlargement of body size took place after severe climatic cooling. These dramatic changes in the hydrodamalines suggest the possibility that environmental changes influenced marine vegetation such as seaweed and sea grass. As a result the Hydrodamalinae changed the types, methods, and amount of food that they consumed. These changes were directly connected to the enlargement of body size and loss of functional teeth. Sirenia must feed during migration, judging from their physiological and functional abilities. Sea algae or seaweed grows abundantly in the shallower (less than 10m) waters that sunlight can penetrate. The opening and closing of seaways and straits related to fluctuations of sea level due to global climatic changes affected the availability of shallow and coastal waters indispensable in the radiation and distribution of sirenians. The migration of hydrodamalines from the eastem North Pacific to the west appears to have occurred when the shallow coastal waters were continuous along the Aleutian Islands because of the lowering of sea level associated with beginning of cooling in Middle Miocene time.
- 日本古生物学会の論文
- 2005-03-25
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