ゲンゴロウブナの種の出現に関する古生物学的資料

現生ならびに化石咽頭歯の組織学的比較

抄録

The crucian carp, Carassius cuvieri (Gengoro-buna) is an endemic species of the Lake Biwa, living in the middle layer of off-shore water and feeding on phytoplankton. The pharyngeal teeth of this fish morphologically have several adaptive characters for the plankton feeder. Each tooth of this crucian carp is composed of a thin enameloid layer and two dentine layers, vis., the outer orthodentine and the inner vasodentine layers. Besid the outer part of the inner dentine layer consists of interglobular dentine. All these tissues become exposed on the occlusal surface by attrition, so that the surface emphasize the masticatory function of phytoplankton. The dentine layer of the pharyngeal tooth of crucian carps except for this species, is almost exclusively composed of orthodentine. The fossil pharyngeal teeth of C. cuvieri were discovered from the Middle Pleistocene Sakawa Clay Member of the Katata Formation of the Kobiwako Group, which is composed of the lake deposits in the ancient Paleo Lake Biwa. The constituent tissues of these fossil teeth were also examined under a microscopy. As the results, it is found that they are composed of a thin enameloid and a orthodentine layer. Furthermore, the interglobular dentine widely occupies the central part of dentine. From these facts it may be infered that the fossil specimens did not acquire the histological organization of the living form, and they were still in the intermediate stage between ancester and recent forms. The old fauna in the Paleo Lake Biwa disappeared, and instead of it the modern fauna appeared contemporary with the first form of C. cuvieri. On the other hand, it is assumed that the vast off-shore of the present lake was formed during age and at the same time the special phytoplankton flora became dominant. It is believed that the events were necessary for the formation of a new species. C. cuvieri.