東部ザイール, 西部地溝帯, シンダ・モハリ地域における地質学的・古生物学的調査

抄録

A joint Japan-Zaire research expedition has been carried out in the Sinda-Mohari region, Haut-Zaire, eastern Zaire since 1989. The purposes of this expedition are to reinvestigate the geological age of the fossiliferous sites and to make clear the paleoenvironment in this region. These are the first step to investigate a possible fossiliferous area from which new evidences of hominoid evolution will emerge. The resultant fossil mammals of the two field surveys indicate that the fossiliferous sediments (Sinda Beds) are of late Miocene to Pliocene in age. Our estimation is younger than that previously considered. The Sinda Beds are divided into the lower, middle, and upper members. All of these members contain fossils, and there seems no unconformity between them. The resultant fossil collection is more than 600 in number which comprises molluscs, fishes, turtles, crocodiles, and mammals, including the first fossil records of carettochelyid (pig-nosed turtle), Carettochelyinae, and Dwarf Crocodile, Osteolaemus, from Africa. The aquatic vertebrates indicate the existence of a large-scaled water mass in that age. The mammalian fauna implies that this area was in humid environment and kept a rainforest during the estimated period. This is a contrast to East Africa where the climate changed gradually into arid conditions. The Sinda-Mohari and adjacent areas offer us an opportunity to investigate the rare fossiliferous sites that contain the mammals living in a humid environment. Furthermore, geological age of the sediments is late Miocene to early Pliocene which is the important period for studying the hominoid evolution.