中部日本群馬県南西部の中新統産出の深海性サメ類化石群とその生物地理学的意義

抄録

Accumulating fossil records of deep-sea sharks are important for reconstruction of their paleoecology and evolution, because these fossils are generally rare everywhere in the world. This paper reports the newly discovered and diverse fossil assemblage of deep-sea sharks from the Miocene deposits in the southwestern part of Gunma Prefecture, central Japan. The specimens are isolated teeth found from seven localities of three Middle Miocene formations. These fossils are identified into twelve species of eleven genera belonging to eight families within three orders: eight species from the Obata Formation (earliest Middle Miocene) of the Tomioka Group, six species from the Haratajino Formation (early Middle Miocene) of the Tomioka Group and four species from the Niwaya Formation (middle Middle Miocene) of the Annaka Group. Four genera, Centrophorus, Deania, Squaliolus and Mitsukurina represent the first fossil record in the Northwest Pacific. Somniosus and Centroscymnus mark the second record from the Miocene in the world, and Etmopterus and Pseudocarcharias indicate the second in the circum-Pacific. On the basis of lithofacies, benthic foraminifers and other megafossils, three formations are thought to have been deposited under outer sublittoral to middle bathyal environments. Since these environments accord with the Recent species habitats of the eleven shark genera, this fossil deepsea shark fauna similar to the recent one in species/generic composition had been habited in bathyal environment. The presence of this Miocene fauna suggests that the outline of the recent deep-sea shark fauna in the Northwest Pacific region would have been already established the Miocene. The establishment of the fauna might have been resulted from the vicariance and the isolation that caused by the closure of Indonesian seaway and its associated expansion of shallow sea in the Oligo-Miocene time.