Earth Science (Chikyu Kagaku)

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Many specimens of deep-sea shark fossils, four genera - Etmopterus, Deania, Somniosus and Centroscymnus - which were recovered from the Yokoo and Iseyama Formations, are systematically described in this study. Therefore, it is concluded that Six genera - Squalus, Etmopterus, Centrophorus, Deania, Somniosus and Dalatias - occurred in the upper part of the Yokoo Formation, and seven genera - Squalus, Etmopterus, Centrophorus, Deania, Centroscymnus, Centroselachus and Dalatias - occurred in the lower part of the Iseyama Formation. It is extremely rare for deep-sea shark fossils such as these to accumulate in almost the same location. In particular, specimens of three genera, Etmopterus, Centroscymnus and Centroselachus, which were found in the lower part of the Iseyama Formation, have retained the shape of part of a symphysial tooth row, and Dalatias licha occurred as remains such as tooth and dermal denticle sets. These findings indicate that the symphysial teeth and tooth sets had not been long detached from living or dead sharks when they were deposited immediately together with muddy sediment. As a result, those that fossilized on one sea bottom retained a shape almost identical to living shark dentition. These fossils and their modes of occurrence are important for research on the paleoenvironment during the Middle Miocene time in Ueda City and the Northern Fossa Magna Region. This investigation led to the following conclusions. 1) Deep-sea shark fossils from the Yokoo and Iseyama Formations were classified into eight genera, Squalus, Centrophorus, Deania, Etmopterus, Centroscymnus, Centroselachus, Somniosus and Dalatias, belonging to five families within the order Squaliformes. Accordingly, the presence of a deep-sea shark assemblage was established in the Middle Miocene Yokoo and Iseyama Formations in Ueda City, Nagano Prefecture, central Japan. 2) The paleoenvironment in this area changes continuously from a continental slope into a continental slope and abyssal plain.

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