Deep-sea elasmobranch fauna with the first descriptions of genera Arynchobatis and Pseudoraja from the Miocene Yatsuo group in Toyama, central Japan

ABSTRACT

Herein, we describe 22 deep-sea elasmobranch species of the early to middle Miocene Yatsuo Group found in central Japan. The deep-sea elasmobranch fauna of the Yatsuo Group comprises various Squaliformes species together with a few scyliorhinid members and batoids species. This study represents the first fossil record of the occurrence of Arhynchobatis and Pseudoraja. The Yatsuo fauna is divided into three assemblages: Kurosedani Formation, lower Higashibessho Formation, and upper Higashibessho Formation assemblages. The Kurosedani Formation and upper Higashibessho Formation assemblages include many Squalus but only few Squaliolus species, and scyliorhinids. The lower Higashibessho Formation assemblage comprises many Squaliolus species, but only few Squalus species. Squalus species inhabit in shallower deep sea, unlike other genera occurring in the Yatsuo Group, and also it is more frequent in the horizon deposited in the shallower deep-sea than in the other horizons. The frequency of demersal scyliorhinids decreases in the dysoxic environment at the bottom layer. Occurrence of the tropical-subtropical batoid species of Pseudoraja and Benthobatis indicates subtropical palaeoenvironments in central Japan during the early to middle Miocene. At present, species of these two genera are not distributed in areas surrounding Japan.

KEYWORDS:

• Elasmobranchs
• sharks
• rays
• deep-sea
• Miocene
• central Japan

Acknowledgments

We thank Y. Iryu (Institute of Geology and Palaeontology, Graduate School of Science, Tohoku University); I. Suto, S. Hayashi, and M. Humblet (Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University); and T. Oji, S. Nishida, and S. Fujiwara (Nagoya University Museum) for their helpful comments. In addition, we thank T. Ichihara (College of Nagoya Bunri University) for providing information on fossil localities. We are grateful to K. Sato (Okinawa Churashima Research Center) and S. Tanaka (School of Marine Science and Technology, Tokai University) for allowing us to inspect the specimens of modern species and for their insightful comments on the taxonomy of Scyliorhinidae and Squaliformes. We specifically thank T. Ishigaki and Y. Matsunaga (Faculty of Education, Graduate School of Education, University of Yamanashi) for providing information of the benthic foraminifera from the Yatsuo Group. This paper was greatly improved by the comments from two anonymous reviewers and Jürgen Kriwet (Department of Palaeontology, University of Vienna).