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Paleontological Research
Abstract
Abstract.
The late early to middle Miocene limestone bodies in the Sagara and Izu Peninsula areas of central Japan are reefal deposits that were deposited under a tropical climate and provide a window into the paleodiversity of tropical volcanic islands in the northeastern Philippine Sea Plate. Specimens from the early middle Miocene Megami Limestone in the Sagara area and from the middle Miocene limestone blocks within the Yugashima Group on the Izu Peninsula are described as Turbo (Marmarostoma) histrioides sp. nov. and Turbo (Marmarostoma) izuensis sp. nov., respectively. Discovery of these two new species raises the number of Marmarostoma species from four to six in the Sagara and Izu Peninsula areas, demonstrating that the subgenus was more diverse in the northeastern Philippines Sea Plate during the middle Miocene than it is today. In particular, there are double the number of Marmarostoma species (four species) from the middle Miocene in the Izu Peninsula compared with the modern fauna on the tropical islands of the Izu-Ogasawara Arc. The highest richness of modern species of Marmarostoma is found in the central Indo-West Pacific, where molecular phylogenetic studies have suggested that the subgenus rapidly diversified in the late Oligocene or early Miocene. Nevertheless, Miocene species of this subgenus are sparse in the central IWP, being incongruent to the diversity pattern expected from the molecular studies. The findings reported herein provide an insight into the origin of the central IPW biodiversity hotspot.
We thank R. W. Jordan (Department of Earth and Environmental Sciences, Yamagata University) for his English improvement of an early version of the manuscript, H. Sano and E. Hosoda for donating specimens to NMNS, A. Salvador (Natural History Museum, London) who provided high-resolution images of the syntypes of Turbo histrio, Y. Okumura (formerly of the Mizunami Fossil Museum) for preparation of some fossil specimens used in this study, and H. Sano (Izu Fossil Research Club, Matsuzaki) for offering the specimens from the Ena Limestone. We also thank K. Amano (Joetsu University of Education) and an anonymous reviewer for their comments and suggestions. This study was funded in part by a Grant-in-Aid for Scientific Research (KAKENHI) to T. Kase (16K05600) from the Japan Society for the Promotion of Science.
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