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Paleontological Research
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Abstract
Abstract.
A possible phylogenetic relationship of two species of Hyphantoceras (Ammonoidea, Nostoceratidae) was proposed, based on newly found specimens with precise stratigraphic occurrences in the Kotanbetsu and Obira areas, northwestern Hokkaido. Two closely related species, Hyphantoceras transitorium and H. orientale, were recognized in the examined specimens from the Kotanbetsu and Obira areas. Specimens of H. transitorium show wide intraspecific variation in the whorl shape. The stratigraphic occurrences of the two species indicate that they occur successively in the Santonian–lowermost Campanian, without stratigraphic overlapping. The similarity of their shell surface ornamentations and the stratigraphic relationships possibly suggest that H. orientale was derived from H. transitorium. The presumed lineage is likely indigenous to the northwestern Pacific realm in the Santonian–earliest Campanian. Hyphantoceras venustum and H. heteromorphum might stand outside a H. transitorium–H. orientale lineage, judging from differences of their shell surface ornamentation.
I sincerely thank Ryoji Wani (Yokohama National University) for his encouragement and critical reading of the manuscript. I also thank Ryuichi Majima, Masahiro Ishikawa, Tomohiko Kikuchi, and Shinji Yamamoto (all, Yokohama National University) for their critical comments; Kenji Ikuno (Museum of Nature and Human Activities, Hyogo) for fruitful discussions and suggestions, and assistance during fieldwork; Yasunari Shigeta (National Museum of Nature and Science) for fruitful discussions and suggestions, and access to the type specimens; Tomoki Karasawa (Mikasa City Museum) for fruitful discussions and assistance during fieldwork; Ken'ichi Kurihara (Hokkaido Museum) for encouragement and fruitful discussions; Kazushige Tanabe and Yusuke Takeda (both, the University of Tokyo) for fruitful discussions and suggestions, and assistance in collecting the literature; Yasuhiro Ito and Takenori Sasaki (both, the University Museum, the University of Tokyo) for access to the type specimens; Yoshinari Hikida (Nakagawa Museum of Natural History) for access to the significant specimens; Kan'ichi Fujiwara, Toshiaki Matsuda, Keichi Mori (all, Volunteer's Association of Mikasa City Museum), and Yasuji Kera (Sapporo City) for donating or letting me observe referable specimens; Yasuhiro Iba (Hokkaido University) for fruitful discussion; Yasuyuki Tsujino (Tokushima Prefectural Museum), Tomohiro Nishimura (Hobetsu Museum) and Haruyoshi Maeda (Kyushu University) for their valuable comments to improve the manuscript on peer review; Fumiko Murakami and Mamoru Murakami (both, Murakami Lodging House, Haboro Town), and Junya Yonemaru (Yonemaru Automobile, Iwamizawa City) for their kind assistance during fieldwork. Thanks are extended to Kotanbetsu and Tappu forestry offices. This study was partly supported by the Sasakawa Scientific Research Grant from the Japan Science Society and the Fukada Research Grant from the Fukada Geological Institute.
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