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Paleontological Research
Abstract
Abstract.
A new periotic of Numataphocoena yamashitai from the upper part of the Horokaoshirarika Formation (lower Pliocene), Numata, Hokkaido, Japan shows younger ontogenetic features in comparison to the matured holotype (NFL 7). The referred specimen of N. yamashitai is smaller (approximately 90% in size) than the physically and sexually matured holotype periotic. The holotype (NFL 7) and referred specimen (NFL 2617) show several differences owing to ontogenetic changes, such as: the holotype (NFL 7) has a more strongly swollen medial part of the anterior process, blunt anterior keel, rounded anterodorsal angle, deeper and clearer anterior incisure, more rounded pars cochlearis, deeper and narrower hiatus epitympanicus rather than a flat area, longer posterior process with an extra edge posterolaterally. N. yamashitai differs from other fossil phocoenids in having a narrower and sharper anterior part of the internal acoustic meatus and a robust anterior process. Haborophocoena toyoshimai similarly has a relatively narrow internal acoustic meatus and a robust anterior process, compared with other fossil phocoenids. Between N. yamashitai and H. toyoshimai, N. yamashitai has a narrower internal acoustic meatus and a more robust anterior process. These similarities imply a close relationship between Numataphocoena and Haborophocoena. The new specimen provides diagnostic features and insight into ontogenetic variation of N. yamashitai.
I thank Masanobu Kato, Takayuki Tanaka and Tadashi Seto for finding and collecting the specimens in August, 1986, and Shigeru Yamashita, Masaichi Kimura, Numata Fossil Laboratory and the Numata Town Board of Education for collecting and preparing the specimens. I particularly thank the two journal peer-reviewers, Robert Boessenecker (University of Otago) and Rachel Racicot (Howard University), for their constructive comments. I also thank Gabriel Aguirre-Fernández (University of Zurich) for providing constructive comments that helped improve the manuscript. Thanks are also due to R. Ewan Fordyce (University of Otago), who gave constructive comments on this research, to Naoki Kohno (National Museum of Nature and Science) for geological discussion, and to Satoshi Shinohara (Numata Fossil Museum) for taking me to fossil localities in Numata town. I am grateful to Hitoshi Furusawa (Sapporo Museum Activity Center) for his permission to examine comparative specimens.
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