要　旨　 埼玉県の中部中新統秩父町層群奈倉層よりケトテリウム類化石が産出した．標本は頭蓋，周耳骨，鼓室胞，下顎骨，椎骨，肋骨などからなる．当該標本は吻部が細長く上顎骨基部には上顎骨を背部と腹部に分離する切痕の発達が見られることで特徴づけられ，本論文ではこれらの特徴から当該標本をDiorocetus sp.として報告した．下顎骨において観察されるいくつかの派生形質により当該標本は，現生するナガスクジラ類に見られるようなのみ込み型の摂餌機構と関連づけられうる筋突起などの形態を獲得しているものの，下顎骨腹側縁の形状はのみ込み型の摂餌機構の発達を支持しない．のみ込み型の摂餌機構は既に前期中新世においてケトテリウム類で獲得されていたことがこれまでに指摘されていることから，当該標本はケトテリウム類において多様な摂餌機構が存在していたこと示唆する．

キーワード：Diorocetus sp.，ヒゲ鯨亜目，ケトテリウム科，秩父町層群奈倉層，中期中新世，摂餌機構

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図4．秩父町層群産Diorocetus sp.，SMNH-VeF-19．吻部背側観．スケールは10cm． 　　　略号：Max, 上顎骨(maxilla); Na, 鼻骨(nasal); Pmx, 前顎骨(premaxilla)．

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A Middle Miocene Cetothere from the Chichibumachi Group, Central Japan

Toshiyuki Kimura
Department of Earth and Planetary Sciences, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan

Osamu Sakamoto
Saitama Museum of Natural History, Nagatoro, Chichibu-gun, Saitama 369-1305, Japan

Yoshikazu Hasegawa
Gunma Museum of Natural History, Kamikuroiwa, Tomioka, Gunma 370-2345, Japan

Abstract
　　A cetothere has been found in the Middle Miocene Nagura Formation of the Chichibumachi Group, central Japan. This specimen is characterized by a strongly tapered rostrum with an incisure which separates the maxilla behind it into a dorsal and ventral plate at the posterolateral angle of the maxillae. This specimen is assigned to Diorocetus, making it the first described specimen of this genus from the western North Pacific. This specimen includes the skull, mandible, tympanic bullae, periotics, vertebrae, ribs, and other elements. The mandible has a posterolaterally curved coronoid process and relatively well-developed inward elevation. Among extant Mysticeti, these structures are characteristically present in balaenopterids that do engulfment feeding. In the present specimen, however, the development of the coronoid process and inward elevation are less than seen in balaenopterids. Furthermore, as in balaenopterids, the ventral margin of the mandible is ridged and a mylohyoid muscle is attached along this ventral ridge. The mylohyoid muscle plays a quite important role in engulfment feeding. In the present specimen, however, the ventral margin of the mandible is well-rounded, not ridged. This mandibular morphology is common in non-engulfment feeders among extant Mysticeti. Therefore, we infer that the present specimen may have acquired a feeding strategy different from any known among extant mysticetes.