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| Megabalaena sapporoensis Tanaka, Kimura, Shinmura, Ohira & Furusawa, 2025 Artwork by Tatsuya Shinmura |
ABSTRACT
The family Balaenidae (right whales) includes two genera and four extant species, all of which are endangered and giant animals measuring approximately 17 to 20 m in length. The history of the Balaenidae spans about 20 million years. Several small sized extinct balaenids from the Pliocene have been identified. However, half of this history remains unknown owing to a 9-million-year gap from 15.2 to 6.1 m.y.a. in the fossil record. A well-preserved fossil balaenid skeleton, designated SMAC 2731, from the Late Miocene approximately 9 m.y.a. in Sapporo, Hokkaido, Japan, is named as Megabalaena sapporoensis gen. et sp. nov. This specimen preserves the skull, periotics in situ, tympanic bullae, right mandible, basihyal-thyrohyal, right stylohyal, sternum, seven cervical vertebrae, nine thoracic vertebrae, and 16 more posterior vertebrae, rib fragments, scapulae, and left forelimb elements. All preserved vertebral epiphyses are fused, indicating that SMAC 2731 was physically mature. Notably, M. sapporoensis can be distinguished from other balaenids by its excavated orbit in dorsal view with a large postorbital process, dorsoventrally high anterior part of the involucrum of the tympanic bulla, long compound posterior process, high coronoid process and deeper subcondylar furrow of the mandible, incipient cervical fusion (C2+C3 only), and its slender forelimb bones, including the humerus, radius and ulna. Based on a bizygomatic width of 2.2 m, the estimated total length of the holotype of M. sapporoensis is 12.7 m. Overall, M. sapporoensis indicates that balaenids diversified prior to the Late Miocene.
Keywords: Balaenidae; new genus; new species; Tortonian; gigantism; Japan
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| Forelimb elements of balaends. Megabalaena sapporoensis (A), Charadrobalaena valentinae, outline taken from Bisconti et al. (2023) and is a mirror image (B), Antwerpibalaena liberatlas, outline taken from Duboys de Lavigerie et al. (2020) and is a mirror image (C), Eubalaena japonica, outline taken from Omura (1958) (D), and are not to scale. |
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| Images based on a 3D model showing preserved skull elements of SMAC 2731, Megabalaena sapporoensis. Deformations are not restored. |
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| Images based on a 3D model showing skull elements of SMAC 2731, Megabalaena sapporoensis in left lateral view (A) and dorsal view (B). Deformations are restored using 3D model editor by T. Shinmura. Settings are the same to Figure 3. |
CETACEA Brisson, 1762
NEOCETI Fordyce and de Muizon, 2001
MYSTICETI Gray, 1864
CHAEOMYSTICETI Mitchell, 1989
BALAENIDAE Gray, 1825
Megabalaena gen. nov
Type species. Megabalaena sapporoensis sp. nov.
Etymology. The generic name, Megabalaena , is named derived from ancient Greek megas meaning great, large and mighty, and the type genus name of the family Balaenidae.
Megabalaena sapporoensis sp. nov.
Locality and horizon. SMAC 2731 was found at a riverbed of Toyohira River in Sapporo City, Hokkaido, Japan, by Kazuhisa Mori on 10 October 2008: Latitude 42°58'1.24"N, longitude 141°13'18.01"E (Figure 1 and Figure 2). SMAC 2731 was found from the upper part of the Toyama Formation. At the fossil area, the diatomaceous siltstone Toyama Formation is distributed. Diatomaceous siltstone of the Toyama Formation is exposed at the type locality ...
Etymology. Named after the fossil locality, Sapporo City.
Diagnosis. Megabalaena sapporoensis is a member of the Balaenidae because it has a combination of these character states such as a posteriorly pointed anterior edge of the supraorbital process lateral to the ascending process of the maxilla with the skull in dorsal view (Character 31, state 0), laterally oriented postorbital process in dorsal view (Character 38, state 1), confluent posterior border of the zygomatic process of the squamosal and exoccipital in dorsal view (Character 67, state 1), dorsoventrally higher than long parietal in lateral view (Character 76, state 1), anterolaterally directed zygomatic process of the squamosal in dorsal view (Character 86, state 2), distinctly higher than long squamosal including the zygomatic and postglenoid processes (Character 92, state 1), short squamosal fossa (Character 96, state 1), foramen pseudovale opening posteriorly between the squamosal and pterygoid (Character 118, state 1), and posteriorly diverging basioccipital crests in ventral view (Character 125, state 0).
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| Restoration of Megabalaena sapporoensis by Tatsuya Shinmura (Ashoro Museum of Paleontology). |
Yoshihiro Tanaka, Toshiyuki Kimura, Tatsuya Shinmura, Hiroto Ohira, and Hitoshi Furusawa. 2025. A New member of a large and archaic balaenid from the Late Miocene of Sapporo, Hokkaido, Japan partly fills a gap of Right Whale Evolution. Palaeontologia Electronica. 28(2): a37. DOI: doi.org/10.26879/1549 [August 2025]
Plain Language Abstract: The right whale family (Balaenidae) includes four extant species in two genera such as the Balaena and Eubalaena, all of which are endangered and giant animals about 17 to 20 m in length. The history of the right whale group spans about 20 million years. Several small-sized extinct fossil right whales from the Pliocene have been identified. However, half of this history remains unknown owing to a 9-million-year gap from 15.2 to 6.1 million years ago in the fossil record. A well-preserved fossil right whale skeleton (SMAC 2731) from the late Miocene (approximately 9 m.y.a.) of Sapporo, Hokkaido, Japan, is named as the new species Megabalaena sapporoensis. This specimen preserves the skull, ear bones, right lower jaw, hyoid bones, sternum, back bones, ribs, scapulae, and left forelimb elements. All preserved vertebral epiphyses are fused, indicating that SMAC 2731 was physically mature. Notably, M. sapporoensis can be distinguished from other balaenids by its large postorbital process of the skull and unfused cervical vertebrae, except for the axis and third cervical vertebra, and its slender forelimb long bones, including the humerus, radius and ulna, which are about twice slenderer than these of extant balaenids. Based on a bizygomatic width of 2.2 m, the estimated total length of the holotype of M. sapporoensis is 12.7 m. Overall, M. sapporoensis enhances our understanding of balaenid diversity, suggesting that it expanded earlier than the late Miocene.
