open access

https://hdl.handle.net/2115/98909

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

英語

Japan

2025-08

Coquina Press

Palaeontologia Electronica

28(2)

a37

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.

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Copyright: August 2025 Society of Vertebrate Paleontology.

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