Megabalaena sapporoensis
- 演化之聲
- Mar 20
- 2 min read
Updated: Apr 9
Age
Neogene（Tortonian）
Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Family: Balaenidae
Genus: Megabalaena
Species: Megabalaena sapporoensis
Morphological description
The total body length of Megabalaena sapporoensis is estimated to be approximately 12.7 m based on a bizygomatic width of 2.2 m. In dorsal view, the orbit is distinctly excavated, and the lateral process of the maxilla at the base of the rostrum extends posterolaterally.
Among the cervical vertebrae, all except the second and third are unfused. The fourth to sixth cervical vertebrae are transversely wide with relatively thin centra, whereas the seventh cervical vertebra has a slightly thicker and more rounded centrum compared to the fourth to sixth.
The long bones of the forelimb are slender, including the humerus, radius, and ulna. The radius and ulna are slightly longer than the humerus, with the radius exceeding the ulna in length. The metacarpals are shaft-like, with a constricted midsection and a nearly circular cross section.
Etymology
The generic name Megabalaena is derived from the Greek mégas meaning great, combined with Balaena, the type genus of the family Balaenidae.
The species name refers to the locality where the fossil was discovered.
Biological description
The holotype specimen SMAC 2731 of Megabalaena sapporoensis was discovered on October 10, 2008, from the riverbed of the Toyohira River in Sapporo, Hokkaido. The fossil includes parts of the skull, mandible, sternum, ribs, left forelimb elements, and a total of 32 vertebrae. All preserved vertebral epiphyses are fused, indicating that the individual was physically mature.
The evolutionary history of the Balaenidae extends for more than 20 million years. However, there exists a gap of approximately 9 million years in the fossil record between 15.2 and 6.1 Ma. The discovery of this species partially fills this gap and helps connect morphological transitions, including changes in forelimb morphology from narrow to broader forms.
Early balaenids were typically small, measuring about 4–6 m in length, whereas Megabalaena sapporoensis exceeds 12 m. This indicates that large body size in balaenids originated earlier than previously assumed, likely before the Late Miocene. However, due to limited fossil evidence, it remains uncertain whether such large body size was common at that time.
Phylogenetic analysis places Megabalaena sapporoensis in a more derived position than Balaenula, within an unresolved polytomy. It forms a monophyletic group together with Antwerpibalaena, Eubalaena, and Charadrobalaena.
（Author: Bai Leng）
Reference
Tanaka, Y., Kimura, T., Shinmura, T., Ohira, H., Furusawa, H. (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.
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