The top fossil is believed to be of the mandible of a baleen whale, as seen in Noshiro, Akita Prefecture, on Nov. 1, 2023. On both sides of the tip are fossils of what appear to be ribs. (Mainichi/Muneo Takahashi)

NOSHIRO, Akita — Baleen whale fossils have been discovered in a 2.7-million-year-old stratum at the foot of the Shirakami-Sanchi mountains in this northern Japan city, and the excavation work was unveiled to the press on Nov. 1.

Shinshu University in Nagano Prefecture and the Fukui Prefectural Dinosaur Museum have been jointly excavating the fossils. Among them is one believed to be an approximately 4-meter-long right mandible, reportedly the first mandible of a baleen whale from this period discovered in Japan. The team will begin research to identify the exact species.

The excavation site is located along a tributary of the Taneume River, about 5 meters below a forest road, revealing that the mountainous area was a seabed in ancient times. In July 2020, a team led by paleontology professor Katsura Yamada of Shinshu University’s Faculty of Science and some of her students, who were conducting a geological survey to learn more about the area’s paleoenvironment, found two 10-centimeter-wide fossil sections protruding about 5 cm from a roadside along the stream.

Yamada said, “From the cross-sectional features and other factors, I guessed that they were the bones of a large mammal.” When she and the students set to work excavating, they were apparently delighted to discover one fossil after another.

In 2021, a tip of the recently unearthed 4-meter-long mandible bone was found. Yamada, a microorganism fossil expert and no expert in large specimens, thought, “Here we are again. I’m confounded.”

A piece of the fossil believed to be part of a baleen whale’s bone is seen in Noshiro, Akita Prefecture, on Nov. 1, 2023. (Mainichi/Muneo Takahashi)

After consulting with a good number of people, Yamada was told by Fukui Prefectural Dinosaur Museum deputy director Hiroto Ichishima, a paleontology expert who is knowledgeable about whales, “It’s probably the mandible of a baleen whale,” and they decided to excavate it jointly.

They had unearthed three approximately 2-meter-long fossils believed to be ribs by last year, and in the excavation that began this past summer, a fossil believed to be the right mandible, two ribs, and a possible scapula or part of the head were found. The work on Nov. 1 revealed to reporters part of a fossil that could be the left mandible, set to be excavated next year or later.

According to Ichishima, the size of the mandible suggests the creature was an 18-meter-long baleen whale. Baleen whales had been at most around 10 meters long until about 3 million years ago, so this whale is thought to have been large for the time. The period 2.7 million years ago falls between the Pliocene and Pleistocene eras. Academic papers published overseas say that whales began getting larger around this period, and the latest finding in Akita Prefecture could strengthen the theory.

Ichishima said, “This provides valuable data on the evolution of baleen whales. We would like to advance efforts to identify the species in parallel with the cleaning work to remove rocks from the fossil.”