Extinct Mammal Fossil Excavated in Akan, Hokkaido, Reidentified; Species Previously Only Found in North America

The Yomiuri Shimbun

1:00 JST, August 25, 2025

OKAYAMA — A fossil discovered three decades ago in Hokkaido has been reidentified by a team of Japanese researchers as belonging to Neoparadoxia, an extinct species of mammal previously found only in the United States.

A team of researchers from Okayama University of Science and the Ashoro Museum of Paleontology made the finding while reexamining fossils kept at the museum.

Neoparadoxia, a mammal that may have looked like a hippopotamus, is believed to have lived about 14 to 16 million years ago. Measuring about 2.5 meters long, it is believed to have inhabited coastal areas. However, much of its ecology remains unknown due to the limited number of fossils. Following the discovery of a complete skeleton on the U.S. west coast, Neoparadoxia was announced to be a new species in 2013.

In response to this announcement, the Japanese research team began examining three skull fragments, which had been excavated in Akan, Hokkaido, which is now part of Kushiro, between 1996 and 2000 from a stratum dating back to 14.9 to 15.9 million years ago and were believed to belong to Paleoparadoxia, a mammal slightly smaller than Neoparadoxia.

Judging from the size of the fossils, the researchers concluded one of them was from Neoparadoxia.

The findings of the research were carried in an overseas online scientific journal.

“The Neoparadoxia conclusion was reached as a result of accumulated data,” said Yuma Asai, 23, of University of Tsukuba’s graduate school, who had conducted research on the fossils as a student at Okayama University of Science. “I am astonished and delighted.”