Photo/Illutration Reconstruction of Neoparadoxia, top left, and Paleoparadoxia, bottom right (© Tatsuya Shinmura & Ashoro Museum of Paleontology)

Researchers have identified the fossil of a rare extinct marine mammal in Japan for the first time, shedding light on the climate-driven Miocene epoch spanning roughly 23 million to 10 million years ago.

The presence of Neoparadoxia, previously known only from fossil sites along the western coast of the United States, was confirmed from a trove of fossils unearthed between 1996 and 2000 in Akan, a town in the city of Kushiro, Hokkaido.

The remains date back about 15 million years, the scientists said.

The Neoparadoxia discovery was the result of a collaborative investigation by Okayama University of Science and the Ashoro Museum of Paleontology in Ashoro, Hokkaido.

Initially, the fossils had been attributed to Paleoparadoxia, a large herbivorous marine mammal resembling a hippopotamus, and a fellow member of the now-extinct order Desmostylia.

These enigmatic creatures are believed to have thrived in the shallow coastal waters of the ancient North Pacific.

Yet the presence of slightly larger specimens suggested the possibility of Neoparadoxia and prompted a re-examination in 2013.

After a detailed morphological analysis, researchers confirmed the identification and the significant expansion of the species’ known range.

The discovery also deepened the scientists’ understanding of the creature’s evolutionary history.

The research team uncovered compelling evidence that members of the Desmostylia order, including Paleoparadoxia and Neoparadoxia, thrived during a period of elevated global temperatures around 15 million years ago.

Their eventual extinction may have been closely linked to dramatic shifts in Earth’s climate, according to the study.

“These fossils had been stored for years without a definitive classification,” said Hiroshi Sawamura, special curator at the Ashoro Museum of Paleontology. “It’s deeply gratifying to finally reveal their true identity.”

Shoji Hayashi, an associate professor at the Department of Biosphere-Geosphere Science at Okayama University of Science, described the discovery as “a rare and valuable case that illustrates the relationship between Earth’s climate and the evolutionary trajectory of marine mammals.”

The findings have also been a personal milestone for Yuma Asai, now a graduate student at the University of Tsukuba.

Asai first became involved in the project during his undergraduate studies at the Okayama University of Science, where the fossils formed the basis of his thesis.

According to Asai, both species likely inhabited shallow coastal waters and fed on marine vegetation, but much about their ecology remains shrouded in mystery.

“I hope to explore their lives more deeply by examining the internal structure of their bones to trace changes in their habitat,” Asai said.

(This article was written by Tomoyuki Suzuki and Tomoyuki Yamamoto.)

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