Ancient microfossils reveal a moving North Pacific Ocean 5 million years ago

A research team led by scientists at Kumamoto University has discovered a new genus of microscopic crustaceans from northern Japan, offering rare insights into how ocean currents in the North Pacific shifted during a key period of Earth’s climate history.

The study focuses on fossil ostracodes—minute, shrimp-like crustaceans enclosed in hard shells—found in Early Pliocene sediments (about 3–4 million years old) of the Takikawa Formation in Hokkaido. By carefully examining their shapes and distribution, the researchers identified a previously unknown genus, which they named Woodeltia.

Although only a few millimeters in size, ostracodes are powerful indicators of past marine environments. Because different species prefer specific water temperatures and depths, their fossils can act as biological “recorders” of ancient ocean conditions.

The newly identified genus Woodeltia is particularly significant because of its close relationship to species previously known only from distant regions of the North Pacific. This unexpected connection suggests that marine organisms were able to migrate across vast ocean areas during the Early Pliocene—a time when global temperatures were warmer than today.

“Our findings indicate that ocean circulation patterns in the North Pacific were more dynamic than previously thought,” said Associate Professor Gengo Tanaka from Center for Water Cycle, Marine Environmental and Disaster Management (CWMD), Kumamoto University. “These tiny fossils provide direct biological evidence that marine pathways linking the northern Pacific regions were already active several million years ago.”

The Early Pliocene is often studied as a natural analogue for future climate scenarios, as atmospheric carbon dioxide levels were similar to those projected for coming decades. Understanding how oceans behaved during this warm period can help scientists better predict how modern marine ecosystems may respond to ongoing climate change.

This discovery also highlights the scientific value of Japan’s fossil-rich sedimentary formations. The Takikawa Formation, long known for its abundant marine fossils, continues to yield new species that deepen our understanding of Earth’s environmental past.

The research demonstrates how detailed paleontological studies—down to microscopic organisms—can illuminate large-scale processes such as ocean circulation and climate evolution. Through this work, Kumamoto University continues to contribute to international efforts to reconstruct Earth’s history and better understand the future of our planet’s oceans.