Bats may navigate using Earth’s magnetic field, and ancient magnetic fossils reveal a surprising internal compass structure
Spearhead-shaped and no bigger than a bacterial cell, they had been sitting in North Atlantic seafloor sediment for 97 million years. Scientists who studied these microscopic magnetic fossils assumed they were structural oddities — perhaps protective spines, perhaps nothing more. Then a new imaging technique looked inside them for the first time, revealing something far more intricate — and possibly the oldest evidence ever found of an animal navigating the open ocean using Earth’s magnetic field.
Fossils with an unexpected secret
Magnetofossils aren’t rare. Scientists have pulled these microscopic magnetic structures from ancient seafloor sediments for decades. They come in shapes resembling spearheads, spindles, bullets, and needles — each no larger than a bacterial cell — and their biological origin has long been accepted. Something made them. The harder question was always: why?
For years, the leading guess was structural. Maybe protective spines, maybe some mechanical purpose that left no other trace. Computer simulations eventually hinted at something more complex, but without a way to look directly inside the fossils, the question stayed open.
What comes next is a search — for the animal, for similar fossils in other geological periods, and for a clearer picture of how this sense evolved across deep time.
A tornado inside a fossil
That changed when researchers from the University of Cambridge and the Helmholtz Zentrum Berlin used magnetic vector tomography — a new 3D imaging method — to map the internal magnetic structure of the fossils for the first time. The measurements were carried out at the Diamond X-ray facility in Oxford, using an approach developed by co-author Claire Donnelly at the Max Planck Institute in Germany. Conventional X-rays had previously failed to penetrate particles this large. This method could.
What the images revealed was striking. Magnetic moments — tiny fields generated by spinning electrons — spiral around a central axis running through each fossil, forming a vortex pattern resembling a tornado. This internal geometry had never been observed in magnetofossils before.
Built to read Earth’s magnetic field like a map
The vortex structure isn’t just visually unusual. It carries specific physical properties that make it well suited to navigation.
Small changes in magnetic field strength cause the vortex to produce a subtle wobble. That wobble, researchers suggest, could allow an organism to detect both the tilt of Earth’s magnetic field — which varies with latitude — and its strength, which can shift with longitude. The particle may therefore have functioned as a two-coordinate position sensor. The geometry is also highly stable, making it resistant to environmental interference.
“If nature developed a GPS,” said Professor Rich Harrison of Cambridge’s Department of Earth Sciences, “a particle that can be relied upon to navigate thousands of kilometers across the ocean, then it would be something like this.”
How this compares to nature’s simpler magnetic compasses
Some bacteria already use magnetic particles to orient themselves — chains of magnetite crystals acting like compass needles, helping cells swim toward preferred water depths. It’s a well-understood system, and it works. But bacterial magnetic particles are just 50 to 100 nanometers wide.
The giant magnetofossils in this study are roughly 10 to 20 times larger. That extra size changes what’s physically possible. Smaller particles make efficient compass needles; larger ones can support the kind of complex vortex structure that encodes richer navigational information. Whatever produced these fossils had moved well beyond bacterial-level magnetoreception — toward something far more precise.
Who made them? The eel hypothesis
The identity of the organism remains unknown — one of the study’s most tantalizing loose ends.
Eels are a leading candidate. They evolved around 100 million years ago, close enough to the fossils’ 97-million-year age to be plausible. Modern European and American eels are extraordinary migrants, traveling thousands of kilometers to reproduce in the Sargasso Sea. Researchers know eels can detect Earth’s magnetic field, though exactly how remains uncertain. Magnetite particles have been found in eels, but scientists haven’t yet directly imaged those particles inside eel tissue.
“The next question is what made these fossils,” Harrison said. “This tells us we need to look for a migratory animal that was common enough in the oceans to leave abundant fossil remains.”
A missing link in the evolution of animal navigation
The findings push the confirmed record of magnetic navigation in animals back to at least 97 million years ago. They also suggest a plausible evolutionary pathway: from the simple compass chains inside bacteria, to the sophisticated dual-coordinate system these fossils represent, to the magnetoreception used today by birds, fish, and insects crossing entire hemispheres.
The results were published in Communications Earth & Environment, with support from the European Research Council and the Royal Society. What comes next is a search — for the animal, for similar fossils in other geological periods, and for a clearer picture of how this sense evolved across deep time. If ancient eels or something like them left these structures behind, more evidence may be waiting in sediments not yet imaged with the right tools. The fossils have already kept their secret for 97 million years.
All the information is available here: Richard J. Harrison, Jeffrey Neethirajan, Zhaowen Pei, Pengfei Xue, Lourdes Marcano, Radu Abrudan, Emilie Ringe, Po-Yen Tung, Venkata S. C. Kuppili, Burkhard Kaulich, Benedikt J. Daurer, Luis Carlos Colocho Hurtarte, Majid Kazemian, Liao Chang, Claire Donnelly, Sergio Valencia. Magnetic vector tomography reveals giant magnetofossils are optimised for magnetointensity reception. Communications Earth, 2025; 6 (1) DOI: 10.1038/s43247-025-02721-3
Daniel García is an Editor-in-Chief with strong expertise in structural work and engineering principles. He combines this technical foundation with deep knowledge of energy, spatial design, and emerging technologies, bringing a forward-thinking and analytical approach to editorial leadership.
