At 50 to 100 nanometers wide, bacterial particles sit in a size range that maximises magnetic efficiency. The fossils examined in the new study are roughly 10 to 20 times larger.

What did the new imaging reveal?

Previous tools could not peer inside these giant magnetofossils because standard X-rays fail to penetrate them. The breakthrough came through a technique developed by co-author Claire Donnelly at the Max Planck Institute in Germany, with measurements carried out at the Diamond Light Source in Oxford.

"That we were able to map the internal magnetic structure with magnetic tomography was already a great result, but the fact that the results provide insight into the navigation of creatures millions of years ago is really exciting," Donnelly said.

The scans showed magnetic moments spiralling around a central axis through each fossil in a pattern resembling a tornado. This vortex geometry, the team reports, could double as a sophisticated navigational instrument.

What makes a vortex-shaped magnet such a good compass?

According to Harrison, the vortex shape responds to two kinds of magnetic information at once. The tilt of Earth's field varies with latitude, while field strength varies with longitude. A particle that senses both could yield positional detail well beyond a simple heading.

"This magnetic particle not only detects latitude by sensing the tilt of Earth's magnetic field but also measures its strength, which can change with longitude," he said.

The geometry is also unusually stable. That stability may help the system shrug off small environmental disturbances, letting it deliver reliable readings over long journeys.

"If nature developed a GPS, a particle that can be relied upon to navigate thousands of kilometres across the ocean, then it would be something like this," Harrison added.

What creature could have made them?

The fossil maker remains unidentified. Harrison and colleagues argue the answer must be a migratory animal abundant enough in ancient seas to leave plentiful traces.

Eels, which evolved around 100 million years ago, fit the profile. Modern European and American eels travel thousands of kilometres between freshwater rivers and the Sargasso Sea to breed. Researchers know eels detect Earth's field, but how remains a mystery. Magnetite particles have been found in eel tissue, though their tiny size has kept them hidden from direct imaging.

Harrison plans to widen the search. "The next question is what made these fossils," he said. "This tells us we need to look for a migratory animal that was common enough in the oceans to leave abundant fossil remains."

What are the limits of the evidence?

The team's case rests on computer simulations and magnetic imaging rather than direct observation of a living animal using such particles. Whether ancient creatures actually relied on the vortex structures as the simulations suggest cannot be confirmed from the geological record alone.

Still, the authors argue the fossils mark an important milestone. "Giant magnetofossils mark a key step in tracing how animals evolved basic bacterial magnetoreception into highly specialised, GPS-like navigation systems," Harrison said.

The study received funding from the European Union, the European Research Council, and the Royal Society. Harrison is a Fellow of St Catharine's College, Cambridge.