Researchers have produced the first 3D images of the magnetic structure inside 97-million-year-old magnetofossils, and the geometry they found — a tornado-shaped vortex of magnetic moments spiraling around a central line — appears optimized for detecting both the direction and the strength of Earth's magnetic field. If the interpretation holds, these microscopic structures represent the earliest direct evidence that an animal navigated using Earth's magnetic field, pushing the documented origin of magnetoreception-based navigation back at least 97 million years.

The findings carry no immediate clinical weight for practicing veterinarians, but they speak directly to one of comparative biology's open questions: how birds, fish, and insects sense magnetic fields over migrations spanning thousands of kilometers. The study, published in Communications Earth & Environment, was led by Professor Rich Harrison of Cambridge's Department of Earth Sciences, with Sergio Valencia of Helmholtz Zentrum Berlin as co-designer and Claire Donnelly of the Max Planck Institute in Germany as developer of the imaging technique used. The work was supported in part by the European Union, the European Research Council and the Royal Society.

The fossils themselves come in shapes resembling spearheads, spindles, bullets, and needles, each no larger than a bacterial cell. They were preserved in ancient seafloor sediments retrieved from the North Atlantic Ocean. Scientists accept a biological origin, but the organism that produced them — and their function — has remained uncertain. One earlier hypothesis held that these unusually large magnetofossils acted as protective spines.

From bacterial compass to animal GPS

The size difference matters. Magnetotactic bacteria living in lakes and other aquatic environments carry chains of magnetic particles 50–100 nanometers wide that let them orient along field lines and swim to their preferred water depth.

"At just 50-100 nanometers wide, these particles are the perfect compass needles," Harrison said. "If you want to create the most efficient magnetic sense, smaller is better."

The giant magnetofossils in this study are roughly 10 to 20 times larger than the particles bacteria use. Computer simulations had hinted the structures could possess more sophisticated magnetic properties than simple compass behavior would require.

"It looks like this creature was carefully controlling the shape and structure of these fossils, and we wanted to know why," Harrison said.

Conventional X-rays could not penetrate particles of this size, so the team turned to magnetic tomography — a technique developed by Donnelly — to map how magnetic moments, the tiny fields generated by spinning electrons, arrange themselves throughout each fossil. Measurements took place at the Diamond X-ray facility 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.

A particle built to read latitude and longitude

The vortex magnetism the images revealed has properties exceptionally well suited to navigation, according to Harrison. Small variations in magnetic field strength cause the structure to produce a subtle "wobble," potentially allowing an organism to extract detailed positional information.

"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 vortex geometry is also extremely stable, which could let the system resist small environmental disturbances that might otherwise corrupt navigational information.

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

"Whatever creature made these magnetofossils, we now know it was most likely capable of accurate navigation," he added.

A candidate culprit

The identity of the organism remains unknown, but the fossils themselves narrow the search. "This tells us we need to look for a migratory animal that was common enough in the oceans to leave abundant fossil remains," Harrison said.

He points to eels as one possible candidate. Eels evolved around 100 million years ago — close to the fossils' age — and remain poorly understood today. Modern European and American eels migrate thousands of kilometers from freshwater rivers to spawn in the Sargasso Sea, and researchers know they can detect Earth's magnetic field, though the mechanism remains uncertain. Magnetite particles have been found in eels, but no one has yet imaged those particles inside their cells and tissues; their extremely small size means they could sit almost anywhere in the body.

For veterinary audiences tracking magnetoreception research — relevant to avian and aquatic species medicine, migration-dependent welfare assessments, and anthropogenic impacts such as electromagnetic interference — the study supplies a deep-time data point. "Giant magnetofossils mark a key step in tracing how animals evolved basic bacterial magnetoreception into highly-specialized, GPS-like navigation systems," Harrison said.

The next step is identifying the organism itself, a search Harrison suggests should focus on abundant, migratory marine animals of the mid-Cretaceous oceans.