The tiny magnetic fossils - dating from 97 million years ago - were buried in ancient seafloor sediments, left behind by a mysterious, unidentified organism.
Shaped like spearheads, spindles, bullets and needles, and no larger than a bacterial cell, scientists believe these 'magnetofossils' are biological in origin, but they don't know what creature made them, or why.
Now, researchers have now solved part of the mystery and found that these fossils may have served as an animal GPS system, enabling organisms to read Earth's magnetic field like a map.
The researchers, from the University of Cambridge and the Helmholtz-Zentrum Berlin, captured the first 3D images of the fossils' magnetic structure, and revealed features optimised to detect both the direction and strength of Earth's magnetic field, which would have aided navigation.
"Whatever creature made these magnetofossils, we now know it was most likely capable of accurate navigation," said Professor Rich Harrison from Cambridge's Department of Earth Sciences, who co-led the research.
The discovery provides the first direct evidence that animals have been navigating using the Earth's magnetic field for at least 97 million years. It may also offer insights into how animals evolved this ability, known as 'magnetoreception'. The results are reported in the journal Communications Earth and Environment.
Life has evolved a range of extraordinary senses, and magnetoreception is one of the most poorly understood. Birds, fish, and insects use the Earth's magnetic field to navigate vast distances, but how they do this is still unclear. One theory is that tiny crystals of magnetite within the body align with the Earth's magnetic field, acting like microscopic compass needles.
Certain bacteria found in lakes and other bodies of water possess a primitive form of magnetoreception. Chains of tiny magnetic particles inside the bacteria allow them to line up with the magnetic field, helping them swim to their preferred depth in the water column.
"At just 50 - 100 nanometres wide, these particles are the perfect compass needles," said Harrison. "If you want to create the most efficient magnetic sense, smaller is better."
But the magnetofossils the team studied for the current study are 10 to 20 times larger than the magnetic particles used by bacteria, and were retrieved from a site in the North Atlantic Ocean. Previously, some researchers had argued that 'giant' magnetofossils may have served as protective spines.
However, model simulations have suggested that they might also possess advanced magnetic properties, something Harrison wanted to explore further. "It looks like this creature was carefully controlling the shape and structure of these fossils, and we wanted to know why," he said.
The researchers applied a new technique to visualise the fossil's internal structure, revealing how magnetic moments (tiny magnetic fields generated by spinning electrons) are arranged inside the magnetofossil.
Until now, scientists had been unable to capture 3D magnetic images of larger particles, such as giant magnetofossils, because X-rays couldn't penetrate them.
The research was made possible using a technique developed by co-author Claire Donnelly at the Max Planck Institute in Germany and carried out 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," said Donnelly.
Their images revealed an intricate magnetic configuration, with magnetic moments swirling around a central line running through the fossil's interior, forming a tornado-like vortex pattern.
This vortex magnetism provides ideal properties for navigation, said Harrison, generating a 'wobble' in response to tiny changes in the strength of the magnetic field that translate into detailed map 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 geometry of this vortex structure is highly stable, meaning it can resist small environmental disturbances that may otherwise disrupt navigation. "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," he said.
In solving the enduring mystery over the fossils' function, the work also helps narrow the search for the animal that made them. "The next question is what made these fossils," said Harrison. "This tells us we need to look for a migratory animal that was common enough in the oceans to leave abundant fossil remains."
Harrison suggests that eels could be a potential candidate, since they evolved around 100 million years ago and remain one of the least understood and elusive animals. European and American eels travel thousands of kilometres from freshwater rivers to spawn in the Sargasso Sea. Though they can sense Earth's magnetic field, how they do so is unclear. Magnetite particles have been detected in eels but not yet imaged directly in their cells and tissues, partly because of their tiny size and the fact they could be hidden anywhere in the body.
Harrison worked closely with Sergio Valencia from Helmholtz-Zentrum Berlin in designing the research. "This was a truly international collaboration involving experts from different fields, all working together to shed light on the possible functionality of these magnetofossils," said Valencia.
Despite their as-yet-unknown host, "giant magnetofossils mark a key step in tracing how animals evolved basic bacterial magnetoreception into highly-specialised, GPS-like navigation systems," Harrison said.
The research was supported in part by the European Union, the European Research Council and the Royal Society. Rich Harrison is a Fellow of St Catharine's College, Cambridge.
Research Report:Magnetic vector tomography reveals giant magnetofossils are optimised for magnetointensity reception
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