Magnetic fossils powered ancient animal navigation

Scanning electron microscopy image of the ‘no-stalk’ spearhead giant magnetofossil (red arrow) surrounded by detrital particles. The spearhead-like microfossil is about 2 micrometer in length and has a radius of about 0.5 micrometer at its largest bottom region.

The paper, published in Nature, was a collaboration between the University of Cambridge, Max Planck Institute, Helmholtz-Zentrum Berlin and several other international partners.  

The fossils, which resembled spearheads, spindles, bullet shapes and needles, were found buried on the ocean floor and are no larger than a single bacterial cell.  Although scientists are confident that these magnetofossils are biological in origin, they were unsure what creature made them - or why. The team found that these fossils may have served as an animal GPS-system, enabling organisms to read Earth’s magnetic field like a map.

At Diamond, the researchers captured the first 3D images of the fossil’s magnetic structure, revealing features optimised to detect both the direction and strength of Earth’s magnetic field, guiding long-distance migration. 

“Whatever creature made these magnetofossils, we now know it was most likely capable of accurate navigation,” said Rich Harrison from Cambridge’s Department of Earth Sciences. With magnetofossils found in sediments dating as far back as far as 97 million years, the discovery provides the first direct evidence that animals have been navigating using the geomagnetic field for at least this long. It may also offer insights into how animals evolved this ability, known as ‘magnetoreception’. 

Life has evolved a range of extraordinary senses, and magnetoreception is one of the most poorly understood. Birds, fish, and insects use Earth’s magnetic field to navigate vast distances, but how they detect it 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. 

Sections of the fossil's 3D magnetic volume at three different heights: The blue/red color indicates the component of the magnetization lying in the plane, with the magnetic moments swirling around a central line, forming a vortex pattern resembling a tornado (see projections at the bottom). The 3D reconstruction was created on the basis of 140 individual images from different angles. © Jeffrey Neethirajan/ MPI CPfs and Sergio Valencia/HZB

Certain bacteria found in lakes and water bodies worldwide possess a primitive form of magnetoreception. Chains of tiny magnetic particles inside the bacteria allow them to line up with Earth’s 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. 

But the so-called ‘giant’ magnetofossils the team studied are 10 to 20 times larger than the magnetic particles used by bacteria, leading some scientists to dismiss their capabilities for navigation. 

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 the team wanted to explore further. 

Harrison worked closely with Sergio Valencia from Helmholtz-Zentrum Berlin in designing the research. He and the team 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.  

This work was made possible using a new technique developed by study co-author Claire Donnelly at the Max Planck Institute in Germany. Part of this work was carried out at Diamond Light Source by I08’s principal beamline scientist Burkhard Kaulich and his team. 

This vortex magnetism provides ideal properties for navigation, 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,” said Harrison. 

He added that the geometry of this vortex structure is very 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.” 

In solving the mystery over the fossils’ function, the work also helped to narrow down the search for the animal that made them. The research implied that it was a migratory animal that was common enough in the oceans to leave abundant fossil remains. 

Eels could be a potential candidate, having evolved around 100 million years ago and remaining 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. They can sense Earth’s magnetic field, but how they do so remains 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. 

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.