Researchers have identified what may be the earliest known evidence of an internal animal navigation system, in magnetic fossils 97 million years old, preserved in ancient seafloor sediments and produced by an organism whose identity is still unknown.
The structures, known as magnetofossils, come in shapes resembling spearheads, spindles, bullets and needles. Each is no larger than a bacterial cell. Their biological origin has long been accepted; their purpose has not.
Scientists from the University of Cambridge and the Helmholtz Zentrum Berlin have now produced the first three-dimensional images of the magnetic structure inside them. The work is reported in Communications Earth & Environment.
“Whatever creature made these magnetofossils, we now know it was most likely capable of accurate navigation,” said Professor Rich Harrison of Cambridge's Department of Earth Sciences, who co-led the research.
Magnetoreception remains one of the least understood senses in nature. Birds, fish and insects use Earth's magnetic field to cross enormous distances, and the biological mechanism is still not fully explained. One leading idea involves magnetite, which can form tiny crystals inside an animal's body that align with the field like compass needles.
Some bacteria already do a simple version of this. “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 fossils in this study, retrieved from a location in the North Atlantic, are roughly 10 to 20 times bigger than the bacterial particles. They had previously been thought to be protective spines.
Imaging them required a technique developed by co-author Claire Donnelly at the Max Planck Institute in Germany, with measurements carried out at the Diamond X-ray facility in Oxford. Conventional X-rays could not penetrate particles that large.
The images showed magnetic moments spiralling around a central line, forming a structure resembling a tornado-shaped vortex. “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,” Harrison said. The geometry is also very stable, which would help it resist small environmental disturbances.
“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,” he said. The next task, he added, is to look for “a migratory animal that was common enough in the oceans to leave abundant fossil remains”. Eels are one candidate: they evolved around 100 million years ago, modern European and American eels travel thousands of kilometres to spawn in the Sargasso Sea, and magnetite has been found in them, though never directly imaged inside their tissues.
The comparison with satellite navigation is the researchers' own, and it survives inspection. Latitude from the tilt of the field, longitude from its strength, a stable reading that resists interference, and the whole apparatus no larger than a bacterial cell.
What the system lacks is the satellites, the ground stations, the atomic clocks and the receiver. It is a global positioning system with no infrastructure, built by an animal nobody has identified, 97 million years before anybody needed to find anything.
It also has a service life that the modern equivalent has not been asked to match. The structures spent 97 million years in seafloor sediment with their magnetic architecture intact, and were read out this year in Oxford by people who were not the intended users.
The Department of Unnecessary Research notes the single specification on which the ancient system falls short. It has no maps. The animal could establish its coordinates to a high standard and had nothing whatever to look them up in.
The next task is to find the creature. It left precise instructions for where it was, and no indication at all of where it was going.