By Will Dunham

(Reuters) -Microscopic magnetic fossils found in North Atlantic seafloor sediments may represent components of an internal "GPS system" for an ancient marine creature that used Earth's magnetic field to navigate long distances, according to scientists.

The researchers said the fossils - about 50 times smaller than the ​width of a human hair - are made of a strongly magnetic iron-bearing mineral called magnetite. They suspect these particles were once part of a marine organism, ‌though its identity remains mysterious.

Scientists have recovered a number of these fossils dating back as far as 97 million years ago. There has been a debate as to whether or not they were biological in origin.

A new study employed three-‌dimensional imaging to determine the magnetic structure of one of the fossils, a magnetite particle shaped like a tiny ice cream cone that dates to 56 million years ago. The researchers identified features optimized to detect the strength and direction of Earth's magnetic field, a dynamic force generated by the motion of molten iron in our planet's core and extending out into space.

The fossil's interior harbored a closed loop of magnetization resembling a vortex. The particle could have been used to create a magnetoreception sense in an animal useful for guiding navigation, the researchers said.

"We show that the large magnetization ⁠means it would be optimized to detect variations in the strength of ‌Earth's magnetic field from location to location, which is a key part of building a natural 'GPS system,' enabling an animal to actually geolocate itself, not just know, for instance, which way is north," said Rich Harrison, a University of Cambridge professor of ‍Earth and planetary materials and co-leader of the study published in the journal Communications Earth & Environment.

A global positioning system provides satellite-based navigation guidance.

While some migratory animals including birds, fish and insects are thought to use Earth's magnetic field to navigate, how they manage this remains unanswered. One hypothesis is that magnetite particles inside their bodies align with Earth's magnetic field, akin to a compass needle.

Identifying the creature ​to which the fossils may have belonged remains a challenge as they were not associated with any other bodily remains.

It would make sense, the researchers said, if they came from ‌a migratory animal common enough to have left behind abundant fossil remains. Eels are a possibility, Harrison said.

"Eels are just one example of a migratory marine organism, famous for having to navigate twice across the Atlantic Ocean. Larval eels are transported by ocean currents associated with the Gulf Stream system from Sargasso Sea breeding grounds to coastal and freshwater habitats from North Africa to Scandinavia. After a decade or more, maturing adults migrate back to the Sargasso Sea, spawn and die," Harrison said.

The organism responsible for the magnetite fossils also may have been simply a microbe of some sort. These particles closely resemble magnetite fossils called magnetosomes produced by certain bacteria, though magnetosomes are around 20 times smaller.

Certain aquatic bacteria possess a form of magnetoreception arising from ⁠chains of magnetosomes inside these unicellular organisms that enable them to line up with Earth's magnetic field, ​helping them navigate to their preferred water depth.

"There is strong evidence suggesting that many other organisms - including ​mammals, birds, amphibians, reptiles and insects - do have magnetic-navigation capabilities. What remains unknown is how they do it," said physicist and study co-leader Sergio Valencia of the Helmholtz-Zentrum Berlin research institute in Germany.

"One major challenge is that if magnetic particles exist in these animals,‍ they are extremely small and sparse, making ⁠them very difficult to locate within the whole organism," Valencia said.

So how would this biological "GPS system" work?

"If these particles were indeed part of a living organism, one intriguing possibility is that they were connected to magnetoreceptive cells, acting as magnetic sensors. As the particle reoriented itself with Earth's magnetic field, it could ⁠have generated a mechanical or electrical signal that the organism used to detect magnetic intensity and direction," Valencia said.

"In this way, the organism might have had a magnetic 'sense' - analogous to our sense of ‌sight, which captures light distribution - allowing it to navigate its environment safely using the local strength and direction of the planet's magnetic field as a ‌guide," Valencia said.

(Reporting by Will Dunham in Washington; Editing by Daniel Wallis)