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A unique 'hyper-eye' consisting of three different levels of vision has been discovered in a prehistoric animal.
Found in a group of ancient animals known as trilobites, the structure is believed to have allowed much better vision in the dark waters of the ocean.
An extinct trilobite has been found to have a type of eye unlike anything seen before or since.
The unique structure in a group of trilobites, dubbed a 'hyper-eye', features lenses that each cover a separate group of receptors that form multiple eyes within each other. Theories about its true nature first emerged in the 1970s, but were dismissed at the time.
The complex 'hyper-eye' is believed to have help the arthropod group see in very low levels of light and may have offered other advantages over ways of seeing.
Lead author, Dr Brigitte Schoenemann, says: ‘This structure is unique in the animal kingdom.
'As well as enabling vision in low light, it is also possible that the individual components of the eye performed different functions, enabling, for example, contrast enhancement or the perception of different colours.'
Others have urged caution on this finding, however, with Dr Greg Edgecombe, a researcher at the Museum who specialises in the evolution of arthropods and who was not involved with the study, saying he is 'unsure' over some details of the paper's claim.
'Whether or not the rosette structures are the cells of the photoreceptors is a bit contentious,' he says. 'Under exceptional conditions photoreceptors of arthropod compound eyes can be preserved but in this particular instance I’m unsure if we’re really seeing the boundaries of cells.
'However, the arrangements are highly suggestive because they’re in the right place.'
The research, led by German scientists, was published in the journal Scientific Reports.
Trilobites are a group of arthropods that lived on Earth for over 250 million years. Most are believed to have been scavengers or predators on the seafloor, with many species believed to have crushed prey to death with its legs. Others, meanwhile, would have survived on nutrients in the sediment.
From over this time period more than 20,000 species of trilobite have been recorded, putting them amongst the most successful of the early arthropods.
This range of species resulted following the Cambrian explosion, a period of history over 500 million years ago when most major animal groups begin to appear in the fossil record. The trilobites are well-represented as fossils and are found in excavations across the world.
Many of the specimens used by the new study come from German quarries, with X-rays taken in the 1970s by radiologist and amateur palaeontologist Wilhelm Stürmer. He was one of the pioneers of using this technology to examine the soft tissue of fossils.
While examining fossils of a group of phacopid trilobites using X-rays, Stürmer and a colleague found structures that they believed represented compound eyes. These are eyes formed of many individual lenses to form an image, and are commonly found in insects and other arthropods.
These compound eyes were connected to what they termed optical 'fibres' which carried signals from the eye to the brain. However, at the time it was generally believed that soft tissues like nerves didn't fossilise, and their finding was dismissed.
'The idea went out of fashion at one point, but by going back to the images the authors have done a convincing job to reassure us they are part of the eye,' says Greg.
'The reason for this is that there is a one-to-one correspondence between the external lenses and the fibres. They occur in multiple specimens and species, so as far as I am concerned they are part of the eye.
'I also think it is quite likely that these fibres are nerves, as over the past 10 years it has been demonstrated in arthropods from different parts of the fossil record that optic nerves can indeed fossilise.'
The phacopids have external lenses that seem too large and too far apart to function as a compound eye as expected. However, the researchers suggest that this is because their eyes were organised differently to other trilobites.
'Each of these eyes consisted of about 200 lenses up to 1 millimetre in size,' says Brigitte. 'Under each of these lenses, in turn, at least 6 facets are set up, each of which together again makes up a small compound eye.
'So we have about 200 compound eyes, one under each lens, in one eye and underneath sat a foam-like nest that was probably a small neural network to process the signals.'
After initially finding these eyes in Phacops fecundus, researchers examined X-ray images of other phacopid species from around the world and found similar structures.
Another compound eye with similar properties, though not the same level of complexity, is found in a living species of amphipod called Ampelisca callopia. It is suggested that the phacopids may have had a similar lifestyle living in the deep sea.
Greg also draws comparisons with strepsipterans, a group of parasitic insects that have similar compound eyes.
'The paper's new idea that you could have multiple photoreceptors associated with each lens in phacopids is possible,' says Greg. 'We should expect a different kind of optical construction in them. The overall argument is something I do think is possible.'
Whatever the case, if these eyes did offer them an advantage against competitors, it wasn't enough to save them from extinction. While other trilobites persevered, the phacopids, and their 'hyper-eyes', died out around 360 million years ago.