The 530-million-year-old fossilized remains of a long-extinct sea creature shows that the structure of the eye hasn’t actually changed all too much in half a billion years.
Circled is what could be the oldest eye ever discovered. [Photo via PA]
Scientists were examining a remarkably preserved trilobite fossil when they made the striking discovery. The hard-shelled trilobite was the ancestor of today’s crabs and spiders, and lived between 541 to 251 million years ago in the coastal waters of the Paleozoic era. The researchers found that this particular trilobite species, Schmidtiellus reetae, possessed an early form of the compound eye. It’s likely that this creature had poorer vision than modern animals, but was probably able to see a predator approach and avoid obstacles.
The fossilized eye, according to a new study, also does not have a lens, unlike modern compound eyes. However, another trilobite species was able to develop a lens in their eyes just a few million years later. Scientists think that this may be the oldest eye we’ll be able to discover.
The full fossil [Photo via PA]
It may be unlikely that we’ll find fossils that are much older than this one and others like it. “Older specimens in sediment layers below this fossil contain only traces of the original animals, which were too soft to be fossilised and have disintegrated over time,” says Brigitte Schoenemann, one of the researchers.
According to fossil records, trilobites were quite successful during their run on Earth. For one thing, they were among the earliest creatures to have been able to develop a sense of vision. The compound eye they possessed was brimming with vision cells, something that we can still see in today’s flies, bees, and other arthropods.
Partial wearing on the eye has given researchers the opportunity to study the eye's internal structures. [Photo by Gennadi Baranov]
This trilobite fossil was so well-preserved that the researchers were able to analyse the eye down to the cellular level. Cambrian trilobite fossils have contained traces of the primitive compound eye before, but these discoveries haven’t been able to show the internal structure of the eye. Because the eye was partially worn away, the researchers were able to study its internal structure in more detail than could have been possible if the eye had been completely intact.
The researchers were also able to count about 100 ommatidia, or the units that make up a compound eye. In modern insects, the ommatidia are densely packed together. In the trilobite, however, the researchers found that the ommatidia were spaced wide apart.
The eyes of modern bees are also comprised of vision cells, like the eyes of trilobites.
Researchers also say that the absence of a lens structure was remarkable. "There are indications of round lens-like discs when the eye is studied from the outside, but from the internal aspect, no convexities that could effectuate any refraction of light can be made out," the researchers write. It’s possible that the trilobite’s exoskeleton didn’t have the materials necessary for forming the lens. Through this fossil however, scientists were able to make educated guesses on how our early animals saw the world.
The lens is the part of the eye that bends light so an image will form clearly on the retina. It can grow fatter when you focus on objects that are close by, and thinner when you focus on objects that are far away.
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