Rod cells and cone cells, the ones in our eyes that help us see, have been detected in a fossil fish at least 300 million years old, showing such cells evolved at least that long ago -- and that ancient fish probably saw in color, researchers say.

The fossilized cells are the oldest photoreceptors in a vertebrate eye ever found, they say.

Rods and cones, cells that contain pigments that absorb light, line the retinas in vertebrate eyes. The long thin rods are the more sensitive to light, but the shorter, triangular cones are what allow color vision.

In an effort to understand the evolution of eyes and vision, Japanese researchers studied the amazingly well-preserved fossil of a fish known as Acanthodes bridgei.

Unearthed in Kansas in a quarry that was once a shallow lagoon, the 4-inch fossil in in the collection of the National Museum of Nature and Science in Tokyo.

The study, using a scanning electron microscope, yielded traces of the earliest fossilized rod and cone cells known in this fish species, the researchers reported in the journal Nature Communications.

"Rods and cones are not usually preserved, because these soft tissues are more fragile," said lead study author and paleontologist Gengo Tanaka of Kumamoto University in Japan, noting that the cells normally begin to disintegrate within days of an animal's death.

This fossil is remarkable well preserved probably because it was very quickly covered in sediment, Tanaka said

The fish likely lived in shallow waters where the full color spectrum of sunlight would have reached, the researchers explain, so color vision would have helped the fish in finding food and avoiding predators.

In that it would have similar to a modern-day fish called Rhinogobius, a small goby fish often kept in aquariums that is similar in size to A. bridgei and also lives in slightly brackish water.

The ratio of rods to cones was about the same in both fish, suggesting A. bridge, like Rhinobobius, was active during the day and relied on its vision to feed and survive, the researchers say.

The findings support previous studies suggesting the development of modern eyes can be traced back hundreds of millions of years, in many species including dinosaurs, bids and other ancient fish.

By studying such examples of fossilized vertebrate eyes, "we can reconstruct what colors extinct animals -- example, dinosaurs ---could see," Tanaka says.

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