Scientists have discovered rod and cone cells while examining the fossil of 300-million-year-old fish eyes. The findings suggest that the fish likely possessed color vision.
Scientists found fossil of a fish that measured about 10 centimeters long. The fossil was found in Hamilton Quarry, near Hamilton in Kansas. Researchers suggest that Hamilton Quarry was a shallow lagoon in ancient times and fossils discovered from the area are usually well preserved as they got buried very swiftly in the sediments of the lagoon.
Gengo Tanaka at the Kumamoto University, Japan, who is also the lead author of the research, suggests that the fossil is of an extinct fish species named Acanthodes bridgei. Tanaka believes that the cells in the fish eyes were well preserved probably due to a bacterial activity that deposited a thin film of phosphate over the eyes before it was buried.
The study suggests that some parts of the visual system are not commonly preserved in the fossil record as the soft tissue found in the brain and eyes decay very rapidly after death. These tissues in the eyes can decay in just 64 days after death.
"Rods and cones are not usually preserved, because these soft tissues are more fragile," says Tanaka.
However, the fossil of the latest fish was so well-preserved that rod and cone cells are still visible in the eyeballs under an electron microscope.
Rods and cones are cells that line the retina in our eyes. Rod cells are long and thin while cone cells are triangular. Even though Rods are more sensitive to light in comparison to cones, it is the cone cells that allow us to have colored vision.
Both the rod and cone cells depend upon pigments to absorb light. Chemical analysis found the evidence of one pigment called melanin in the fossil of the fish.
The study also compared the eye of the fish fossil with modern day Rhinogobius fish species, which is very similar in size to the Acanthodes bridgei. The researchers found that rod and cone cells ration in both the fish species were similar, which indicates that the Acanthodes bridgei was active in the day and depended on its vision to survive.
Scientists had long believed that modern eyes developed millions of years ago and the latest discovery provides a definitive proof.
The study has been published in the journal Nature Communications.