Scientists have discovered a fossilized fish with its well-preserved eyeball containing rods and cones that are visible under a microscope. The discovery of the 300 million-year-old fish eye helped scientists determine that the ancient fish likely possessed color vision.

According to scientists, this is the first time that rods and cones -- also called photoreceptor cells -- have been discovered in a fossil vertebrate eye. Rods and cones are found in the retina of the human eye. The rods -- some 120 million -- are more sensitive than the cones, but are not sensitive to color. The 6 million to 7 million cones, on the other hand, provide color sensitivity to the eyes and are much more concentrated.

“This is the first discovery of fossil rods and cones in general and… also the first record of receptors, in a fossil vertebrate eye. These findings indicate that such receptors have been conserved in vertebrate eyes for at least 300 million years,” the scientists said, in a study published in the journal Nature, on Tuesday.

Scientists knew that the biological vision system, which consists of light receptors and image-processing capabilities, has existed for at least 520 million years. But, exactly when the optical system developed the capacity to see the world in color was still unknown. Scientists said that the new findings of the well-preserved photoreceptors have suggested that fish might have been enjoying color vision for at least 300 million years.

The newly discovered fossilized fish, called “Acanthodes bridge,” is about 4 inches long and was found in the Hamilton quarry in Kansas, which was once a shallow lagoon. Fossils that have been excavated from the lagoon are well preserved because they were buried very quickly in sediments, the Los Angeles Times reported, citing the study’s lead author Gengo Tanaka of Kumamoto University in Japan.

According to Tanaka, the discovery could help scientists better understand the evolution of many vertebrates, including dinosaurs, birds and other fossil fish.