Fossilized rods and cones have been found in a 300 million year old fish, according to a paper published in the December 23rd, 2014 issue of Nature Communications.  The photoreceptors were discovered using a scanning electron microscope.  The fish, Acanthodes bridgei was found in the Hamilton Quarry in Kansas, which was a shallow lagoon in the Pennsylvannian.  Because there were both rods and cones suggests that fish have been seeing in color for 300 million years.  Gengo Tanaka of Kumamoto University in Japan, the lead author, said fossils from this quarry are very well preserved because they were buried very quickly in the sediments in the lagoon.

In the case of this fish, an extinct species called Acanthodes bridgei, the preservation process probably also got some help from bacterial activity that left a thin film of phosphate over the eyes before it was buried.

Tanaka said that gills and pigments on other parts of the fish were also preserved. However, he had not looked to see whether organs and nerves were intact as well.

The researchers compared the fossilized fish eye to the modern-day fish Rhinogobius, which is similar in size to A. bridgei and which also lives in slightly salty water. They found that the ratio of rods to cones was similar in both fish, which suggests A. bridgei was more active during the day and relied on its vision to make a living.

Tanaka said the discovery could inform the study of many vertebrates like dinosaurs, birds and other fossil fish. Scientists had thought that modern eyes had developed hundreds of millions of years ago. Now, they have definitive proof.