Scientists have discovered a fossilized fish so well preserved that the rods and cones in its 300-million-year-old eyeballs are still visible under a scanning electron microscope. It is the first time that fossilized photoreceptors from a vertebrate eye have ever been found.
The researchers say the discovery also suggests that fish have been seeing the world in color for at least 300 million years. Rods and cones are cells that line the retina in our eyes. Rods are long and thin, and more sensitive to light than cones. However, cones, which are triangular, allow us to see in color. Both these cells rely on pigments to absorb light. Using chemical analysis, the scientists found evidence of one of these pigments, melanin, in the fossilized eye as well.
The fish pictured above is about 10 centimeters long. It was found in the Hamilton Quarry in Kansas, which was once a shallow lagoon. Fossils from this area are remarkably well preserved because they were buried very quickly in sediments in the lagoon, said Gengo Tanaka of Kumamoto University in Japan, the lead author of the paper.
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.