The first discovery of fossilised rods and cones, receptors in the visual system, is reported online this week in Nature Communications. The discovery in a 300-million-year-old fish, called Acanthodes bridgei, indicates that these visual receptors have been conserved in vertebrate eyes for at least 300 million years and that the vertebrate in question could likely see in colour.
The evolution of vision in vertebrates is an important theme in the history of animal life, however, aside from the calcified lenses of fossilised arthropods, other parts of the visual system are not usually preserved in the fossil record because the soft tissue of the eye and brain decays rapidly days after death.
Gengo Tanaka and colleagues examined the fossilised remains of a fish called Acanthodes bridgei from the Upper Carboniferous Hamilton Formation in Kansas, USA. A. bridgei is the last common ancestor of modern jawed fishes and supposedly lived in quite shallow brackish waters. They find that the original colour, shape and presence of eumelanin (a retinal pigment that absorbs light) have been preserved in the fish.
They also find the preserved tissues of the eye, which provides the first record of mineralized rods and cones in a fossil. The existence of rods, cones and melanin pigments in the eye suggests that retinomotor activity (light dependent vision seen in fish today: daylight vision by cones and twilight vision by the more sensitive rods) probably already existed 300 million years ago. The presence of cones also indicates that the 300-million-year-old A. bridgei likely also possessed colour vision, although conclusive evidence would require the recovery of photoreceptor proteins called opsins.