Like ours, the fish visual system relies on light-sensitive receptors in the retina called rods and cones: Rods for twilight vision, cones for daylight vision. Researchers examining a 300-million-year-old fish have discovered fossilized rods and cones for the first time ever. The findings, published in Nature Communications this week, suggests that these visual receptors -- and color vision -- have been around in vertebrate eyes for at least as long.
Other than calcified lenses of trilobites and some crustaceans, most parts of the vertebrate visual system haven’t been preserved in the fossil record. Vision may have existed for at least 520 million years, but soft tissue in both the eye and the brain decays too fast -- usually within 64 and 11 days after death, respectively.
Now, a team led by Gengo Tanaka from Kumamoto University studied the well-preserved remains of a fish called Acanthodes bridgei from the Upper Carboniferous Hamilton Formation in Kansas. This fish was buried in oxygen-low sediment immediately after dying, Science reports, which prevented microbes from breaking down all of its tissues. This species is the last common ancestor of modern jawed fishes (from sharks and rays to all bony fishes), and It went extinct around 250 million years ago at the end of the Permian. This fish had a long, streamlined body with spines, and when it was alive, it probably swam in shallow, brackish waters like Rhinogobius, a fish with similar sized eyes that’s alive today.
The original color and shape of its dark brown eumelanin (a light-absorbing retinal pigment) were preserved, and the team also found mineralized rods and cones, which means that the fish likely could see in color. That would be important for finding food and spotting predators in shallow water, light-penetrating water. Taken together, these findings suggest that retinomotor activity -- light-dependent vision seen today -- already existed 300 million years ago.