An amazingly well preserved 10 centimeter long fossilized fish found in the Hamilton Quarry (Kansas) suggests that the fish have been seeing the world in color since at least 300 million years. Since the fossils from this region get buried in the sediments in the lagoon very quickly, they are usually very well preserved.
In this particular case, the 300 million year old fish is so well preserved that even the rods and cones in its eyeballs can still be seen very clearly under a scanning electron microscope. This is the first time fossilized photo receptors from a vertebrate eye have ever been found.
Rods and cones in the eyes line the retina. The long and thin rods are more sensitive to light than the cones which are triangular in shape and help us see color. Both rods and cones rely on pigments to absorb light. The scientists studying this fish have found, using chemical analysis, the evidence of one of these pigments — melanin — in the fossilized eye as well.
It is rare for paleontologists to find eye remains, as the soft tissue generally decays within 64 days, the authors of the study said.
“This is the first discovery of vertebrate retinal fossils,” said Gengo Tanaka from Kumamoto University, who co-authored the study in the journal Nature Communications.
With an entire ecosystem having rapidly buried under sediment, the Hamilton Quarry in Kansas is a treasure trove of unusually well-preserved fossils including the extinct fish Acanthodes bridge which is among the oldest known vertebrates with jaws.
It had a long, streamlined body and fins with spines, is believed to have lived in shallow, brackish water, and died out at the end of the Permian period about 250 million years ago when nearly 90 percent of species disappeared in the largest extinction in Earth’s history.
The remains of the fish had been preserved under a thin coating of phosphate, told Tanaka. An analysis of the tissue “provides the first record of mineralised rods and cones in a fossil,” said the study.
These, combined with light-absorbing melanin pigments, suggested the fish was “probably” able to see in low light using highly sensitive rod cells, and by day using cone cells.
This is one of the oldest direct evidence of color sensitive receptors though scientists believe that vision has existed for at least 520 million years