Fossil eye pigments reveal ancient differences in fish vision
A comparison of living and fossil fishes found distinct shapes in the pigment structures of their eyes. The peer-reviewed study links those differences to visual adaptations across major fish groups. Fossils around 306–311 million years old preserve some of the patterns, while interpretations of their function depend on comparisons with living species.
Science··Evening
Pigment shapes differ between fish groups
Ancient fish eyes retained microscopic structures that can help trace the evolution of vision. A peer-reviewed comparison of living fishes and fossils found that eye pigments follow distinct patterns across major fish groups. The structures are melanosomes, organelles that store the pigment melanin inside cells and sometimes survive fossilisation. Their shape and location provide evidence about tissues that have otherwise disappeared.[1], [2]
Bony fishes had cylindrical as well as approximately spherical melanosomes in their eyes. Only the near-spherical form appeared in the eyes of the jawless and cartilaginous fishes examined. The researchers associate these differences with strategies for regulating light, protecting light-sensitive cells and maintaining visual sensitivity. Internal tissues showed more similar melanosome shapes across the groups.[1]
Living fishes anchor the comparison
Electron-microscope measurements covered the size and shape of melanosomes in eyes, skin and internal tissues. The living-fish sample contained 36 specimens from 12 species, providing 432 tissue samples. Larger and more abundant melanosomes in lungfish were interpreted in connection with adaptations to life spanning aquatic and terrestrial environments. These functional explanations draw on comparisons among living organisms.[1]
Sharks and rays provide a contrasting light-regulation strategy: rounded pigment structures allow light to reach reflective crystals behind the retina, while pupil changes adjust light entry. In bony fishes, elongated melanosomes are associated with slower adjustment within the retina.[1], [2]
Carboniferous fossils preserve part of the pattern
The fossil material came from the Mazon Creek biota, about 306–311 million years old. Three of the initial 20 fossils lacked enough preserved material for measurement. Some lineage-specific eye patterns remained visible in the other specimens. Comparing these with modern fishes suggests that the distinctions may reach back to the emergence of major fish lineages about 445 million years ago. That older date is an evolutionary inference rather than a direct measurement of eyes of that age. Preservation and the species sampled constrain how far the comparison can reconstruct ancient visual function.[1]
