Melanosome evolution shaped sensory biology and terrestrial adaptations in early aquatic vertebrates.

Eye melanosome morphology in early aquatic vertebrates is lineage-specific, with cylindrical bony-fish melanosomes and spheroidal jawless/cartilaginous melanosomes preserved in Mazon Creek fossils.

• Why it matters: Tracing how fish sensory and ecological adaptations evolved through deep time is difficult, and melanosomes can fossilise while reflecting physiology.
• What they did: A comparative analysis of melanosomes from internal tissues of extant fishes and from late Carboniferous Mazon Creek fossils was used to infer ecological and physiological adaptation links.
• The result: The preserved taxonomic differences indicate that melanosome morphology–function links were established by the Ordovician, approximately 445 million years ago, enabling vertebrate sensory evolution and ecological diversification.