Evolutionary constraints on anuran melanin for 45 million years

Fossil evidence of melanin is a remarkable palaeobiological resource, but the evolution of melanin through deep time is poorly resolved, partly because few studies have integrated data from fossils and their extant relatives. Here, we use data on melanin-rich organelles - melanosomes - in extant and fossil anurans to show that the evolution of melanin in the skin during the last 45 million years is decoupled from that in the eyes and internal tissues. Skin melanosomes differ in geometry (i.e., length, width, and aspect ratio) between extant and fossil species. In contrast, melanosomes from the eyes and internal tissues show strongly conserved geometries in all fossil and extant species. These tissue-level trends in melanosome evolution are probably controlled by physiological constraints on melanosome functions such as photoprotection, visual signaling, thermoregulation, and metal homeostasis, that have been in place at least since the Eocene. Skin melanosomes are not subject to such constraints on geometry, which may reflect evolutionary tradeoffs linked to the diversification of integumentary coloration and/or habitat.