Abstract
Melanosome (melanin-containing organelles) geometries sampled from fossil Jurassic fur have recently indicated uniformly dark colors consistent with a proposed nocturnal bottleneck early in the mammal lineage. Here, we use a distinct dataset of ~8000 melanosomes from 60 species (11 mammalian orders) including a Jurassic species to ask more generally what color or environmental variables may explain melanosome geometry in mammals. We confirm support for limited melanosome variation in Jurassic mammals. Crown therian mammalian melanosome diversity as extreme as that in birds is recovered within placental lineages. More environmental variables substantially explain aspects of melanosome geometry than do color variables, but the explanatory power of both sets of variables is limited. Prediction of color from melanosomes in fossil mammals merits caution, given observed lineage-specific trends. Potentially comparatively weak selection on the melanosome geometry-color relationship in mammals or correlations with other life history variables may further explain the evolution of these geometries.