High melanosome diversity exhibits weak correlation with color and environmental variables in the early evolution of therian mammals

TL;DR

The study finds that melanosome diversity in early mammals shows a weak link to color and environment, suggesting limited evolutionary pressure on color traits.

Contribution

The research expands the analysis of melanosome geometry across a large dataset of mammals, revealing limited correlations with color and environmental variables.

Findings

Jurassic mammals show limited melanosome variation, suggesting uniform dark colors.

Environmental variables explain melanosome geometry better than color variables, though both have limited explanatory power.

Lineage-specific trends suggest caution in predicting color from melanosomes in fossils.

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.

Melanosome geometry in Jurassic and therian mammals reflected a limited color/environment link unlike that in dinosaurs.

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