Interpreting melanin-based coloration through deep time: a critical review

Abstract

Colour, derived primarily from melanin and/or carotenoid pigments, is integral to many aspects of behaviour in living vertebrates, including social signalling, sexual display and crypsis. Thus, identifying biochromes in extinct animals can shed light on the acquisition and evolution of these biological traits. Both eumelanin and melanin-containing cellular organelles (melanosomes) are preserved in fossils, but recognizing traces of ancient melanin-based coloration is fraught with interpretative ambiguity, especially when observations are based on morphological evidence alone. Assigning microbodies (or, more often reported, their ‘mouldic impressions’) as melanosome traces without adequately excluding a bacterial origin is also problematic because microbes are pervasive and intimately involved in organismal degradation. Additionally, some forms synthesize melanin. In this review, we survey both vertebrate and microbial melanization, and explore the conflicts influencing assessment of microbodies preserved in association with ancient animal soft tissues. We discuss the types of data used to interpret fossil melanosomes and evaluate whether these are sufficient for definitive diagnosis. Finally, we outline an integrated morphological and geochemical approach for detecting endogenous pigment remains and associated microstructures in multimillion-year-old fossils.

Chemical, experimental, and morphological evidence for diagenetically altered melanin in exceptionally preserved fossils

“…By combining SEM imaging and TOF-SIMS experiments, we demonstrate that the organic residues in a diverse sample of vertebrates and cephalopods are consistent with the hypothesis that they comprise fossilized, altered melanin. These observations corroborate the hypothesis that the oblong and spheroidal microbodies in these carbonaceous compressions are indeed melanin-bearing melanosomes (1) rather than bacteria (10,11,36). A recent review argued that the ubiquity of bacteria, their supposed propensity to fossilize, and their similarity in size and scale to melanosome microbodies (36) cast doubt on morphology-based identification of fossil melanosomes.…”

Molecular evidence of keratin and melanosomes in feathers of the Early Cretaceous bird Eoconfuciusornis

“…However, melanosomes and microbes overlap completely in size and shape (16)(17)(18), and thus these hypotheses are equally plausible. Furthermore, the majority of the data presented to support a melanosome origin are based not on the morphology of the bodies themselves but rather on their impressions within a structural matrix (9)(10)(11)(12)(13)(14)(15)19) that is presumed, but not demonstrated, to be keratin.…”

FANTASY VS REALITY: A Critique of Smith et al.'s Bird Origins

“…As Lindgren et al [124] showed, "differentiation of melanosomes from microorganisms is necessary because they overlap in size, shape and distribution. Moreover, microbes are always associated with decaying carcasses, and are known to fossilize as both organic and inorganic traces."…”

“…Moreover, microbes are always associated with decaying carcasses, and are known to fossilize as both organic and inorganic traces." Lindgren et al [124] summarize, new and innovative "approaches will ultimately facilitate more rigorous interpretations and reduce the risk of spectacular yet insufficiently supported claims propagating in the literature." Finally, even if melanosomes can be identified, the overall technique is totally deficient.…”