Melanosomes or Microbes: Testing an Alternative Hypothesis for the Origin of Microbodies in Fossil Feathers

Abstract

Microbodies associated with fossil feathers, originally attributed to microbial biofilm, have been reinterpreted as melanosomes: pigment-containing, eukaryotic organelles. This interpretation generated hypotheses regarding coloration in non-avian and avian dinosaurs. Because melanosomes and microbes overlap in size, distribution and morphology, we re-evaluate both hypotheses. We compare melanosomes within feathers of extant chickens with patterns induced by microbial overgrowth on the same feathers, using scanning (SEM), field emission (FESEM) and transmission (TEM) electron microscopy. Melanosomes are always internal, embedded in a morphologically distinct keratinous matrix. Conversely, microbes grow across the surface of feathers in continuous layers, more consistent with published images from fossil feathers. We compare our results to both published literature and new data from a fossil feather ascribed to Gansus yumenensis (ANSP 23403). ‘Mouldic impressions’ were observed in association with both the feather and sediment grains, supporting a microbial origin. We propose criteria for distinguishing between these two microbodies.

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

“…[55,64,65]). Most critically, many alleged melanosomes occur only as imprints ('mouldic melanosomes' [53]), a preservation mode that implies preferential degradation of the bodies relative to the surrounding substrate [12]. To complicate matters further, impressions indistinguishable from those attributed to melanosomes are occasionally found also in clay minerals, on silica crystals, and other sedimentary grains associated with, but clearly distinct from, the fossilized tissue structures (e.g.…”

“…Nevertheless, such interpretations are inherently equivocal, and further complicated by the fact that microbes: (1) overlap in size, shape and preservation potential with melanosomes; (2) can produce melanins; and (3) are innately associated with decaying organics [12]. Judicious elimination of alternative hypotheses for the origin of microbodies in fossils is therefore necessary prior to extrapolations of colour or function.…”

“…To complicate matters further, impressions indistinguishable from those attributed to melanosomes are occasionally found also in clay minerals, on silica crystals, and other sedimentary grains associated with, but clearly distinct from, the fossilized tissue structures (e.g. [9,12,65]). …”

“…Since then, a number of investigations have used chemical markers and presumed fossil melanosomes to explore aspects of the biology and ecology of extinct animals, including colour [8,9,[53][54][55][56][57], behaviour [8,56] and physiology [10]. However, studies reporting remnant melanosomes have been met with controversy, and an alternative hypothesis has been put forth favouring a more conservative interpretation of the fossil microbodies as microbes colonizing the degrading tissues prior to burial [12,58]. Such criticism has sparked intense debate (see Edwards et al [11] for review), that is further aggravated by the dearth of unequivocal molecular indicators for ancient melanic pigments, which thus far have only been recorded from cephalopod ink sacs [59,60], a fish 'eye spot' [61] and marine reptile integuments [56].…”

“…This is particularly pertinent when only SEM image data are considered, as these are demonstrably inadequate for discriminating between remnant melanosomes and pervasive bacteria [12]. Differentiation of melanosomes from microorganisms is necessary because they overlap in size, shape and distribution [12][13][14].…”