USING TAPHONOMIC EXPERIMENTS TO DISTINGUISH MICROBIAL GROWTH FROM MELANOSOMES IN EXTANT AND FOSSIL FEATHERS

Elongate microbodies associated with fossil feathers were originally attributed to microbial biofilms. Microbes, as well as the exoplymeric substance they secrete, are known to persist in the fossil record. However, recent papers have reinterpreted these features as intracellular, melanin-containing organelles (melanosomes). Based upon these interpretations, coloration in non-avian and avian dinosaurs has been hypothesized. While melanin is highly resistant to degradation and has high preservation potential, the intracellular organelles storing the pigment have not been shown to be equally resistant. Moreover, as organelles, melanosomes cannot be microbially derived, although melanin is a common microbial pigment. Because melanosomes and microbes overlap in size and morphology, morphological data are insufficient to robustly support either claim. Some FTIR data have been put forth. Here, we re-evaluate the source of these microbodies using taphonomic experiments and electron microscopy.

We tested the hypothesis that microbial growth, as a participant in feather degradation, is distinguishable from melanosomes. Additionally, because the beta-keratin matrix of feathers is highly resistant to degradation, claims for the presence of melanosomes should also support the presence of molecular keratin, identifiable by immunohistochemical methods. Extant Gallus gallus (chicken) feathers were degraded in environmental (Neuse River) mud over six weeks and compared with feathers exposed to a pure culture of Bacillus cereusfor 4 days. Melanosomes are sparse and non-overlapping and limited to internal regions surrounded by and embedded in the keratinous matrix of the feather. Conversely, microbes grow across the surface of feathers in dense, over-lapping aggregations. A black film formed in the mud sediment surrounding the feather rachis.

Fossil feathers have been found in diverse depositional environments, and a correlation between environment and modes of preservation have been proposed. The most common preservational mode reported for fossil feathers is as black carbonaceous films. This is the first attempt to mimic this process in a laboratory setting. The natural degradation processes of feathers must first be understood before the preservation processes can be addressed in fossils.