The evolution of the feather: Sinosauropteryx, a colourful tail

Abstract

A recent development in the identification of feathers in fossils by means of melanosomes was used to suggest that structures observed in an SEM of a filament in the basal theropod dinosaur, Sinosauropteryx, were phaeomelanosomes and that they represented conclusive evidence that the filaments were early feathers. At the most basic level, the claims of phaeomelanosomes are shown here to be founded on an optical illusion created when the SEM is reproduced at low image size—viewed at larger image size (~2× original) the structures are nondescript in both size and shape and impossible to equate with phaeomelanosomes. At a higher level of investigation, the study is seriously questioned for ignoring standard scientific protocol: despite size and shape being critical to the identification of the phaeomelanosomes, no statistically viable measurements of the structures (particles) were made—the measurements, which are simply conjectured, are shown here to be incorrect in the speculated sizes, and in shapes; inferences made on vital characters from birds and advanced non-avian dinosaurs, e.g. with respect to colour banding, are without confirmation in the test animal but conjectured on circular argumentation; alternative arguments, e.g. that the particles might be bacteria or colour from the overlying skin, are peremptorily dismissed or not considered; suggestions that the particles are embedded within the filament are without support since there is no evidence of cross-sections or tangential sections either made or occurring serendipitously—only a single section is reported, apparently of the filament’s surface. False dichotomies such as, if the structures are not bacteria they must be melanosomes, are questioned given that one of the most important factors in the taphonomy of ancient (structures in question, ~130 MYR) fossilised filaments i.e., decomposition—that the structures might reasonably represent the degraded remains of the filaments—is not even considered. Here, from experiments on the decomposition of native collagen in fish and reptilian dermis, SEMs of their ultrastructure show that distinctive spherical, elliptical or oblate particles, even more so than those figured in Sinosauropteryx, typically form during degradation. This is confirmed in SEMs of degraded collagen fibres in a 225-MYR ichthyosaur fossil, virtually point by point. In addition numerous small bead-like structures in the filament of Sinosauropteryx bear a striking resemblance to the unique 67-nm D-bands of collagen, in both shape and size. This paper does not question the value of scientifically meritorious identifications of melanosomes, as indeed of collagen and keratin, in interpreting the integumental structures of fossil animals. However, allegations of phaeomelanosomes in Sinosauropteryx are shown to be without scientific merit.