SYNCHROTRON-BASED CHEMICAL IMAGING REVEALS PLUMAGE PATTERNS IN ARCHAEOPTERYX

Feather and integument color depend on both chemical and structural characteristics. The combination of melanosome morphology (structural) and trace metal biomarkers (chemical) has been used to infer color and pigment patterns in a range of extant and fossil organisms. Melanin is the most widely used pigment in birds and consist of several covalently linked indoles. Melanins are considered unusually large polymers compared to most natural pigments. The sheer size and complexity of these molecules determines their precise structure and physical properties and also controls their bonding to other components (e.g. proteins, metal ions). In this study, three key specimens of Archaeopteryx were subjected to non-destructive chemical analysis in order to investigate the potential for pigment preservation in feathers of this early bird. Synchrotron Rapid Scanning X-ray Fluorescence maps are combined with sulfur X-ray Absorption Near Edge Structure spectroscopy to provide the first map of organic sulfur distribution within whole fossils, and demonstrates that organically derived endogenous compounds are present. The distribution of copper and organic species of sulfur in two Archaeopteryx specimens are strongly controlled by feather structure, but only lighter elements (phosphorus, sulfur) are comparable with a third specimen analyzed in this study. The bonding environment of copper is consistent with organic chelation and when combined with the presence of organic sulfur supports that remnant endogenous eumelanin pigments are preserved in the feathers of Archaeopteryx. We argue the uneven preservation of organic sulfur that correlates with copper is a result of the biocidal properties of chelated metals, as seen in extant feathers. Copper is a powerful taphonomic control reducing local breakdown of organic sulfur in the original keratin. This work also suggests that keratin has a higher preservation potential than collagen. The distribution of organometallic compounds can be used to predict the complete pigment pattern and show that the distal tips and outer vanes of feathers were more heavily pigmented than inner vanes. This pigment adaptation might have impacted upon the structural and mechanical properties of early feathers, steering plumage evolution in Archaeopteryx and other feathered theropods.