Synchrotron-based chemical imaging reveals plumage patterns in Archaeopteryx
A key collaborator on this project was Dr. Daniela Schwartz-Wings (Curator at the Museum für Naturkunde, Berlin), who kindly loaned two of the priceless Archaeopteryx fossils scanned in the course of this study.
The accepted paper addresses some serious issues relating to the recently published paper by Li et al (2012) in Science (353, 1215-1219). The morphological variation observed in the pigment generating organelle, the melanosome, in living species is now being used to predict the colour of fossilised organisms. However, because it relies on point sampling, it can only do this at discrete locations on a fossil and therefore cannot currently provide a complete picture of colour or patternation. The approach taken by our team focused on chemistry, using synchrotron rapid scanning x-ray fluorescence imaging (SRS-XRF) and this is presented in our paper to chemically resolve and map pigment biomarkers over whole specimens of one of the rarest fossils in the world, one of the first fossil birds Archaeopteryx. This technique can be used to fill in the vast gaps in the prior structural approaches and provides a complete picture of an organism.
The distribution of copper, nickel, and organic sulphur in two Archaeopteryx specimens (MB.Av.100 and HMN 1880) are strongly controlled by feather structure, but only lighter elements (P & S) are comparable with an additional third Archaeopteryx specimen (the third being WDC CSG100). Our work further elevates the significance of MB.Av.100 among all known Archaeopteryx specimens, given its preservation affords a more compete view of the elemental inventory for this iconic taxa. It is important to note that the subtle variation in pigment concentration and distribution can only be spatially resolved using SRS-XRF mapping, coupled with an understanding of the oxidation state of sulphur present that was determined using XANES spectroscopy. Our study shows how chemical methods augment structural methods (Zhang et al 2010; Li et al 2010; Carney et al 2011; Carney et al 2012) for diagnosing coloration.
This study shows that the distribution of trace-metals and organic species of sulphur in Archaeopteryx can be used to predict the complete feather pigment pattern and show that the distal tips and outer vanes of feathers were more heavily pigmented than inner vanes, contrary to recent studies (Carney et al 2012). This pigment adaptation might well have impacted upon the structural and mechanical properties of early feathers, steering plumage evolution in Archaeopteryx and other feathered theropod dinosaurs….this is a BIG leap forward in terms of our understanding of the evolution of plumage.
This study crucially shows that our non-destructive chemical approach should augment structural methods when diagnosing pigment density and distribution in the fossil record. The quantitative synchrotron-based techniques used by our current study will undoubtedly prove valuable in distinguishing between endogenous and exogenous components for a range of studies involving paleontological and archaeological samples, biological tissues, and forensic investigations. The article is entitled: Synchrotron-based chemical imaging reveals plumage patterns in a 150 million year old early bird. J. Anal. At. Spectrom., DOI:10.1039/C3JA50077B.