Abstract
A feather from the Eocene Messel Formation, Germany, has been demonstrated to have been originally structurally colored by densely packed sheets of melanosomes similar to modern iridescent feathers exhibiting thin-film diffraction. The fossil itself currently exhibits a silvery sheen, but the mechanism for generating this optical effect was not fully understood. Here we use scanning electron microscopy, electron probe microanalysis, and dual-beam focused ion beam scanning electron microscopy to investigate the source of the silvery sheen that occurs in the apical feather barbules. Focused ion beam scanning electron microscopy provides a powerful tool for studying three-dimensionality of nanostructures in fossils. Use of the method reveals that the flattened apical barbules are preserved almost perfectly, including smooth structural melanosome sheets on the obverse surface of the fossil feather that are identical to those that cause iridescence in modern bird feathers. Most of each apical barbule is preserved beneath a thin layer of sediment. The silvery sheen is generated by incoherent light diffraction between this sediment layer and melanosomes and, although related to the original iridescence of the feather, is not a feature of the feather itself. The reddish and greenish hues frequently exhibited by fossil feathers from the Messel Formation appear to be due to precipitates on the surface of individual melanosomes.
Kurzfassung
Bei einer Feder aus der eozänen Messel Formation von Deutschland wurde eine originale strukturelle Färbung aus dicht gepackten Melanosomen nachgewiesen, die modernen irridisierenden Federn ähnlich ist, die Dünnschichtdiffraktion zeigen. Das Fossil selbst zeigt in seinem derzeitigen Zustand einen silbrigen Glanz, aber es ist unklar, durch welchen Mechanismus dieser entsteht. Mit Hilfe von Rasterelektronenmikroskopie, Elektronenstrahlmikroanalyse und doppelt fokussierter Ionenfeinstrahlmikroskopie untersuchen wir die Herkunft der glänzenden Färbung, die an den apikalen Hakenstrahlen auftritt. Ionenfeinstrahlmikroskopie stellt eine leistungsfähige Methode zur Untersuchung dreidimensionaler Nanostrukturen in Fossilien dar. Ergebnisse dieser Untersuchungsmethode zeigen, dass sowohl die abgeflachten, apikalen Hakenstrahlen, als auch die einzelnen Schichten aus Strukturmelanosomen, die mit denen heutiger Vogelfedern nahezu identisch sind, annähernd unversehrt überliefert sind. Ein Großteil der apikalen Hakenstrahlen ist unter einer dünnen Sedimentschicht erhalten, die aufgrund der Interferenz mit den Melanosomen zu dem silber glänzenden Farbeffekt führt. Dieser Effekt wird durch die irridisierende Färbung der Feder verstärkt, trat allerdings nicht im Originalzustand auf. Dies deutet darauf hin, dass die rötliche und grünliche Färbung, die typisch für Federn aus Messel ist, aufgrund von Ablagerungen auf der Oberfläche der einzelnen Melanosome zustande kommt.
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Acknowledgments
We thank Evira Brahm and Stephan Schaal from the Senckenberg Research Institute and Natural History Museum Department for Messel Research, and Gerald Mayr of the Senckenberg Department of Ornithology, for access to this exceptional specimen and for permission to carry out the destructive sampling that was necessary for this research. We thank Jim Eckert and Zhenting Jiang (Yale) for technical assistance. Martin Wagener and Hubert Schulz (Zeiss) facilitated analyses at the Zeiss Electron Microscopy Demonstration Center in Oberkochen, Germany. We thank Michael Wuttke and an anonymous reviewer for comments that improved the manuscript, and Stephan Lautenschlager for his translation of the abstract. We acknowledge the Virginia Tech Institute of Critical Technology and Applied Science for providing access to analytical facilities and resources to James Schiffbauer. The research was supported by NSF EAR 0720062, and a National Geographic Scientific Research Grant to Richard Prum.
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Vitek, N.S., Vinther, J., Schiffbauer, J.D. et al. Exceptional three-dimensional preservation and coloration of an originally iridescent fossil feather from the Middle Eocene Messel Oil Shale. Paläontol Z 87, 493–503 (2013). https://doi.org/10.1007/s12542-013-0173-5
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DOI: https://doi.org/10.1007/s12542-013-0173-5