Chemical, experimental, and morphological evidence for diagenetically altered melanin in exceptionally preserved fossils
Caitlin Colleary a,b, Andrei Dolocan c, James Gardner d, Suresh Singh a, Michael Wuttke e, Renate Rabenstein f, Jörg Habersetzer f, Stephan Schaal f, Mulugeta Feseha g, Matthew Clemens h, Bonnie F. Jacobs h, Ellen D. Currano i, Louis L. Jacobs h, Rene Lyng Sylvestersen j, Sarah E. Gabbott k, and Jakob Vinther a,d,l,1
aSchool of Earth Sciences, University of Bristol, Bristol BS8 1RJ, United Kingdom;
bDepartment of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060;
cTexas Materials Institute, University of Texas at Austin, Austin, TX 78712;
dJackson School of Geosciences, University of Texas at Austin, Austin, TX 78712;
eDepartment for the Conservation of the Cultural Heritage of Rhineland-Palatinate, 55116 Mainz, Germany;
fDepartment of Palaeoanthropology and Messel Research, Senckenberg Research Institute, 60325 Frankfurt am Main, Germany;
gPaleoanthropology and Paleoenvironment Program, School of Earth Sciences, College of Natural Sciences, Addis Ababa University, Addis Ababa, Ethiopia;
hRoy M. Huffington Department of Earth Sciences, Southern Methodist University, Dallas, TX 75275;
iDepartments of Botany and Geology & Geophysics, University of Wyoming, Laramie, WY 82071;
jDivision of Natural History, Muse®um, 7800 Skive, Denmark;
kDepartment of Geology, University of Leicester, Leicester LE1 7RH, United Kingdom;
lSchool of Biological Sciences, University of Bristol, Bristol BS8 1TQ, United Kingdom
Edited by Donald E. Canfield, Institute of Biology and Nordic Center for Earth Evolution, University of Southern Denmark, Odense M., Denmark, and approved August 26, 2015 (received for review May 19, 2015)
Significance
Melanin is a widespread pigment that provides black to reddish brown hues to organisms. Recent evidence has shown that melanin is retained in exceptionally preserved fossils, including feathered dinosaurs, allowing the reconstruction of ancient color patterns. However, little is known about the chemical preservation of melanin or its distribution in the fossil record. Here, we show that melanin is preserved in a number of soft-bodied fossils, but its burial under high pressure and temperature for millions of years alters its original chemistry. The widespread occurrence of melanin substantiates the applicability of reconstructing aspects of original color patterns and allows us to dismiss the alternative suggestion that these structures are microbial in origin.
Abstract
In living organisms, color patterns, behavior, and ecology are closely linked. Thus, detection of fossil pigments may permit inferences about important aspects of ancient animal ecology and evolution. Melanin-bearing melanosomes were suggested to preserve as organic residues in exceptionally preserved fossils, retaining distinct morphology that is associated with aspects of original color patterns. Nevertheless, these oblong and spherical structures have also been identified as fossilized bacteria. To date, chemical studies have not directly considered the effects of diagenesis on melanin preservation, and how this may influence its identification. Here we use time-of-flight secondary ion mass spectrometry to identify and chemically characterize melanin in a diverse sample of previously unstudied extant and fossil taxa, including fossils with notably different diagenetic histories and geologic ages. We document signatures consistent with melanin preservation in fossils ranging from feathers, to mammals, to amphibians. Using principal component analyses, we characterize putative mixtures of eumelanin and phaeomelanin in both fossil and extant samples. Surprisingly, both extant and fossil amphibians generally exhibit melanosomes with a mixed eumelanin/phaeomelanin composition rather than pure eumelanin, as assumed previously. We argue that experimental maturation of modern melanin samples replicates diagenetic chemical alteration of melanin observed in fossils. This refutes the hypothesis that such fossil microbodies could be bacteria, and demonstrates that melanin is widely responsible for the organic soft tissue outlines in vertebrates found at exceptional fossil localities, thus allowing for the reconstruction of certain aspects of original pigment patterns.
paleocolor melanosome mass spectrometry diagenesis pigmentation
Footnotes
1To whom correspondence should be addressed. Email: jakob.vinther{at}bristol.ac.uk.
Author contributions: J.V. designed research; C.C., A.D., and J.V. performed research; A.D., J.G., M.W., R.R., J.H., S. Schaal, M.F., M.C., B.F.J., E.D.C., L.L.J., R.L.S., and S.E.G. contributed new reagents/analytic tools; C.C., A.D., S. Singh, and J.V. analyzed data; and C.C., A.D., and J.V. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1509831112/-/DCSupplemental.
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