Monday, March 28, 2011
New ways of looking at fossils
“You can see a lot just by looking.” --Yogi Berra
by George Wilkinson
The formation of a large body fossil is a complicated process involving rapid burial of the remains and chemical and physical interaction of the body with the forming rock bed. The final, discovered, fossil contains an amalgamation of chemical signatures of the original creature and of the bedrock in which it is embedded. Recent application of imaging methods derived from analytical chemistry has accentuated the composite nature of these fossil objects. If the fossilization process preserves the analyte in question, these methods can reveal structures that are not apparent in visible light, show the distribution of trace metals or biogenic compounds-- and of course, a positive result reflects on the fossilization process itself. These palaeometric methods further allow the team to map fossils non-destructively, which means they can take a fresh look at even precious or fragile specimens.
Ultraviolet light has been used for analysis of organic compounds and microscopic fossils for some time. Fossils from some rock beds will fluoresce under UV illumination, yielding a much greater contrast with the surrounding rock compared to visible wavelengths. Improvements in the ultraviolet illumination and in fluorescence detection have allowed the use of UV light to detect otherwise hidden features of fossils, including traces of soft tissues. In the example at the link, ultraviolet light imaging of a feathered non-avian dinosaur fossil was able to show preserved attachments between flight like feathers and the legs, raising the possibility that this creature glided using all four limbs. A great profile of Helmut Tischlinger, the scientist behind many ultraviolet spectrum images, is here.
Another of my favorite recent examples used synchrotron generated X-ray imaging to confirm that the fossilized impression of Archeopteryx feathers contains chemical residue of the feathers themselves. The outlines of the feathers were previously simply conceived of as deformations of the rock matrix, but these areas in fact have residual chemical signatures consistent with known composition of modern bird feathers. The shafts of the feathers show readily detectable phosphorous and iron signatures.
Finally, infrared imaging can reveal the presence of amides and thiols, remnants of proteins, within well preserved samples. Fossilized reptilian skin, but not fossilized leaves from the same rock bed, shows characteristic infrared absorption peaks—as does skin from modern amphibians.
The attractiveness of this body of work for me comes from both its analytical advances and its ability to render the resultant data in pictorial form. The idea of unsuspected information hiding in plain sight is intrinsically glamorous, whether in a detective show or a Dan Brown . Beyond that, the heavy presence of metaphors for sight and seeing in the language of discovery touches on an important feature of the human intellect.
Figure: Synchrotron rapid scanning X-ray fluorescence (SRS-XRF) map of the phosphorous distribution in an Archaeopteryx fossil. This map shows the splay of the rachises from the flight feathers (Blue Arrows) and the reconstructed areas (Yellow Arrows). Open access image from Bergmann et al. (see reference below).
Bergmann, U., Morton, R. W., Manning, P. L., Sellers, W. I., Farrar, S., Huntley, K. G., Wogelius, R. A. and Larson, P. Archaeopteryx feathers and bone chemistry fully revealed via synchrotron imaging. Proc Natl Acad Sci U S A 107, 9060-5.
Edwards, N. P., Barden, H. E., van Dongen, B. E., Manning, P. L., Larson, P. L., Bergmann, U., Sellers, W. I. and Wogelius, R. A. Infrared mapping resolves soft tissue preservation in 50 million year-old reptile skin. Proc Biol Sci.
Hone, D. W., Tischlinger, H., Xu, X. and Zhang, F. The extent of the preserved feathers on the four-winged dinosaur Microraptor gui under ultraviolet light. PLoS One 5, e9223.
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