Saturday, 5 April 2014

The Chemistry Of Fossils

Here the fossil is preserved as a carbonaceous smudge on the rock,
carbon derived in part from the original creature; but to what extent?
Fossilisation turns living flesh to stone, tissues into minerals and cells into crystals. Compounds from the surrounding environment seep into the corpse preserving it for potentially billions of years, and molecules which once made up the body, are replaced by those from the non-living world. Yet to what extent does transformation take place?

Some fossils are completely replaced. I have ammonites which have transformed into calcite petrifactions; mineral facsimiles of the original creature. Others are rather different. A small amphibian fossil is preserved as a dark smudge, contrasting beautifully against the beige shale on which it sits.

The reason for this coloration is the high carbon content making up the fossil: carbon derived from the proteins and lipids which once made up the amphibian's body. Recent fossils, including mammoth bones and hair have the majority of their original molecules, albeit altered by hundreds of thousands of years of burial in the ground. The spectrum regarding how much a fossil is replaced by other compounds is vast. Yet recent studies are beginning to tap into this new field of research, to reveal secrets which were previously unobtainable through simple physical examination.

Fossils from the 50 million year old Green River formation are remarkably well preserved, but nobody had thought to examine their chemical composition. Researchers from Britain's University of Manchester, Diamond Light Source and the Stanford Synchrotron Radiation Lightsource in the US, used synchrotron radiation to conduct an x-ray spectroscopic analysis of a number of plant fossils, including relatives of modern day palms. This built up a chemical map.

The elemental (metallome) map of the Green River fossil
P. wyomingensis. The above image includes optical copper,
zinc and nickel tracer views and a composite map on the
bottom right. The orange indicates the presence of copper.
The results were astonishing. By comparing them to chemical maps of similar living leaf the team was able to show that the majority of the elements making up the fossil were derived from the original leaf. The major portion of the chemical map was copper, derived from the electron transfer complex and respiratory enzymes within the cells.

'In one beautiful specimen, the leaf has been partially eaten by prehistoric caterpillars, just as modern caterpillars feed, and their feeding tubes are preserved on the leaf,' said Professor Roy Wogelius from Manchester. 'The chemistry of these fossil tubes remarkably still matches that of the leaf on which the caterpillars fed.'

'We think that copper may have aided preservation by acting as a 'natural' biocide, slowing down the usual microbial breakdown that would destroy delicate leaf tissues,' added  Dr Phil Manning from Manchester. 'This property of copper is used today in the same wood preservatives that you paint on your garden fence before winter approaches.' Indeed it is only recently that the biocide properties of copper have become apparent. A study has shown that if all the door handles in hospitals are replaced with solid copper versions, the rate of germ transfer drops radically.

A close up of the metallome map which helps display previously
invisible structures, including the micro-venation on the leaf
What is more, it is possible that transition group metals may aid the preservation of molecular signatures. A team led by Dr Mary Schweitzer found that DNA and proteins had been preserved within the bones of a specimen of Tyrannosaurus rex. Analysis revealed that the iron from the red blood cells had helped stabilise the molecular material to the extent that Schweitzer and her team could reverse engineer both the protein and the DNA to find its genetic code and expressed amino acids respectively.

'The synchrotron has already shown its potential in teasing new information from fossils, in particular our group's previous work on pigmentation in fossil animals,' said Dr Nicholas Edwards from Manchester. 'With this study, we wanted to use the same techniques to see whether we could extract a similar level of biochemical information from a completely different part of the tree of life.' We have a myriad of techniques to examine the surface and even the internals of fossils. Being able to extract information from their chemical make-up is an exciting addition to palaeontology.