Snapshot Issue 33 April 2007
Tyrannosaurus rex and collagen
Fossils are old because they are made out of stone. Until recently, as far as science was concerned, organic tissues – such as bone matrix – had little chance to survive the passage of time. Organic soft tissues – such as cells and blood vessels for instance – had almost no chance at all. Two years ago though, a 68 million year-old Tyrannosaurus rex was unearthed from one thousand cubic metres of sandstone. The mineral from the bone of one of its femurs was removed and, to the scientists’ astonishment, they found minute traces of organic soft tissue which had survived millions of years.
Since collagen is the main organic tissue which makes up bone, the researchers set out to find some. And they did, thanks to biomolecular techniques for extraction and analysis that have been perfected over the years. A number of collagen peptide sequences were identified and, when compared with known sequences, they turned out to be similar to those of chicken collagen – which came as a relief since birds and dinosaurs are believed to be directly related according to phylogenetic studies. Unfortunately, no comparisons could be made with closely related existing species such as alligators and crocodiles, since their collagen sequences are still not available.
Organic tissue has been known to last thousands of years but was not expected to last over a million. Moreover, organic soft tissues were expected to persist even less time since they are the first to suffer degradation by bacterial enzymes once an organism has died. However, it is less of a surprise to discover protein fragments since they are far more resistant than DNA.
The fact that this dinosaur soft tissue has lasted so long brings controversy to the existing theories of the fossilization process. So far, theoretical extrapolations had been made on how long it took organic tissue to be replaced by mineral. However, fossilization depends not only on the type of organic tissue involved but also hugely on the chemical environment in which an organism is buried. In the case of this particular T. rex, scientists suggest that irreversible cross-links between the collagen fragments may well have been initiated by iron components that resulted from the degradation of iron-containing entities such as haemoglobin for instance. Suggesting that the very process of degradation can preserve organic tissue…
Collagen alpha-1(I) chain, Tyrannosaurus rex (Tyrant lizard king): P0C2W2
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