New method tracks down organic molecules inside fossils
9 June 2009, by Sara Coelho
Taking genetic information from fossils is a tantalising idea for palaeontologists. In practice this is difficult because organic molecules are destroyed when bones turn to rock.
Now, a new method shows scientists where to look for preserved organic material inside old bones.
Fossils of bones and teeth are often the only material palaeontologists have to study extinct animals, such as dinosaurs. Bones are made of bioapatite mineral crystals bound within a matrix of collagen protein. As soon as the animal dies, the collagen starts to break down and the bone begins to crumble. But if the skeleton is buried, the space left behind by the collagen can be filled up by new minerals as the bone interacts with chemicals in underground water. If enough mineral grows, the overall structure of the bone is preserved as a fossil.
The replacement of collagen, called recrystallisation, does not happen uniformly or at the same time - its rate and extent depends on how underground water interacts with the bone. Dr Clive Trueman from the National Oceanography Centre, Southampton and colleagues from the United States found a new way to understand how recrystallisation proceeds.
The team used special mass spectrometry equipment to measure the concentration and distribution of the so-called Rare Earth chemical element group - the REEs - inside fossils. Since the REEs do not occur naturally in bones, their presence in the fossil is 'entirely due to the interaction between the bone and groundwater,' says Trueman.
'As soon as the bones come into contact with water, the collagen matrix begins to break down and the REEs start to diffuse into the bone,' he explains.
The team analysed six unidentified large reptile bones from the Cretaceous, the time of the dinosaurs. The analysis, published this month in Geology, shows that REEs can indeed be used to show how recrystallisation works inside bones.
If the REEs have a uniform distribution throughout the bone, this means that recrystallisation was relatively slow and that REEs had enough time to infiltrate all parts of the bone. In this case, it's not very likely to find organic molecules.
But if the REEs are concentrated around the rims or bone cracks, it means that recrystallisation was too fast for elements to diffuse to all parts of the bone. In this scenario, 'it is possible that small amounts of collagen might be preserved, trapped between recrystallised material,' says Trueman.
Bones with unequal distributions of REE fossilised fast and are the ideal candidates to sample for collagen. 'In theory, the collagen amino acid sequence could be used, for instance, to get phylogenetic information and test evolutionary relationships between organisms,' Trueman says. 'Not,' he adds, 'to recover DNA from fossil bones.'
The technique has other uses beyond sampling for organic material in dinosaur bones. The relative abundance and type of the REEs found (there are 15) are a 'link between the bone and its original burying ground,' says Trueman, because each site has a unique groundwater chemistry. The REE pattern in the bone works like a 'fingerprint' and can be used to track down illegal fossils collected from protected places.
Alan E. Koenig, Raymond R. Rogers, and Clive N. Trueman. Visualizing fossilization using laser ablation-inductively coupled plasma-mass spectrometry maps of trace elements in Late Cretaceous bones. Geology, Jun 2009; 37: 511 - 514.
Interesting? Spread the word using the 'share' menu on the top right.