New Technique Peeks Inside Opaque Amber

Opaque amber looks like a stone. The naked eye cannot see anything in this material. But, because it is a fossil resin, it can incorporate fossils like any other amber. So far, palaeontologists have found in amber from fossil insects and microbes to small vertebrates (like frogs), feathers, plant organs and pollen.

Now, a team at the University of Rennes (France) and the ESRF has used a synchrotron X-ray imaging technique called propagation phase contrast microradiography, managing to overcome the issue.

The results were spectacular: 356 fossil animals found in 640 pieces of opaque amber (meaning 2 kg (4.4 pounds) of material) from mid-Cretaceous sites of Charentes (France). In Charentes, 80% of the amber is opaque.

"Researchers have tried to study this kind of amber for many years with little or no success. This is the first time that we can actually discover and study the fossils it contains," said co-author Paul Tafforeau, an ESRF paleontologist.

The new fossils went from wasps, ants and flies to spiders and acarians, most of them tiny (like a 0.8 mm acarian or a 4 mm wasp). 53% of the fossils were assigned to families.

"The small size of the organisms is probably due to the fact that bigger animals would be able to escape from the resin before getting stuck, whereas little ones would be captured more easily," explained co-author Malvina Lak from the University of Rennes.

To abolish the surface traits (like cracks) of the amber pieces, which stood out in the images compared to the fossil organisms, the amber pieces were soaked in water, which has a density similar to amber.

When the organisms were spotted on the radiographs, they were 3D imaged and virtually extracted from the amber. The highly detailed 3D reconstructions made possible the study and description of the fossil organisms.

"Opaque amber hosts many aspects of past life on our planet that are still unknown, and the use of third generation synchrotron sources will continue to play an important role in unveiling them," said Lak.