Seeing the light
Palaeontologists can now look inside fossils without damaging them
UNTIL recently, techniques for studying the quick and the dead evolved together. Biologists started by cutting things and looking inside them. So did palaeontologists (though they did their “cutting” with grindstones rather than scalpels). Then X-rays were discovered. These penetrate stone, as well as flesh, so fossil-lovers took to them with enthusiasm. Thus was the field of palaeoradiology born. X-rays evolved into computerised tomography (CT scanning)—a boon to doctors and palaeontologists alike. But now things have advanced further still. These days fossil-scientists at the cutting edge, so to speak, are using machines called synchrotrons to look inside their specimens.
CT scanning has achieved a lot. It works by using X-rays to cut virtual layers through a specimen (indeed, the field is sometimes known as virtual palaeontology) and then adds those layers together to produce a three-dimensional image. But a synchrotron can do far more. Synchrotrons are machines that accelerate subatomic particles (in this case electrons) in a circle. That produces X-rays so powerful that they knock the socks off those from a CT scanner. And because the beam is powerful, not all of it need be used. Instead, a single wavelength can be selected to produce the X-ray equivalent of a pure-colour beam of light. That allows more precise pictures to be taken, as Paul Tafforeau, a palaeontologist at the European Synchrotron Radiation Facility (ESRF) in Grenoble, has just shown. He has created a remarkable series of images of previously invisible invertebrate fossils preserved in amber.
Slices of life
Dr Tafforeau first saw the light, as it were, in 2000, when he found himself unhappy at having to cut up the teeth of fossil primates in order to study the fine details of their enamel. This led him to ESRF, where he has since worked on “every kind of fossil you can imagine”. He has made images of algae, of the bones of embryonic dinosaurs hidden inside petrified eggs, of the jaws of Neanderthal man and even of a tiny 580m-year-old fossil embryo from China, preserved after only a few cellular divisions, and one of the earliest known examples of multicellular life. In 2006 he pictured the Toumai skull, from a fossil ape thought to have lived about 7m years ago and reckoned to be the oldest known ancestor of humans. He and his colleagues collected 53 gigabytes of data (enough to fill 75 CDs) from this specimen alone.
It is amber, though, that has been his most recent venture. And in only a few days of scanning he has revealed hundreds of new fossils for the first time.
Amber is fossilised tree-resin. When that resin first leaked, it often trapped insects, spiders and other small creatures, which are rarely preserved by other kinds of fossilisation. Amber fossils are thus crucial to an understanding of how insects and other invertebrate critters have evolved. The amber which Dr Tafforeau studied comes from Charentes and dates from the Cretaceous period, the last in which the dinosaurs flourished and also the time when flowering plants first appear in the fossil record. Charentes amber is thus an important archive. Unfortunately, 80% of it is opaque—at least to light. But X-rays can see through it, as Dr Tafforeau has demonstrated brilliantly.
In collaboration with Malvina Lak, a research student, and a group of colleagues from the University of Rennes, he took pictures of 640 pieces of amber from Charentes, and found 356 tiny fossils, including fragments of plants as well as wasps (pictured above), flies, ants and spiders.
The blocks of amber were run past the synchrotron twice. The first run was a rough and ready two-dimensional screening designed to work out which pieces contained interesting inclusions. The second studied the fossils in detailed slices taken while the sample was rotated. From this, the researchers extracted a “virtual insect” that can be viewed on screen from any direction. It can even be sent to one of the new generation of so-called 3D printers, which produce scale models in plastic, layer by layer.
That is important, for when a scientist discovers a new species, the original specimen (or holotype) on which his description is based has to be deposited in a museum, so that other researchers can examine it. In the case of an insect buried in a cloudy piece of amber, this plastic printout can be deposited, alongside the otherwise invisible original.
While Dr Tafforeau has focused on the insects, Phil Donoghue, a geologist at Bristol University, has been using another synchrotron (at the Paul Scherrer Institute in Villigen, Switzerland) to look at fossil plants. In doing so, he hopes to solve what Darwin himself referred to as an “abominable mystery”: the evolutionary origin of flowering plants.
Dr Donoghue observes that a great deal of what is known as molecular phylogeny has been applied to this problem, but without success. (Molecular phylogeny uses genes to try and work out evolutionary relationships.) Synchrotron images mean that the tried and trusted methods of comparative anatomy, with which Darwin would have been familiar, can be brought back into play, since X-rays can reveal minute details of the reproductive organs of fossil plants, right down to individual cells and pollen grains.
A side effect of all these digital data is that they encourage the sharing of specimens, albeit virtual ones. The University of Texas's digital library, known as Digimorph, for example, contains hundreds of specimens and nearly a terabyte of data. Virtual palaeontology thus promises to shake up a profession that once jealously hoarded items that were too precious or fragile to share, or were too rare to sacrifice to destructive techniques. It may even allow the study of valuable specimens that are held by private collectors. So, as the tools multiply to make the invisible visible, an entire world of evidence is available without anyone having to lift a geological hammer.