Beneath the surface

Fossils are a valuable insight into the evolutionary past, although the quality of their preservation can be highly variable. But focusing chiefly on the specimens that look nice means we might have overlooked a valuable source of material to study. Thomas van de Kamp, from the Karlsruhe Institute of Technology wondered whether a synchrotron X-ray beam could enable him and his team to see inside what are called mineralised specimens of insects, which are often dismissed as being of much lower scientific value. The results, from a fossil collection that hadn't seen the light of day since the 1940s, are impressive...

Thomas - Our groups worked quite a lot on the fossil insects also before but until then we scanned amber specimens. So it’s like inclusions in fossil tree resin where you can see the insects from the exterior quite nicely. But these insects were mineralised fossils. Three-dimensional insect that looked much worse than these nice looking amber fossils from the exterior and there were x-ray methods we employ here. We wanted to have a look inside. We wanted to create virtual three-dimensional images and cut virtually through their fossils to see if there is something inside.

Chris - In other words, in these insect specimens, the tissues have been almost replaced with the minerals, making them effectively into a stone replica of the insect as it once was.

Thomas - Yes, this is true. So parts of the exoskeleton of the original insects were replaced by minerals and parts were replaced by air-filled spaces. While most of the soft tissue parts were replaced by minerals.

Chris - How big were the insects we’re talking about?

Thomas - They’re mediocre in size. So, our beetles, we scanned 8 specimens. All of them were around 3 millimetres in length and 2 millimetres in width and in height.

Chris - Thomas, how did you do this? How did you image these tiny, effectively grains of stone to work out what the anatomy was?

Thomas - So first, start with a more or less standard micro CT setup at a synchrotron light source so we’re producing intense x-rays. We place our objects inside a sample holder on a rotary stage. Then we radiate the specimen with x-rays and during the scan, we rotate the sample for 180 degrees. During the rotation, we acquire a few thousand, I think, 3,000 images in this case, of the specimen. After the scan, we reconstruct a three-dimensional volume from the 3,000 projections we took.

Chris - What do you compare them to? Do you have any beetles that have not been mineralised in order to ask how do they hold up to the same interrogation and so you can work out whether there's any effect of the mineralisation on the actual imaging you were able to achieve?

Thomas - Yes. What we did for this study was we compared the extinct species with close related species that still is alive nowadays in Europe. So we have roughly 30 million Europe beetle and we compared it with our existing beetle species using the same tomographic setup.

Chris - Does this turn out to be a valid way to study these fossils?

Thomas - Yeah, definitely. What we also did is we scanned two specimens of the existing species - one that was fixed in ethanol where we could see all the organs in the more or less natural state and then with the dried museum specimen. We really can conclude that the preservation of the fossilised specimen is better than the preservation of the air dried specimen with respect to the soft tissues. So from this comparison, we can conclude that the fossilisation had to be very fast because an air dried specimen from the museum is really degraded so you cannot see much of their soft tissue anymore while in the fossil, you can see some glands, the genitals, almost like in the alcohol fixed specimen.

Chris - Does the fact that you’ve solved this for this particular species of beetle, does this mean you can now extrapolate this technique to other species? It means you can unlock a lot more fossil information – not just about these beetles, but many other types of fossilised insect which could perhaps be lurking in collections of specimen museums totally unobserved and unstudied because people thought that there was a limit to the data they're going to get from them.

Thomas - Yeah, that's exactly what we wanted to show with this study because now, we really see that there are thousands of neglected fossils inside museum collections. You can see that on this collection, we investigated was more or less dormant in the museum of Brazil for over 70 years. The exterior shape was studied in 1944 the last time and with our fast setup, we are able to screen them in a very short time, so effectively, we need 16 seconds for a scan, 3 minutes if we include all the coping. So let’s say, 3 minutes per scan. So you can imagine, we can really image a lot of samples from the collections in a short time to get a better idea about evolutionary history of insects.