April 1, 2016

Elsa Panciroli

Scanning is leading to huge breakthroughs. For example, we’ve now found the world’s oldest chameleons and know why giant wombats were air-heads

Palaeontologists seem to be CT-scanning everything these days. A paper published in March by an international team led by Juan D. Daza, used scans to get a better look at fossil lizards encased in 99 million year old amber. These tropical lizards from Myanmar can be seen in their orange, glass-like tombs with the naked eye, but not in great detail. Using CT-scanning meant scientists were able to examine not only the scaled skin of these fossils, but the internal bones and soft tissues, allowing them to identify five major groups of reptile. This includes the oldest chameleon ever found, complete with the long projectile tongue so characteristic of their feeding. This pushes the palaeontological record for chameleons back an extra 80 million years – an astonishing discovery from these little glowing time capsules

But what exactly is a CT scanner? Why has this medical machine become a powerful tool in modern palaeontological research?

We’re all familiar with the CT scanner from television medical dramas, where hard-to-love doctors lay semi-naked patients on a table that glides them into a circular chamber, like a giant polo-mint. In the room next door their organs and bones are revealed in monochromatic slices on a computer monitor, and more often than not – eureka! - the problem is identified, and treatment advised. Yet this is only the end result of a history of technological advances coupled with the latest imaging technology.

The full name for the technology is x-ray computed tomography. It has its origins 100 years ago with the development of mechanical tomography, which created images of individual slices through the human body using x-rays and film. Like many early medical apparatus, the first tomographic machines look like torture devices from the Saw franchise, but they were a technological breakthrough; allowing physicians to see inside the body without invasive surgery. They didn’t effectively capture soft tissue images to begin with - this was not possible until the 1970s, when computational techniques put the ‘C’ into CT scan. As computer technology advanced, so did tomographic scanning, and today we can effectively reconstruct the inside of living and fossilised structures in three dimensions using digital software.

Palaeontologists usually use slightly different CT scanning technology from the medical profession. Micro-CT (written µCT) uses higher doses of x-rays than can be used on living things, allowing the beams to penetrate denser materials like rock. You wouldn’t want to climb inside one of these machines; the dangerous dose of radiation would badly damage your body tissues. Instead, you slide open a thick shielded door and place your specimen inside, before shutting it and turning on the x-rays (the x-rays won’t turn on unless the door is shut, so you can’t accidentally irradiate yourself).

The name tomography comes from the Greek tomos, slice, and graphos, to write, and this is wonderfully illustrative. CT scanners take x-ray images in the form of thousands of individual radiographs through the fossil. Each image is a single projection, from one angle, before the object is rotated by a degree or so and the next image taken. Once this process is finished, software takes all of the images and reconstructs the fossils, generating ‘slices’ through the object then knitting them together into a three-dimensional graphic. Then the magic of digital palaeontology can begin.

However it isn’t always plain sailing. There are drawbacks for this wonder technique - cost being the most obvious. The machine requires a substantial financial outlay to buy and set up. Not every institution can afford a scanner, and those that do usually charge hefty fees for their kit as well as their expertise. Researchers often travel long distances and fork out a good portion of their budget to acquire a scan. Even then, you aren’t guaranteed good results: if the rock encasing your fossil is especially dense, crystallised, or contains metals, the x-rays may not penetrate, or the images may be low quality. Micro-CT is only suitable for small objects, but larger scans often mean lower quality images. When you’ve handed over upwards of £700 for a single scan, these are costly You could always build your own. The University of Edinburgh’s School of Geosciences have done just that with great results. In a paper just out this month, Steven Brusatte and colleagues announced a new tyrannosaur, Timurlengia euotica, based on the results of work done on this in-house scanner. Access to Edinburgh’s own scanner opened up this technology to the next generation of students. The difficult reconstructions were the work of co-author Amy Muir, who processed the CT data as part of her Masters degree at the University – a unique palaeontological opportunity not offered in many UK universities.

Whether you fork out a fortune or do it DIY, it only takes a browse of a website like DigiMorph to appreciate the worth of CT scanning. This online resource from the University of Texas gives you access to hundreds of reconstructed fossil and living species, collected by researchers around the world: for example, the fossil dinosaur egg digitally excavated by Amy Balanoff and colleagues in 2008. Lack of bone mineralisation makes reconstruction of dinosaur egg contents from CT scans a tricky task; the embryo can be hard to pick out against the infilled rock and broken shell fragments. However, Balanoff’s fossil from the Gobi Desert in Mongolia was recently identified as a Cretaceous bird, from the clade enantiornithes; a group which retained a lot of the features we typically associate with dinosaurs. Fossil eggs are normally physically broken open or digested with acid to reveal the embryo inside – a destructive but necessary process – but micro-CT offers a chance to explore inside non-destructively.

Reconstructions from CT scans can also be used in FEA – that’s finite element analysis to the uninitiated. Put simply, because you can calculate volumes, accurately measure the fossil, and manipulate it in 3D, you can use engineering principles and formulae to work out things like bite force and the stress and strain a fossil experiences under different conditions. Alana Sharp and Thomas Rich used it to figure out why the largest marsupial that ever existed, the three-metre long wombat-like Diprotodon, had an unexpectedly thin skull full of sinuses. In their recent paper in the Journal of Anatomy, they scanned the massive Pleistocene skull using a medical scanner in a hospital – it was just too big for a micro-CT machine. They then made a digital skull model with the sinuses ‘filled-in’ (that is, with solid bone), and another model with no sinuses, but extra bone for increased muscle attachments. They compared the stresses experienced by these different models when biting, with the original thinner, sinus-filled skull. The results suggest that the cranial sinuses of Diprotodon lightened yet strengthened the skull, dissipating stress across a larger surface area and allowing the animal to bite hard without risking a breakage.

All of these studies highlight a veritable revolution in palaeontological research over the last decade. The original skills of excavation and anatomical description remain as important as ever, yet the adoption of computed tomography and its unique ability to let us peer through solid rock and inside once-living organisms, saves valuable specimens from destructive exploration, and provides data previously undreamt of. As with all technology, it is constantly improving and being used in novel ways – who knows what astonishing applications we’ll see in the future?

https://www.theguardian.com/science/2016/mar/30/getting-under-a-fossils-skin-how-ct-scans-have-changed-palaeontology-dinosaur-lizard