You may have seen the Museum’s work in the news recently, when our scan of a catshark helped University of Sheffield researchers understand how shark teeth evolved. In this blog, Brett Clark from the Museum’s Vertebrates Palaeobiology department shows us the method used.
Our research, led by Dr Zerina Johanson, investigates the evolution and development of teeth in jawed vertebrates – in particular, the tooth arrangement of present day sharks.
But how exactly do you scan a shark?
One thing you can count on is that we won’t be using a live shark for this type of research. The specimen needs to remain completely still for long periods of time during scanning. And as with all living things, sharks like to move around.
So the most practical option is to scan preserved shark specimens. The best place to find a preserved shark is in the spirit collection, a temperature-regulated archive in the Darwin Centre of the Museum.
Is it always necessary to scan a whole shark? That depends on the area of interest. If we’re looking at skin denticles, the tiny v-shaped scales that cover a shark’s body, it may be crucial to scan the entire specimen.
However, the limiting factor is the size of the scanner, and sacrifices have to be made with some of the larger sharks, where we scan only the head. Even then, some of the larger sharks we scan (40-60 centimetres long) can be tricky to handle, as they are stored in large glass containers filled with industrial-strength methylated spirit.
Anything dense can be a problem in the scanning world. Glass containers pose the biggest problem, as they are much denser than the soft tissues of the shark inside. So the glass container has to go or the results will be blurry. Blurry results are no use to anyone, because a lot of research now focuses on the small features, such as teeth and spines.
This also means the specimens have to undergo some degree of preparation before being exposed to the X-ray. But the specimens can’t be left out of the spirit for any great length of time because they would dry out. This means the whole process has to be performed quickly and correctly to preserve the shark.
The clock is ticking from the moment the shark is removed from the glassware. We use sealable plastic bags to prevent the sample from drying out during the scan, which can take around 40 to 60 minutes. A cloth soaked in methylated spirit is a low-tech but effective method to keep the shark moist during its brief stint in the bag. Sealed bags are ideal because of their low material density. Minimal density interference during X-ray scanning means better results.
During scanning, we target particular features of the shark depending on the research we’re doing. For example, the bowmouth guitarfish (Rhina ancylostoma) shown in these photos has unique denticle patterns located along its dorsal side.
The higher the image resolution achieved during scanning the better, as we can make more meaningful interpretations from the data. Putting extra care and attention in at this stage can be hugely beneficial for later 3D digitisation. Based on previous experience, this can save as much as 10 hours of an analyst’s time.
We then mount the shark using various low density materials, such as foam, plastic containers and polystyrene, before placing it inside the X-ray chamber. It also has to remain absolutely still during scanning. If a specimen moves even slightly during its 360-degree rotation in the scanner, the reconstruction process can produce terrible data.
The process is similar to Victorian photography, where if the person moved during the exposure then the photo would be blurry. Every scanning technician has encountered with great frustration a specimen that moved even after it was securely mounted. This can be a very tiresome process, considering the length of time required to perform a single scan. However, when the scans are processed the outputs can be visually stunning and well worth the effort.
Once the scan is finished, we can manipulate the data using 3D imaging software to create striking images that are also important for our research. Using this type of technology, researchers can now explore below the surface of a shark without physically destroying it. They can measure certain features and compare them with similar features in other shark species, or look at morphological similarities between groups – for example, does each shark family have similarly shaped teeth? And in this new age of research, a digital copy can be made and stored for future reference or additional study.
Scanning a shark is not easy so it is not something we do every day. The whole process can be time consuming, so it requires careful planning. We also need to be selective about which preserved shark species we scan – no matter how much a researcher would like to scan a specified shark species, it may just simply be too big. However, X-ray scanning is a rapidly developing technology, so in the future we may be able to scan nature’s giants such as the whale shark.