In September 2015, I went to Compton Bay on the Isle of Wight to hunt for dinosaur bones. It was equinox tides all week, so an ideal time to get out on the furthest rocks of the Wessex formation, dating from the Barremian stage of the early Cretaceous (about 130mya) also famous for the bone debris beds, which are highly fossiliferous. Time passed and I hadn’t had a great amount of luck. So, deciding today was not my day, I decided to head home. As I turned, I glanced down to see a beautiful piece of rib bone with the most amazing internal structure I’ve ever seen (Fig. 1). But also nothing like I’ve ever seen before.
I took it show my tutor, David Martill, at the University of Portsmouth. He was quick to identify it as being from a sauropod, due to the large air cavities now filled in with a clear mineral banded by pyrite. He then followed the identification by: “how’d you fancy cutting it in half for a thin section?” I was dubious about the idea at first: I’d never looked at a bone and thought ‘you know what, that would be better cut in half’. But I went along with it and handed over my prize.
What is a thin section?
A thin section is an approximately 30µm thick slice of rock, or in this case, dinosaur bone attached to a glass slide with epoxy, a very strong, transparent glue. The thinness of the specimen makes it transparent, so can be viewed under a microscope. This allows you to identify individual minerals and microstructures of the bone.
Making a thin section: the process
First, the bone is cut in two using a rock cutting saw, which has diamonds embedded in the metal. The cutting is performed slowly and in stages to reduce the vibrations to the bone, reducing the risk of crumbling.
Once the bone has been cut, the chosen end is embedded with proxy resin (like superglue). This hardens the bone and ensures it will stay intact during the next stage of the process.
The chosen piece is then attached to an arm, which holds it securely in place to the side of a small diamond tipped circular saw. This slowly cuts a slice of bone away approximately 1 to 2mm thick. The slice of bone has one side polished to remove the saw marks, and the same side is then stuck to a glass slide using epoxy.
However, the bone is still too thick to see through. So it is thinned down by grinding using successively finer sandpapers until it is roughly 30µm. A cover slip is then stuck on using epoxy, which protects the specimen from damage, as well as increasing the clarity under the microscope.