Clockwise from top right: Terror Bird (cast), Triceratops skull (real and cast), Stegosaur (cast), Sabertooth Cat (real), Mastodon (cast), Ichthyosaur (real), Xiphactinus (real), Whale (real)
What is a cast?
A cast is an exact copy of a fossil, typically made from polyester resin or plaster. To make a cast, each bone of the original fossil is encased in silicone and then removed, leaving an empty space – a mold. The mold is then filled with liquid casting material that takes the same shape and size as the original. Once the casting material hardens, it can be safely removed from the mold and used in mounted skeletons.
Why use casts? Aren’t real bones cooler?
Yes, real bones are cool, but occasionally cast bones are the only option. For example, sometimes only a partial skeleton was found during excavation, so fill-in bones are necessary in order to display a complete specimen. An example of this is the Allosaurus on display at the museum:
This dinosaur is a mix of real fossil bones from different Allosaurus individuals and some fill-in casts. It can be hard to tell the difference! In this case, the real hind legs and the cast tail bones look very similar.
Another reason why we might display a cast is that the real fossil of a particularly rare specimen is on display at another museum and exhibiting a cast is the only way to make it available for our visitors to see. This Elasmosaur is a cast of the one-of-kind complete specimen found on Vancouver Island. Because the real specimen is currently being studied by scientists in British Columbia, the Burke is only able to display it as a cast.
Next time you visit a natural history museum, see if you can figure out which bones are real and which are casts. Can you tell the difference?
Posted by: Julia Swan
