The specimen that defines a species
This is a ‘holotype’, meaning it is the specimen which is described in scientific literature as best representing its species. So if someone wanted to check that the ammonite Aulacostephanus (aulacostephanus) undorae is a correct identification, they will come and look at this very specimen as it defines the species.
If you look closely, you might detect small amounts of the shell’s original iridescent nacre (mother of pearl) remaining upon its surface.
Find out more about ammonites on the molluscs pages.