Pachycephalosaurus means “thick-headed lizard” in Greek, a particularly apt name given that the prehistoric herbivore that features on the penultimate coin in the Supersaurs series had a skull roof up to 25 cm thick. As the second part of its name implies, Pachycephalosaurus wyomingensis lived in what is now the north-western United States and the province of Alberta in Canada during the late Cretaceous period, some 70 million years ago.

Unusually curved and with short, blunt spines protruding along its edge, the skull of Pachycephalosaurs was unique among dinosaurs. Towards the front it tapered into...