The current arthropods (crabs, spiders, flies...) constitute a very numerous and diverse group that has managed to colonize practically all the planet's ecosystems. The group also includes extinct forms, such as trilobites and abundant primitive marine arthropods during the Paleozoic Era. They are, without a doubt, the most characteristic and well-known fossils of this Era.

An external skeleton or exoskeleton protects the body of any arthropod. To ensure their growth, they must shed this rigid cover and replace it with a larger one called exuvia or moult. Trilobites shed their exoskeleton several times throughout their lives, so exuvia is found relatively frequently in the fossil record. This specimen is an excellent example of moulting.