Cambrian

The Cambrian was an epoch of Earth history.

When Peri Brown suggested that horseshoe crabs were from space, the Sixth Doctor quickly corrected her, stating that the Cambrian Epoch in which they originated made them just as terrestrial as she was. (PROSE: Blue Box)

Behind the scenes
In actuality, the Cambrian was a period, not an epoch. As for horseshoe crabs, their earliest fossils only date back to the Ordovician period.