Cambrian

The Cambrian was a period of Earth history which ended around 500,000,000 BC. It was a part of the Palaeozoic era, which in turn was in the Phanerozoic eon. (AUDIO: Power Play)

According to another account, it was an epoch. (PROSE: Blue Box)

Biology
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)

The Ediacara biota went extinct at the end of the Cambrian period. (AUDIO: Power Play)

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.