Cambrian

The Cambrian was a period of Earth history which spanned from 542 million to 488 million BCE; it followed the Precambrian period and was itself followed by the Ordovician period. (PROSE: Prehistory Repeating Itself ^[+]Loading...["Prehistory Repeating Itself (feature)"]) According to another account, it 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)

The start of the Cambrian period was known as the Cambrian Explosion of life; after billions of years of single-celled organisms, evolution gave rise to thousands of species of hard-shelled multicellular creatures, most of which lived on the floor of the warm shallow seas that dominated the planet. There were three main continental groups: Laurentia and Baltica in the north and Gondwana in the south, but the only life on the surface consisted of lichen and algae; plants would not evolve for millions more years. (PROSE: Prehistory Repeating Itself ^[+]Loading...["Prehistory Repeating Itself (feature)"])

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.