Palaeozoic

The Palaeozoic was an era of Earth history which was part of the Phanerozoic eon. It included the Cambrian and Permian periods, and ranged from at least 500 million to 260 million BC. (AUDIO: Power Play)

Biology
Horseshoe crabs were arthropods from the Palaeozoic. (PROSE: The Southwell Park Mermaid)

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

History
The Fifth and Tenth Doctor once stranded a Cyber-scout in the Palaeozoic, keeping it away from the 18th century Paris it was trying to influence. Upon its arrival, the scout's scans noted the "irregular" atmosphere composition, including reduced oxygen. (AUDIO: The Gates of Hell)

Behind the scenes
The Palaeozoic Era contained the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods.