Ordovician

The Ordovician was an period of Earth's history, dating from 488 million to 443 million BC, which was bookended by two extinction events: the Cambrian-Ordovician extinction event and the Ordovician-Silurian extinction event.

Concluding the Cambrian, the Cambrian-Ordovician extinction event was likely due to a change in sea level, flooding portions of the two super-continents.

The dominant lifeforms throughout the Ordovician were hard-shelled sea creatures such as trilobites, brachiopods, sea stars and corals. Between ice ages, the atmosphere was hot and heavy in carbon dioxide, together with plenty of sulphur. During the glaciated periods, the temperature droppped radically.

Leading into the Silurian period, the cause of the second set of extinctions was less clear, with hypotheses ranging from more glaciation to a change in carbon dioxide levels, or even a gamma ray burst from a dying star that destroyed the ozone layer. It could even have been the result of alien meddling, providing a mystery which time travellers may have potentially investigated. (PROSE: Prehistory Repeating Itself ^[+]Loading...["Prehistory Repeating Itself (feature)"])