Silurians

Intelligent reptilian lifeform which ruled the Earth during the Cretaceous (136 to 65 million years BC), and not the Silurian (430 to 395 million years BC). They, and their underwater "cousins", the so-called "Sea Devils", were eventually driven to hibernate in underground shelters by some undetermined cosmic event. Theories about the nature of this event included: a wandering planetoid causing a change in the Van Allen belt; movements of the Moon; departure of Earth's twin planet, Mondas; or possibly the quasi-psychic awareness of the forthcoming crash of a space liner from the future.
The Silurians of Wenley Moor were reawakened from suspended animation in 1971 because of power leaks from the neighboring British atomic research center. The Old Silurian met Dr. Quinn, then the Third Doctor, and became convinced that his people could share Earth peacefully with Mankind. However, the Young Silurian tried to wipe out Mankind with a deadly virus. After that had failed, he and the Silurian scientist, Icthar, attempted to use a molecular disperser to destroy the Van Allen Belt, and thus render Earth inhabitable for humans. Thwarted by the Third Doctor, they returned to suspended animation. Their shelter was then blown up by UNIT.
Icthar survived, and reawakened in 2084 AD. He reanimated a Sea Devil commando, led by Sauvix, and attacked Sea Base Four. His plan was to launch nuclear missiles to provoke a war between the two blocs, and thus eradicate mankind (which had been the Young Silurian's last wish). His efforts were defeated by the Fifth Doctor, who used deadly hexachromite gas to wipe out the Silurians.

Silurian cousins The Sea Devils: Intelligent reptilian lifeform which ruled the Earth during the Cretaceous (136 to 65 million years BC), and not the Silurian (430 to 395 million years BC). They, and their "cousins" the Silurians, were eventually driven to hibernate in underground shelters by some undetermined cosmic event. A commando of Sea Devils awoke from suspended animation in 1973 and began sinking ships in the British Channel. They were contacted by the Master who offered to ally himself with them. The Third Doctor and the Chief Sea Devil talked peace, but they were thwarted by the interference of the Master and Walker, a civil servant who ordered a depth charge attack. In the end, the Doctor was forced to engineer an explosion which destroyed the Sea Devils' undersea base before any more Sea Devils could be awakened.
The Silurian Icthar reanimated a Sea Devil commando led by Sauvix in 2084 AD, and used them to attack Sea Base Four. His plan was to launch nuclear missiles to provoke a war between the two blocs, and thus eradicate mankind. His efforts were defeated by the Fifth Doctor, who used deadly hexachromite gas to wipe out the Silurians and the Sea Devils.