Cretaceous

The Cretaceous was a period in early Earth history in which the dinosaurs, such as tsintaosaurus, dsungaripterus, protoceratops, velociraptor, ankylosaurs, tyrannosaurs, bactrosaurs and Alioramus, thrived. (PROSE: Living in the Past)

Ace once asked the Seventh Doctor if they could visit Earth during the upper Cretaceous, a time which the Doctor, considered a "very good time for dinosaurs". (TV: The Happiness Patrol) Later, the Doctor dropped off Ace in the Cretaceous period, then travelled on his own for a time. (COMIC: Train-Flight) He eventually returned to Mongolia in the upper Cretaceous, 75,000,000 BC. Along the way he encountered a pit of dholes and an alien slave who had escaped from a nearby construction site. Ace arrived with a herd of dinosaurs to trample the site, destroying it and freeing the slaves. (PROSE: Living in the Past)

On another occasion, the Sixth Doctor guessed the Rani had popped back to the Cretaceous age to collect embryos of the Tyrannosaurus rex. (TV: The Mark of the Rani) Later, the Time Lady meant to use a Time Manipulator to come back to this age and avert the extinction of the dinosaurs. (TV: Time and the Rani)