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.
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 Cretaceous to pick her up. 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)