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.

The Doctor arrived in Mongolia during the Cretaceous period to pick up Ace, whom he had dropped off there. 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. (DWM: Living in the Past)