In the prehistory of the Earth, the Cretaceous period was a vast period of time starting approximately 145 million years ago (at the end of the Jurassic period) and ending approximately 66-65 million years ago. It was the last era of the Age of the Dinosaurs, before an asteroid, the Chicxulub impactor, struck the Earth at what became the Chicxulub crater in what is now Mexico's Yucatan Peninsula, triggering a mass-extinction event that wiped out most of the dinosaurs. The only dinosaurs that survived this cataclysm were the birds. In subsequent time periods, mammals (including humanity) would occupy the dominant niche that dinosaurs once occupied. Mammals had also lived during the Cretaceous, but at the time were smaller, far less dominant lifeforms.
In some Teenage Mutant Ninja Turtles continuities and timelines, the Cretaceous is a time period sometimes visited by Renet using the Time Scepter. In the shared Mirage-IDW prehistory, the Cretaceous is where Time Master Savanti Romero was marooned, and from where the IDW continuity's Utroms harvested the dinosaur species triceratops to create the mutant Triceraton race to serve the Utrom Empire.