The last of the Mesozoic periods, the Cretaceous spaned a period in Earth's history that last roughly 80 million years. It signified the beginning of great change on the planet. Continents, for example, moved into recognizable positions, similar to those of toady. Many new animal groups, as well as plant groups, arrived on the scene for the fist time. Many of these organisms, some of which are still existing today, are very familar. It also harbored the second largest extinction event in Earth's history, at the very end of this time frame