The Upper Cretaceous is the last period of the Cretaceous era, spanning from about 100.5 to 66 million years ago. It is known for significant geological and biological changes, including the formation of major mountain ranges and the diversification of dinosaurs. This period ended with the mass extinction event that wiped out many species, including the non-avian dinosaurs. During the Upper Cretaceous, the climate was generally warm, and sea levels were high, leading to widespread shallow seas. Fossils from this time include various marine life, such as ammonites and bivalves, as well as land-dwelling creatures like Tyrannosaurus rex and Triceratops.