In the Early and Late Cretaceous periods, changing geological and ecological conditions created opportunities for the expansion of dinosaurs. These worldwide geologic and climate shifts of the Cretaceous period allowed for the evolution of Saurischian - lizard-hipped - dinosaurs and Ornithischian - bird-hipped - dinosaurs, including several families that appeared for the first time during this span. However, these same ecological changes also contributed to the eventual extinction of dinosaurs large and small. ""Last of the Dinosaurs"" explores the theories for what may have caused the mass extinction that ended the reign of the dinosaurs and many other terrestrial and marine creatures of the time. The non-dinosaurian reptiles of the Mesozoic era are also examined.