No one knows the exact number of dinosaur species living before the end of the Cretaceous period, as we do not have a complete dinosaur fossil record. The nature of fossilization—or that not all animals were in the right place at the right time in order to become a fossil—and the process of erosion have wiped away much of the evidence over time. Although scientists know there were probably many more dinosaurs, only a few species were still alive toward the end of the Cretaceous period, and most of them lived on the North American continent.