The Jurassic is a geologic perio that extends from about 199.6± 0.6 Ma (million years ago) to 145.5± 4 Ma, the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the "Age of Reptiles". The start of the period is marked by the major Triassic-Jurassic extinction event. The start and end of the period are defined by carefully selected locations; the uncertainty in dating arises from trying to date these horizons.