The Jurassic is a geologic period and system that extends from about 199.6± 0.6 Mya (million years ago) to 145.5± 4 Mya, that is, from 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. However, the end of the Jurassic Period did not witness any major 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.The chronostratigraphic term "Jurassic" is directly linked to the Jura Mountains. Alexander von Humboldt recognized the mainly limestone dominated mountain range of the Jura Mountains as a separate formation that was not at the time included in the established stratigraphic system defined by Abraham Gottlob Werner and named it “Jurakalk” in 1795. The name “Jura” is derived from the celtic root “jor” which was Latinised into “juria”, meaning forest (i.e. “Jura” is forest mountains).