The Jurassic is a geologic period and system that extends from 201.3± 0.6 Ma (million years ago) to 145± 4 Ma; 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 period did not witness any major extinction event. The Jurassic is named after the Jura Mountains within the European Alps, where limestone strata from the period was first identified.
By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses, Laurasia to the north and Gondwana to the south. This created more coastlines and shifted the continental climate from dry to humid, and many of the arid deserts of the Triassic were replaced by lush rainforests. Dinosaurs dominated the land, and reached their peak in this period as they diversified into a wide variety of groups. The first birds also appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. The oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates. Mammals also existed during this time; however, overshadowed by the dinosaurs, they constituted only a small and relatively insignificant part of the biosphere.