Jurassic (from Jura Mountains) is a geological period that lasted from about 200 to 145 million years ago, during the middle of the Mesozoic Era. 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; two other significant events occurred during the period: the Pliensbachian/Toarcian turnover, about 183-175 million years ago, and Carnian Pluvial Event around 216 million years ago.
The Jurassic was named by Alexandre Brongniart for the extensive limestone deposits of the Jura Mountains region of Switzerland and France. By definition it extended from 201.3 Ma to 145 Ma (million years ago), although some authors prefer an age range with a slightly different beginning date, 200 Ma, due to significant changes in fauna at that time.
During this time span dinosaurs flourished in many parts of Pangaea (the single continent which then existed). The break-up of Pangaea occurred during late Jurassic/early Cretaceous times, approximately 175 million years ago; consequently most dinosaur genera had become extinct by this time (although some would linger on into Cretaceous times). Nevertheless, there were many notable exceptions: birds evolved from small feathered dinosaurs during late Jurassic times; pterosaurs were also present throughout much of this period; stegosaurs and ceratopsians thrived in what are now western North America and eastern Asia respectively until their demise at or near the end Cretaceous extinction event 65 mya; and lastly crocodilians were common throughout most land habitats except for polar regions until relatively recent times.
All these groups shared one common ancestor: archosaurs (“ruling reptiles”), a group which included not only crocodilians and dinosaurs but also flying reptiles such as pterosaurs (“winged lizards”). Archosauriforms first appeared about 247 mya in Early Triassic times but their great radiation did not begin until Late Triassic periods when they began to replace earlier tetrapod stock such as prolacertiforms and rhynchosaurs. Amongst archosauromorphs (“near ruling reptiles”), a group thought to be closely related to archosaurs but which excludes them, several important lineages appeared during Middle Jurassic periods including lagerpetids (small bipedal running reptiles), ichnofossil evidence suggests that early members of Dinosauromorpha may have already been present during Middle Triassic times but they only became abundant after Late Triassic extinctions decimated other tetrapod groups such as crurotarsans (“crocodile like”) reptiles).