Jurassic

Jurassic. The Jurassic is a geologic period and system that spanned from the end of the Triassic to the beginning of the Cretaceous, approximately 201.3 to 145 million years ago. It constitutes the middle period of the Mesozoic Era, often called the "Age of Reptiles," and is renowned for the dominance of dinosaurs, the proliferation of diverse marine life, and the emergence of the first birds. The period's name originates from the Jura Mountains, a range straddling the border of France and Switzerland, where its characteristic limestone strata were first studied.

Etymology and history of study

The term "Jurassic" was formally introduced into geological nomenclature by the French geologist Alexandre Brongniart in 1829, deriving from the Jura Mountains. These mountains, which extend through Switzerland and France, are composed largely of limestone and marl deposits laid down during this period. Early foundational studies of Jurassic strata were conducted by pioneering geologists like William Smith in England and Leopold von Buch in Germany, who began correlating rock layers based on their fossil content. The period was later subdivided into the Early, Middle, and Late Jurassic epochs, with further divisions into stages such as the Hettangian, Sinemurian, and Tithonian, often defined by the evolutionary succession of ammonite species.

Geology

The Jurassic System is represented globally by a wide variety of sedimentary rock formations, including the classic marine limestones of the Jura Mountains, the shallow-water deposits of the Swabian Alb in Germany, and the terrestrial sandstones of the Morrison Formation in the western United States. The base of the Jurassic is formally defined by the first appearance of the ammonite species Psiloceras spelae at the Kuhjoch section in the Karwendel mountains of Austria, a Global Boundary Stratotype Section and Point ratified by the International Commission on Stratigraphy. Significant geological events include extensive marine transgressions that created vast, warm epicontinental seas across continents like Laurasia and Gondwana, depositing thick sequences of shale and limestone rich in fossils.

Paleogeography and climate

During the Jurassic, the supercontinent Pangaea continued to rift apart, initiating the formation of the Atlantic Ocean and separating Laurasia from Gondwana. This tectonic activity created extensive coastlines and shallow continental shelves, such as the Sundance Sea in North America and the Tethys Ocean between the northern and southern landmasses. The climate was generally warmer and more humid than today, with higher atmospheric carbon dioxide levels and no polar ice caps, supporting lush forests from the poles to the equator. Evidence from fossil plants, clay minerals, and oxygen isotopes suggests a greenhouse world with seasonal rainfall patterns, though some intervals like the Toarcian experienced oceanic anoxic events and significant climatic perturbations.

Flora and fauna

Terrestrial ecosystems were dominated by sauropod dinosaurs like Diplodocus and Brachiosaurus, theropod predators such as Allosaurus and Ceratosaurus, and armored ornithischians like Stegosaurus. This period witnessed critical evolutionary innovations, including the appearance of the first true bird, Archaeopteryx, from theropod ancestors in the Solnhofen Limestone lagoons. Marine environments teemed with ichthyosaurs, plesiosaurs, and marine crocodyliforms, while ammonite and belemnite cephalopods diversified extensively. Flora was characterized by cycads, conifers, ginkgos, and ferns, with the first appearance of Bennettitales; the understory saw the rise of early angiosperms by the Late Jurassic.

Major fossil sites

Exceptionally preserved Jurassic fossil assemblages have been discovered on nearly every continent. The Solnhofen Limestone in Bavaria, Germany, is a Lagerstätte famous for exquisitely detailed fossils of Archaeopteryx, pterosaurs, and crustaceans. In North America, the Morrison Formation, stretching from Montana to New Mexico, has yielded a spectacular array of dinosaur skeletons, including Apatosaurus, Camarasaurus, and Tyrannosaurus ancestors. Other significant sites include the marine Oxford Clay of England, rich in ichthyosaur and plesiosaur remains, the Yixian Formation in Liaoning, China, which provides early evidence of feathered dinosaurs, and the Cañadón Asfalto Formation in Patagonia, Argentina, documenting Gondwanan Jurassic fauna.