Jurassic (geological period)

Stratigraphy and subdivisions

Stratigraphically, the Jurassic is divided into three major epochs: Early (Lower), Middle and Late (Upper) Jurassic. Each epoch contains a series of stages that are used worldwide for correlation. These subdivisions are identified by characteristic fossil assemblages and rock types, and are tied to the international geologic period timescale through biostratigraphy and radiometric calibration.

Climate, sea level and plate tectonics

The Jurassic climate was generally warmer and more humid than present, with little or no polar ice and higher global sea levels. Extensive shallow seas formed epicontinental shelves that supported rich marine ecosystems and carbonate deposition. During the Jurassic, the supercontinent Pangaea continued to fragment: rifting widened ocean basins, including the early Atlantic, and the Tethys Seaway separated major landmasses. These tectonic changes rearranged coastlines and continental connections, influencing migration and evolution.

Life and ecosystems

Terrestrial ecosystems were dominated by dinosaurs, which diversified into a wide range of forms from large sauropods to predatory theropods. The Late Jurassic yields the first clear bird-like fossils, indicating the early stages of avian evolution. Plant life was dominated by gymnosperms such as conifers, cycads and ginkgoes, with ferns and seed ferns common in many environments.

Marine environments supported abundant ammonites and other molluscs that are important index fossils. Marine reptiles such as plesiosaurs and ichthyosaurs were common, and numerous fish groups flourished. Flying reptiles (pterosaurs) occupied aerial niches, often along coastlines and inland lakes.

Fossils, significance and research history

The Jurassic period is among the best documented intervals in the fossil record, with extensive fossil-bearing formations across Europe, North America, Africa and Asia. Famous deposits include lithographic limestones and continental sediments that have yielded complete skeletons and exceptional preservation. Iconic localities such as Solnhofen in Germany and the Morrison Formation in North America have provided key specimens that illuminate dinosaur anatomy, behaviour and environment. Research in the 19th and 20th centuries established many stratigraphic frameworks; modern studies combine fieldwork with geochemistry, isotope analysis and refined dating techniques.

Economic and scientific importance

Jurassic rocks are important as sources of building stone and host sediments that can form hydrocarbon reservoirs and mineral deposits. Scientifically, Jurassic strata and fossils are central to understanding Mesozoic life, major evolutionary transitions (including the origin of birds) and the environmental consequences of plate-tectonic reconfiguration.

Key points

• Often called the "age of dinosaurs" because dinosaurs were the dominant large terrestrial animals.
• Marked by high sea levels and extensive marine deposits rich in ammonites and other index fossils.
• Important interval for studying the breakup of Pangaea and early Atlantic rifting.

For general context, see entries on the geologic period concept and the international geological timescale. The Jurassic sits between the Triassic and the Cretaceous in the Mesozoic Era, and is a major unit within the Phanerozoic eon, whose numeric ages are constrained by fossils and radiometric methods.