Jurassic Period

In 1795, the Prussian naturalist Alexander von Humboldt toured the Jura Mountains along the France–Switzerland border and recognised a series of carbonate formations as distinct from the underlying Triassic strata. He published his observations in 1799, applying the term 'Jura-Kalkstein' (Jura limestone) to these deposits. Three decades later, in 1829, French geologist and mineralogist Alexandre Brongniart formally coined the term 'Terrains Jurassiques' in his work Tableau des terrains qui composent l'écorce du globe, correlating Humboldt's Jura limestone with similarly aged oolitic limestones in Britain and thereby establishing the Jurassic as a recognised geological system. In 1832, German geologist Leopold von Buch introduced the three-fold subdivision of the Jurassic into the Lias (Early), Dogger (Middle), and Malm (Late), based on the folded limestone successions of the Jura. This fundamental framework persists in modern stratigraphy.

According to the ICS International Chronostratigraphic Chart (v2024/12), the Jurassic extends from 201.4 Ma (±0.2) to 143.1 Ma (±0.6). The base of the Jurassic System (= base of the Hettangian Stage) has a formally ratified GSSP at the Kuhjoch section, Karwendel Mountains, Northern Calcareous Alps, Tyrol, Austria, ratified by the IUGS in 2010 and inaugurated in 2011. The primary marker is the first appearance datum (FAD) of the ammonite Psiloceras spelae tirolicum. The upper boundary (Jurassic–Cretaceous boundary) has not yet been formally ratified by GSSP — it remains the only period boundary in the Phanerozoic Eon without an accepted GSSP. Many textbooks still cite ~145 Ma for this boundary, while the latest ICS chart places it at 143.1 Ma.

The Jurassic is divided into three epochs (series) and eleven ages (stages):

• Early Jurassic (Lower Jurassic / Lias): Hettangian (~201.4–199.5 Ma), Sinemurian (199.5–192.9 Ma), Pliensbachian (192.9–184.2 Ma), Toarcian (184.2–174.7 Ma)
• Middle Jurassic (Dogger): Aalenian (174.7–170.9 Ma), Bajocian (170.9–168.2 Ma), Bathonian (168.2–165.3 Ma), Callovian (165.3–161.5 Ma)
• Late Jurassic (Upper Jurassic / Malm): Oxfordian (161.5–154.8 Ma), Kimmeridgian (154.8–149.2 Ma), Tithonian (149.2–143.1 Ma)

The Jurassic witnessed the ongoing fragmentation of the supercontinent Pangaea. By the beginning of the period, the supercontinent had already begun to separate into the northern landmass Laurasia (comprising North America and Eurasia) and the southern landmass Gondwana (South America, Africa, India, Antarctica, and Australia), divided by the east–west equatorial seaway known as the Tethys Sea. During the Jurassic, new spreading centres and oceanic rifts opened between North America and Eurasia, between North America and Gondwana, and between segments of Gondwana itself. The proto-Atlantic Ocean basin began to form, accumulating thick sequences of flood basalts and marine sediments — including economically significant salt deposits in the Gulf of Mexico and oil-bearing shales of the North Sea.

Subduction zones were active along virtually all the western margins of the Americas, initiating the early development of the Rocky Mountains and the Andes. In western North America, the collision and accretion of terranes (island arcs and microcontinents carried by moving oceanic plates) added to continental growth. The Nevadan orogeny produced massive granitic intrusions from Alaska to Baja California, including the granites now exposed in Yosemite National Park. In the Early Jurassic, western North America was covered by a vast erg (sand sea) that left behind the Navajo Sandstone, spectacularly exposed today in Zion National Park, Utah.

The Jurassic was predominantly a warm greenhouse world. Atmospheric CO₂ concentrations are estimated to have been at least four times modern pre-industrial levels, with some estimates reaching approximately 2,000 ppmv. Global mean temperatures were roughly 5–10°C above present-day values. There is no evidence of polar ice caps; warm-adapted plants extended to at least 60° palaeolatitude in both hemispheres. Geochemical analyses of marine fossils suggest low-latitude sea-surface temperatures of approximately 20°C, with deep-water temperatures around 17°C — far warmer than today's deep ocean.

The temperature gradient from equator to poles was substantially reduced compared to today, likely weakening zonal wind patterns. The Tethys Sea served as a conduit for warm tropical water circulation around the globe. Climate varied somewhat through the period: the coolest interval occurred during the Middle Jurassic, while the warmest conditions prevailed in the Late Jurassic. A cooling trend is recorded at the Jurassic–Cretaceous boundary (the Tithonian–early Barremian cool interval). Large salt deposits indicate arid belts, particularly on the western side of Pangaea, while extensive coal deposits in other regions indicate high precipitation, reflecting a complex climatic mosaic.

The Jurassic began in the aftermath of the end-Triassic (Tr–J) mass extinction, one of the five largest extinction episodes in Earth history. Approximately 76% of all marine and terrestrial species — and about half of marine invertebrate genera — perished. The most widely accepted cause is massive volcanism associated with the Central Atlantic Magmatic Province (CAMP), linked to the initial rifting of Pangaea. CAMP eruptions released enormous quantities of CO₂ and SO₂, disrupting the carbon cycle, driving global warming, and triggering ocean acidification. The ecological vacancies left by this extinction were rapidly colonised by the survivors — dinosaurs, pterosaurs, crocodilians, turtles, mammals, and diverse marine invertebrates — setting the stage for the Jurassic radiation of life.

Terrestrial Fauna:

Dinosaurs rose from a relatively minor component of Triassic ecosystems to become the dominant land vertebrates. Early Jurassic dinosaurs were often small and morphologically simple, such as the heterodontosaurids. By the Late Jurassic, dinosaurs had diversified into an extraordinary range of body plans and sizes. Giant sauropods — including Brachiosaurus (over 12 m tall), Diplodocus (up to ~27–30 m long), Apatosaurus, and Camarasaurus — dominated herbivorous niches. Top predators included Allosaurus (9–12 m long), Ceratosaurus, and Torvosaurus, while small theropods such as Compsognathus and Ornitholestes occupied different ecological roles. The iconic armoured dinosaur Stegosaurus, with its dorsal plates and tail spikes (thagomizer), is emblematic of the Late Jurassic. By the end of the period, dinosaurs accounted for all terrestrial animals exceeding one metre in body length.

The Earliest Birds:

Archaeopteryx, discovered in the Solnhofen Limestone of Bavaria, Germany, in 1861, dates to the Late Jurassic (~150 Ma). It is a transitional fossil exhibiting both avian features (feathers, flight capability) and theropod dinosaur traits (teeth, a long bony tail, clawed hands). It remains one of the most important specimens in palaeontology for demonstrating the evolutionary link between non-avian dinosaurs and modern birds. Recent findings of very bird-like footprints in Middle Jurassic rocks in Morocco (described in 2023 by Maidment and colleagues) raise the possibility that birds or very close relatives evolved earlier than previously thought.

Marine Life:

Jurassic seas hosted a rich and modernising fauna. Ammonites, having nearly been wiped out by the end-Triassic extinction, radiated rapidly and serve as the principal index fossils for Jurassic biostratigraphy. Belemnites (squid-like cephalopods), bivalves (including oysters and scallops), and gastropods diversified greatly. Among the vertebrates, large marine reptiles — ichthyosaurs, plesiosaurs, and pliosaurs — were apex predators. Jurassic pliosaurs rank among the largest marine carnivorous reptiles ever discovered. The period also saw the origination of coccolithophores and planktonic foraminifera, planktonic organisms whose skeletons would come to dominate deep-sea carbonate sediments, fundamentally transforming ocean geochemistry. Reef ecosystems were built not only by scleractinian corals but also by siliceous sponges, serpulid worms, and microbial communities (stromatolites). The diversification of effective predators (crabs, snails, echinoderms, marine vertebrates) during this interval marks the beginning of the Mesozoic Marine Revolution — an evolutionary arms race between predators and prey that reshaped benthic communities.

A notable interruption in marine biodiversity occurred at the Pliensbachian–Toarcian boundary (~183 Ma) during the Early Jurassic: the Toarcian Oceanic Anoxic Event caused widespread low-oxygen conditions in epicontinental seas, leading to the extinction of the last spiriferid brachiopods and up to 84% of bivalve species in some regions.

Plants:

The Jurassic was the age of the gymnosperms — conifers, cycads, ginkgoes, and ferns dominated terrestrial vegetation. Bennettitales, superficially resembling cycads but a distinct lineage bearing cones inside their trunks, were an important component of many ecosystems. The equatorial and low-latitude tropics were relatively arid, and most plant diversity was concentrated at mid-latitudes. No unambiguous evidence exists for the presence of angiosperms (flowering plants) in the Jurassic; their major diversification is a Cretaceous phenomenon.

Mammals:

Jurassic mammals were overwhelmingly small (mostly mouse-sized or smaller) and occupied marginal ecological niches — typically nocturnal, insectivorous roles — in ecosystems dominated by dinosaurs.

Unlike its dramatic beginning, the end of the Jurassic was not marked by a catastrophic global mass extinction. In the marine realm, a notable faunal turnover has been documented, particularly among ammonites and other invertebrates, though its magnitude falls short of the top five mass extinctions. On land, the transition was gradual: stegosaurs disappeared from North America but persisted in Europe for some time. The Morrison Formation of western North America, the world's best-known Late Jurassic terrestrial sequence, is followed by a stratigraphic gap of at least 5 million years (and up to 10–15 million years in some areas), making it difficult to track the precise tempo and mode of faunal change. Regional mountain-building events in North America may have destroyed dinosaur habitats locally, but the picture elsewhere is less clear. The Jurassic–Cretaceous boundary remains formally undefined by GSSP — it is the only system boundary in the Phanerozoic without ratified international standardisation. This is largely due to the problem of provincialism: endemic ammonite populations across different regions left uneven or ambiguous fossil markers in the stratigraphic record.

• Morrison Formation (western USA): The world's richest Late Jurassic terrestrial fossil assemblage, preserving Allosaurus, Diplodocus, Stegosaurus, Apatosaurus, and many other taxa from floodplain and river-channel environments.
• Solnhofen Limestone (Bavaria, Germany): Exceptionally fine-grained Late Jurassic lagoon deposits famous for exquisitely preserved fossils, including Archaeopteryx, pterosaurs, and a wide variety of invertebrates.
• Jurassic Coast (Dorset–Devon, England): A ~150 km stretch of coastline exposing continuous Triassic through Cretaceous strata, celebrated as the site where Mary Anning discovered pioneering ichthyosaur and plesiosaur fossils in the early 19th century. It is a UNESCO World Heritage Site.
• Tendaguru Formation (Tanzania): An important Late Jurassic site yielding African sauropods including Giraffatitan (formerly referred to Brachiosaurus) and Dicraeosaurus.

Michael Crichton's novel Jurassic Park (1990) and Steven Spielberg's film adaptation (1993) transformed the Jurassic into the most culturally recognisable geological period. However, many of the most iconic dinosaurs featured in the franchise — Tyrannosaurus rex, Velociraptor, Triceratops, and Parasaurolophus — actually lived during the Cretaceous Period, not the Jurassic. Among the franchise's dinosaurs, Brachiosaurus and Dilophosaurus are genuine Jurassic taxa.