During the Jurassic period, Earth was a warmer, wetter, ice-free world where a single supercontinent was splitting apart, shallow seas flooded vast lowlands, and dinosaurs dominated every major landmass. The period spanned from about 201 million to 143 million years ago, lasting roughly 58 million years. Almost nothing about the planet’s surface would look familiar to a modern observer: the continents were in different positions, the atmosphere held far more carbon dioxide, and flowering plants didn’t yet exist.
A Supercontinent Breaking Apart
At the start of the Jurassic, most of Earth’s land was still joined in the supercontinent Pangaea, a massive C-shaped landmass surrounded by a single global ocean called Panthalassa. Wedged into the curve of that C was the Tethys Ocean, a warm tropical sea that separated the northern block of land (Laurasia) from the southern block (Gondwana). If you could look down at the planet from space, you’d see one enormous continent stretching from pole to pole, with ocean everywhere else.
That arrangement didn’t last. Around the start of the Jurassic, roughly 201 million years ago, the crust began stretching and cracking. Rift valleys opened between what would become North America, Europe, and Africa. These rifts slowly filled with seawater, creating narrow, shallow marine basins that were the earliest ancestors of the Atlantic Ocean. In northwestern Europe, faulting between ancient continental cores produced a patchwork of sedimentary basins with connections running north toward the Arctic and south toward the Tethys. The Arctic and proto-Atlantic rift systems likely remained separate until the Late Jurassic or even later, meaning the Atlantic was still more of a chain of elongated bays than a true ocean.
By the Late Jurassic, around 150 million years ago, the split was well underway. South America was beginning to pull away from Africa. India, Antarctica, and Australia were still fused together as part of Gondwana but starting to show the first signs of separation. The overall picture was a planet in transition: one world becoming many, with new seaways opening and ocean currents shifting.
A Greenhouse Climate With No Ice Caps
The Jurassic was dramatically warmer than today. Atmospheric carbon dioxide levels sat around 900 parts per million for much of the period, more than double today’s roughly 420 ppm. Some estimates for other Jurassic stages push even higher, toward 1,000 ppm or above. That thick blanket of greenhouse gas meant global temperatures were significantly elevated, and there is no evidence of permanent polar ice caps at either pole. Tropical and subtropical conditions reached much farther north and south than they do now.
Oxygen levels were closer to modern values, hovering around 20 to 21 percent of the atmosphere. So the air would have been breathable for a modern human, just noticeably hotter and more humid in most places. Rainfall patterns were strongly seasonal in many regions, potentially monsoonal. Ancient soils from the western United States show cycles of swelling during wet seasons and deep cracking (some cracks over 1.5 meters deep) during dry spells, pointing to alternating floods and droughts rather than steady year-round rain.
What the Landscape Looked Like
Without flowering plants or grasses, which hadn’t evolved yet, the Jurassic landscape looked alien. Forests were built from conifers, ginkgoes, tree ferns, and especially cycads, palm-like plants with thick trunks and feathery fronds that were so dominant the Jurassic is sometimes called the “Age of Cycads.” These plants produced cones rather than flowers, and their leaves resembled fern fronds closely enough that some species were originally misidentified as ferns.
Ground cover would have been ferns, horsetails, mosses, and club mosses rather than grass. Open areas weren’t prairies. They were bare floodplains, mudflats, or scrubby expanses of low-growing ferns. Ancient soil studies from what is now Utah describe open floodplains that periodically turned into shallow lakes, with no evidence of forest canopy or grassland. In wetter regions, dense conifer forests probably lined river valleys and coastal lowlands, while drier interior areas may have been more sparsely vegetated.
The overall color palette would have been green and brown, heavy on deep conifer greens and fern fronds, with none of the bright floral colors we associate with modern landscapes. No fruits, no berries, no blossoms.
Giants on Land
The Jurassic is best known for its dinosaurs, and the Late Jurassic in particular produced some of the largest land animals that ever lived. Sauropods, the long-necked herbivores, were the defining animals of the period. Diplodocus and Apatosaurus fed at heights below about 3 to 5 meters, browsing on ferns and low vegetation. Camarasaurus reached between 3 and nearly 6 meters. Brachiosaurus towered above them all, feeding at heights between 5 and 9.4 meters, stripping leaves from the tops of conifers that other species couldn’t reach.
This wasn’t random. The different feeding heights meant multiple giant species could coexist without competing directly for the same food, much like modern African herbivores partition their savanna. Alongside the sauropods, armored dinosaurs like Stegosaurus and the early ankylosaur Gargoyleosaurus browsed at ground level. Stegosaurus, with its distinctive back plates, was a purely Jurassic animal. It disappeared from the fossil record before the Cretaceous period began.
Predators matched the scale of their prey. Large theropods, the two-legged carnivores, patrolled these ecosystems. Allosaurus was the apex predator of Late Jurassic North America, while smaller theropods filled niches as mid-sized hunters and scavengers.
Life in the Oceans
The Jurassic seas were warm, shallow in many areas due to the flooding of continental margins, and packed with life. Marine reptiles were the top predators. Plesiosaurs, large-bodied carnivorous reptiles that had returned to the sea from land-dwelling ancestors, were important components of marine ecosystems worldwide throughout the Jurassic. They came in two basic body plans: long-necked forms like Cryptoclidus that likely fed on fish and smaller prey, and short-necked, large-skulled “pliosauromorphs” like Liopleurodon that were apex predators capable of taking large prey.
Ichthyosaurs, which resembled modern dolphins in body shape, had already been thriving since the Triassic and remained abundant in Jurassic seas. Ammonites, coiled-shelled relatives of modern squid, were extraordinarily diverse and are now among the most common Jurassic fossils. Coral reefs grew in the warm Tethys Ocean, and the shallow seaways created by Pangaea’s breakup provided vast new habitats for marine life to colonize.
Early Mammals in the Shadows
Mammals existed during the Jurassic, but they were small, inconspicuous, and nothing like their modern descendants. Most were roughly the size of shrews or rats. They occupied ecological niches that kept them out of direct competition with dinosaurs: nocturnal insect hunters, burrowers, and small omnivores.
Recent research using synchrotron imaging of fossilized tooth layers has revealed that these early mammals grew more slowly, lived longer, and reached sexual maturity later than modern mammals of similar size. Early crown mammals (the lineage that includes all living mammals) did develop faster growth rates in their youth compared to more primitive relatives, hinting that the metabolic machinery for warm-bloodedness was already being assembled. But by modern standards, Jurassic mammals were slow-growing and long-lived for their tiny bodies. Groups like docodonts and early relatives of modern placental mammals were present, quietly diversifying in ways that wouldn’t become apparent for another 80 million years.
Three Phases of the Jurassic
The Jurassic is divided into three epochs, each with a distinct character. The Early Jurassic (201.4 to 174.7 million years ago) followed a major mass extinction at the end of the Triassic. Dinosaurs were already present but still diversifying, and the first true plesiosaurs appeared in well-preserved formations like the Lias Group of the United Kingdom. Pangaea was largely intact, though rifting had begun.
The Middle Jurassic (174.7 to 161.5 million years ago) saw the supercontinent’s breakup accelerate. Shallow seas spread across parts of Europe, and sauropod dinosaurs began reaching enormous sizes. Marine ecosystems diversified as new ocean basins created additional habitat.
The Late Jurassic (161.5 to 143.1 million years ago) was the period’s climax. This is when the Morrison Formation of western North America was deposited, preserving the famous fauna of Diplodocus, Apatosaurus, Stegosaurus, and Allosaurus. Global temperatures were high, sea levels were elevated, and biodiversity on land and in the oceans was rich. By the time the Jurassic ended, the Atlantic was a narrow but real ocean, and the world’s continents were recognizably moving toward their modern positions.