The Cretaceous period came directly after the Jurassic, beginning 145 million years ago and lasting until 66 million years ago. At roughly 79 million years long, it was the longest period of the Mesozoic Era (the broader “Age of Dinosaurs”) and ended with the most famous mass extinction in Earth’s history. The Cretaceous brought dramatic changes to life on land, in the oceans, and in the sky, many of which set the stage for the world we live in today.
When the Cretaceous Fits in the Timeline
The Mesozoic Era is divided into three periods: the Triassic (252 to 201 million years ago), the Jurassic (201 to 145 million years ago), and the Cretaceous (145 to 66 million years ago). The Cretaceous picks up right where the Jurassic leaves off and runs all the way to the asteroid impact that wiped out the non-avian dinosaurs. After the Cretaceous, Earth entered the Paleogene period, the beginning of the Cenozoic Era, sometimes called the “Age of Mammals.”
A Warmer, Higher-Sea World
The Cretaceous was one of the warmest stretches in Earth’s history. Atmospheric carbon dioxide levels ranged from roughly 4 to nearly 15 times the pre-industrial level, driven largely by intense volcanic activity along mid-ocean ridges. That extra CO₂ pushed global average temperatures an estimated 6 to 14 degrees Celsius higher than today’s, warm enough that there were no permanent ice caps at either pole.
The supercontinent Gondwana, which had included South America, Africa, India, Antarctica, and Australia, was actively breaking apart. The southern Atlantic Ocean was opening as South America pulled away from Africa, and India was drifting northward. Sea levels were far higher than they are now, flooding large portions of the continents. In North America, a shallow inland sea called the Western Interior Seaway split the continent roughly in half for tens of millions of years.
Dinosaurs Reached Peak Diversity
While the Jurassic is famous for long-necked sauropods like Diplodocus and plated dinosaurs like Stegosaurus, those groups largely faded early in the Cretaceous. In their place came an explosion of new forms. Ankylosaurs (armored, tank-like herbivores) and ornithopods (two-legged plant-eaters) diversified quickly. Sickle-clawed theropods, relatives of Velociraptor, became important small predators.
The final 10 million years of the Cretaceous, particularly in North America, produced some of the most recognizable dinosaurs of all time: Tyrannosaurus, Triceratops, the duck-billed hadrosaurs, and bone-headed pachycephalosaurs. In what is now Big Bend, Texas, paleontologists have found tyrannosaurs, horned dinosaurs, hadrosaurs, and the giant sauropod Alamosaurus, many described only in recent decades. Even Alaska preserves abundant hadrosaur tracks at Denali National Park.
Flowering Plants Changed Everything
One of the most consequential events of the Cretaceous had nothing to do with dinosaurs. Flowering plants, or angiosperms, first appeared in the Early Cretaceous and then diversified rapidly over a span of about 20 to 30 million years. No convincing angiosperm fossils have been found in Jurassic or older rocks anywhere in the world, so this was genuinely new.
Early flowering plants were relatively simple, but by the Late Cretaceous a group called eudicots (the lineage that includes most modern flowers, from roses to oaks) had come to dominate fossil floras. This botanical revolution reshaped ecosystems from the ground up: it created new food sources, new habitats, and new relationships between plants and the insects that pollinated them. The forests of the Late Cretaceous would have looked far more familiar to a modern observer than anything from the Jurassic.
Life in the Seas and Skies
The Cretaceous oceans teemed with giant marine reptiles. Mosasaurs, some exceeding 12 meters in length, were apex predators in the warm inland seas. Plesiosaurs and sea turtles shared these waters alongside a huge diversity of fish and ammonites (coiled-shell relatives of modern squid).
Overhead, pterosaurs reached their most extreme sizes. Quetzalcoatlus, which lived roughly 68 to 66 million years ago, had a wingspan that may have exceeded 10 meters, making it one of the largest animals ever to fly. Other pterosaurs filled a range of ecological roles: Anhanguera had toothy jaws suited for gripping fish, Pteranodon was toothless and also ate fish, and Istiodactylus from the Isle of Wight may have been a vulture-like scavenger. Pterosaurs survived all the way to the end of the Cretaceous, 66 million years ago, when they went extinct alongside the non-avian dinosaurs.
How It All Ended
The Cretaceous ended abruptly. An asteroid roughly 10 kilometers in diameter struck what is now the Gulf of Mexico, carving out the Chicxulub crater, a scar 180 to 200 kilometers across. The impact launched enough dust and soot into the atmosphere to block sunlight on a global scale. Models show that even a 10 percent reduction in sunlight would have effectively eliminated viable habitat for large animals, and at 15 percent dimming, habitat suitability dropped to zero.
The result was an “impact winter” that collapsed food chains on land and in the sea. All non-avian dinosaurs, pterosaurs, mosasaurs, ammonites, and many other groups vanished. Research published in the Proceedings of the National Academy of Sciences confirmed that the asteroid, not volcanic activity from India’s Deccan Traps eruptions, was the primary driver of this extinction.
What Survived and What Came Next
Not everything died. Small mammals, crocodilians, turtles, and one lineage of dinosaurs made it through: birds. Research in Science Advances found that bird lineages originating near the extinction boundary underwent a burst of genetic and physical change, with shifts in body size, developmental patterns, and metabolic rates. Some lineages experienced a “Lilliput effect,” trending toward smaller body sizes in the aftermath. From that bottleneck, modern birds diversified into the more than 10,000 species alive today.
Mammals, which had spent the entire Mesozoic as mostly small, nocturnal creatures, quickly radiated into the ecological space left empty by dinosaurs. Within a few million years of the extinction, the ancestors of whales, bats, primates, and hoofed mammals had begun to appear. The Cretaceous didn’t just follow the Jurassic; it set up the modern world by ending with a catastrophe that cleared the stage for entirely new kinds of life.