What Are the Triassic, Jurassic and Cretaceous Periods?

The Triassic ran from about 252 to 201 million years ago, the Jurassic from about 201 to 145, and the Cretaceous from about 145 to 66 million years ago. The Triassic and Cretaceous each ended in a mass extinction, and the Jurassic in a smaller one.

The whole span is about 186 million years. For comparison, the entire time since the dinosaurs disappeared is only 66 million years, which gives a sense of just how long these creatures dominated the planet.

The Triassic opened in the aftermath of the largest extinction in Earth's history, which had wiped out most marine species. The continents were joined in the supercontinent Pangaea, and much of the interior was hot desert.

The first dinosaurs appeared during this period, but they were not yet dominant. They were mostly small and shared the land with other reptile groups. The first pterosaurs and the earliest mammals also appear in the Triassic.

Pangaea began breaking apart, creating coastlines and wetter climates, and forests of conifers, ferns and cycads spread widely. Dinosaurs became the dominant land animals.

This is the period of the giant long necked sauropods such as Diplodocus and Brachiosaurus, along with plated Stegosaurus and the predator Allosaurus. It is also when the first known bird, Archaeopteryx, appears in the fossil record.

The Cretaceous was the longest of the three and the most familiar. Flowering plants appeared and spread rapidly, changing ecosystems and supporting new insects. Continents took on shapes closer to the modern map.

The most famous dinosaurs come from late in this period:

• •Tyrannosaurus rex in western North America
• •Triceratops, sharing the same landscape
• •Velociraptor in what is now Mongolia
• •Spinosaurus in North Africa, a specialist fish eater

Grouping dinosaurs into one era hides enormous gaps. Stegosaurus died out tens of millions of years before Tyrannosaurus rex existed, so the two never met. In fact more time separates them than separates T. rex from us.

Keeping the Triassic, Jurassic and Cretaceous straight also makes the story of dinosaur evolution readable. You can watch groups appear, diversify, decline and get replaced, which is far more interesting than a single undifferentiated dinosaur age.

The names come from nineteenth-century European geology and are refreshingly literal. Triassic refers to a threefold division of rock layers identified in Germany. Jurassic is named after the Jura Mountains on the border of France and Switzerland.

Cretaceous comes from the Latin for chalk, referring to the thick chalk deposits laid down across northern Europe during that period, of which the white cliffs of southern England are the best known example. Those chalk beds are made largely of the microscopic shells of marine algae, which is why the period is named for a rock formed by plankton rather than by anything to do with dinosaurs.

The periods were first recognised from the order of rock layers and the fossils they contained, long before anyone could give them ages in years. Geologists noticed that certain distinctive fossils, such as particular ammonites, appeared only in a narrow band of rock and could be used to match layers in different places. These index fossils let researchers say that one rock was older or younger than another, but not by how many millions of years. That kind of relative dating built the framework of the time scale.

The numbers came later with radiometric dating. Some minerals, especially zircon crystals in volcanic ash, contain radioactive uranium that decays into lead at a steady, known rate. Measuring the ratio of the two in a crystal gives the time since it formed. When an ash layer lies just above or below a boundary, it brackets the age of that boundary. This is why the dates for the periods are quoted with 'about' and are occasionally adjusted as new ash layers are found and measuring techniques improve.

The end of the Triassic is linked to enormous volcanic eruptions as Pangaea began to split apart, pouring out lava across regions now on both sides of the Atlantic and releasing large amounts of carbon dioxide. Many large reptile groups that had competed with dinosaurs disappeared, which helped dinosaurs take over in the Jurassic. The end of the Cretaceous is marked worldwide by a thin clay layer rich in iridium, an element rare in Earth's crust but common in asteroids. It led researchers to the Chicxulub crater in Mexico, created by a massive impact 66 million years ago.

Massive volcanic eruptions in India, known as the Deccan Traps, were also under way around that time, and scientists still debate how much they added to the impact's effects. The change from the Jurassic to the Cretaceous was much gentler, with no mass extinction on the scale of the other two boundaries. The periods also explain a famous mix-up. The film Jurassic Park features Tyrannosaurus rex, Velociraptor and Triceratops, all of which lived in the Cretaceous. Only a few of its stars, such as Brachiosaurus, truly belong to the Jurassic.

Many animals popularly called dinosaurs were not dinosaurs at all. Dinosaurs were land animals whose legs were held directly beneath the body, a posture visible in the hip socket, which is open in the middle. Pterosaurs, the flying reptiles, were close relatives but formed a separate branch. The great marine reptiles were further removed still. Ichthyosaurs looked strikingly like dolphins and gave birth to live young at sea. Plesiosaurs had broad bodies and four flipper-shaped limbs, often with very long necks. Mosasaurs, which dominated the seas only in the Late Cretaceous, were giant swimming lizards related to modern monitor lizards and snakes.

The three periods tell different stories for these groups. Ichthyosaurs appeared in the Early Triassic, flourished in the Jurassic and had disappeared well before the end of the Cretaceous, while plesiosaurs and mosasaurs lasted until the final extinction. Some of the earliest discoveries of these animals came from the Jurassic cliffs of Lyme Regis in Dorset, where the fossil collector Mary Anning found ichthyosaur and plesiosaur skeletons in the early nineteenth century, years before the word dinosaur was coined in 1842. Her finds helped strengthen the case that whole groups of animals had once lived and then become extinct.

For most of the Mesozoic the Earth was warmer than it is today, and for long stretches there were no large permanent ice sheets at the poles. That allowed dinosaurs to live at latitudes that would now be frozen for much of the year. In the Early Cretaceous, what is now Victoria in south-eastern Australia was joined to Antarctica and lay well inside the Antarctic Circle. Small plant eating dinosaurs such as Leaellynasaura, found at a coastal site called Dinosaur Cove, lived there through long winters with weeks of continuous darkness, even though the climate was mild compared with polar regions now.

In the Late Cretaceous, northern Alaska also lay far inside the Arctic Circle, and its rocks have produced duck-billed hadrosaurs, horned dinosaurs and tyrannosaurs. Palaeontologists long debated whether these animals migrated south for the winter, as caribou do today. The discovery of bones and teeth from hatchlings, and even from young still inside the egg, belonging to several dinosaur families showed that many of them nested in the far north, which makes year-round residence likely for at least some species. Those winters still brought freezing temperatures and probably snow, so these finds raise real questions about how the animals coped with cold and darkness.

The Triassic, Jurassic and Cretaceous periods are the three chapters of the dinosaur age, and separating them reveals that the most famous dinosaurs were often separated from each other by tens of millions of years.