What Are the Triassic, Jurassic and Cretaceous Periods?
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The Triassic, Jurassic and Cretaceous periods are the three divisions of the Mesozoic era, better known as the age of dinosaurs. They span roughly 186 million years, and the animals in each are strikingly different. Knowing which period a dinosaur belongs to prevents one of the most common mistakes in prehistory.
The three periods in order
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. Each ended with a significant extinction event.
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
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.
The Jurassic
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
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
Why the periods matter
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.
How the periods were named
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 takeaway
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.