Learn About the Different Dinosaur Periods

Prehistoric Life During the Mesozoic Era

Andrew Bret Wallis/The Image Bank/Getty Images

The Triassic, Jurassic, and Cretaceous periods were marked out by geologists to distinguish among various types of geologic strata (chalk, limestone, etc.) laid down tens of millions of years ago. Since dinosaur fossils are usually found embedded in rock, paleontologists associate dinosaurs with the geologic period in which they lived—for example, "the sauropods of the late Jurassic."

To put these geologic periods in the proper context, bear in mind that the Triassic, Jurassic, and Cretaceous don't cover all of prehistory, not by a long shot. First came the Precambrian period, which stretched from the earth’s formation to about 542 million years ago. The development of multicellular life ushered in the Paleozoic Era (542–250 million years ago), which embraced shorter geologic periods including (in order) the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. It's only after all that that we reach the Mesozoic Era (250-65 million years ago), which includes the Triassic, Jurassic and Cretaceous periods.

Ages of the Dinosaurs (The Mesozoic Era)

This chart is a simple overview of the Triassic, Jurassic, and Cretaceous periods, all of which were part of the Mesozoic era. In brief, this incredibly long period of time, measured in "mya" or "millions of years ago," saw the development of dinosaurs, marine reptiles, fish, mammals, flying animals including pterosaurs and birds, and a huge range of plant life. The largest dinosaurs did not emerge until the Cretaceous period, which started over 100 million years after the start of the "age of dinosaurs."

Period Land Animals Marine Animals Avian Animals Plant Life
Triassic 237–201 mya

Archosaurs ("ruling lizards");

therapsids ("mammal-like reptiles")

Plesiosaurs, ichthyosaurs, fish Cycads, ferns, Gingko-like trees, and seed plants
Jurassic 201–145 mya

Dinosaurs (sauropods, therapods);

Early mammals;

Feathered dinosaurs

Plesiosaurs, fish, squid, marine reptiles


Flying insects

Ferns, conifers, cycads, club mosses, horsetail, flowering plants
Cretaceous 145–66 mya

Dinosaurs (sauropods, therapods, raptors, hadrosaurs, herbivorous ceratopsians);

Small, tree-dwelling mammals

Plesiosaurs, pliosaurs, mosasaurs, sharks, fish, squid, marine reptiles


Flying insects;

Feathered birds

Huge expansion of flowering plants

Key Words

  • Archosaur: Sometimes called “ruling reptiles,” this group of ancient animals included dinosaurs and pterosaurs (flying reptiles)
  • Therapsid: A group of ancient reptiles that later evolved to become mammals
  • Sauropod: Huge long-necked, long-tailed vegetarian dinosaurs (such as the Apatosaur)
  • Therapod: Two-legged carnivorous dinosaurs, including raptors and Tyrannosaurus Rex
  • Plesiosaur: Long-necked marine animals (often described as similar to the Loch Ness monster)
  • Pterosaur: Winged flying reptiles that ranged from the size of a sparrow to the 36-foot-long Quetzalcoatlus
  • Cycad: Ancient seed plants that were common during the time of the dinosaurs and are still common today

The Triassic Period

At the start of the Triassic period, 250 million years ago, the Earth was just recovering from the Permian/Triassic Extinction, which witnessed the demise of over two-thirds of all land-dwelling species and a whopping 95 percent of ocean-dwelling species. In terms of animal life, the Triassic was most notable for the diversification of archosaurs into pterosaurs, crocodiles, and the earliest dinosaurs, as well as the evolution of therapsids into the first true mammals.

Climate and Geography During the Triassic Period 

During the Triassic period, all of the Earth's continents were joined together into a vast, north-south landmass called Pangaea (which was itself surrounded by the enormous ocean Panthalassa). There were no polar ice caps, and the climate at the equator was hot and dry, punctuated by violent monsoons. Some estimates put the average air temperature across most of the continent at well above 100 degrees Fahrenheit. Conditions were wetter in the north (the part of Pangaea corresponding to modern-day Eurasia) and the south (Australia and Antarctica).

Terrestrial Life During the Triassic Period

The preceding Permian period was dominated by amphibians, but the Triassic marked the rise of the reptiles—notably the archosaurs ("ruling lizards") and therapsids ("mammal-like reptiles"). For reasons that are still unclear, the archosaurs held the evolutionary edge, muscling out their "mammal-like" cousins and evolving by the middle Triassic into the first true dinosaurs like Eoraptor and Herrerasaurus. Some archosaurs, however, went in a different direction, branching out to become the first pterosaurs ( Eudimorphodon being a good example) and a wide variety of ancestral crocodiles , some of them two-legged vegetarians. Therapsids, in the meantime, gradually shrank in size. The first mammals of the late Triassic period were represented by small, mouse-sized creatures like Eozostrodon and Sinoconodon.

Marine Life During the Triassic Period

Because the Permian Extinction depopulated the world's oceans, the Triassic period was ripe for the rise of early marine reptiles. These included not only unclassifiable, one-off genera like Placodus and Nothosaurus but the very first plesiosaurs and a flourishing breed of "fish lizards," the ichthyosaurs. (Some ichthyosaurs attained truly gigantic sizes; for example, Shonisaurus measured 50 feet long and weighed in the vicinity of 30 tons!) The vast Panthalassan Ocean soon found itself restocked with new species of prehistoric fish, as well as simple animals like corals and cephalopods.

Plant Life During the Triassic Period

The Triassic period wasn't nearly as lush and green as the later Jurassic and Cretaceous periods, but it did see an explosion of various land-dwelling plants, including cycads, ferns, Gingko-like trees, and seed plants. Part of the reason there were no plus-sized Triassic herbivores (along the lines of the much later Brachiosaurus) is that there simply wasn’t enough vegetation to nourish their growth.

The Triassic/Jurassic Extinction Event

Not the most well-known extinction event, the Triassic/Jurassic extinction was a fizzle compared to the earlier Permian/Triassic extinction and the later Cretaceous/Tertiary (K/T) extinction. The event, nevertheless, witnessed the demise of various genera of marine reptiles, as well as large amphibians and certain branches of archosaurs. We don't know for sure, but this extinction may have been caused by volcanic eruptions, a global cooling trend, a meteor impact, or some combination thereof. 

The Jurassic Period

Thanks to the movie Jurassic Park, people identify the Jurassic period, more than any other geological time span, with the age of dinosaurs. The Jurassic is when the first gigantic sauropod and theropod dinosaurs appeared on Earth, a far cry from their slender, man-sized ancestors of the preceding Triassic period. But the fact is that dinosaur diversity reached its peak in the ensuing Cretaceous period.

Geography and Climate During the Jurrasic Period 

The Jurassic period witnessed the breakup of the Pangaean supercontinent into two big pieces, Gondwana in the south (corresponding to modern-day Africa, South America, Australia, and Antarctica) and Laurasia in the north (Eurasia and North America). At about the same time, intra-continental lakes and rivers formed that opened new evolutionary niches for aquatic and terrestrial life. The climate was hot and humid, with steady rainfall, ideal conditions for the explosive spread of lush, green plants.

Terrestrial Life During the Jurassic Period

Dinosaurs: During the Jurassic period, relatives of the small, quadrupedal, plant-eating prosauropods of the Triassic period gradually evolved into multi-ton sauropods like Brachiosaurus and Diplodocus. This period also saw the concurrent rise of medium- to large-sized theropod dinosaurs like Allosaurus and Megalosaurus. This helps explain the evolution of the earliest, armor-bearing ankylosaurs and stegosaurs.

Mammals: The mouse-sized early mammals of the Jurassic period, only recently evolved from their Triassic ancestors, kept a low profile, scurrying around at night or nesting high up in trees so as not to get squashed under the feet of bigger dinosaurs. Elsewhere, the first feathered dinosaurs began to appear, typified by the extremely bird-like  Archaeopteryx and Epidendrosaurus. It's possible that the first true prehistoric birds had evolved by the end of the Jurassic period, though the evidence is still sparse. Most paleontologists believe that modern birds descend from the small, feathered theropods of the Cretaceous period.

Marine Life During the Jurassic Period

Just as dinosaurs grew to bigger and bigger sizes on land, so the marine reptiles of the Jurassic period gradually attained shark- (or even whale-) sized proportions. The Jurassic seas were filled with fierce pliosaurs like Liopleurodon and Cryptoclidus, as well as sleeker, less frightening plesiosaurs like  Elasmosaurus. Ichthyosaurs, which dominated the Triassic period, had already begun their decline. Prehistoric fish were abundant, as were squids and sharks, providing a steady source of nourishment for these and other marine reptiles.

Avian Life During the Jurassic Period

By the end of the Jurassic period, 150 million years ago, the skies were filled with relatively advanced pterosaurs like PterodactylusPteranodon, and Dimorphodon. Prehistoric birds had yet to fully evolve, leaving the skies firmly under the sway of these avian reptiles (with the exception of some prehistoric insects).

Plant Life During the Jurassic Period

Gigantic plant-eating sauropods like Barosaurus and Apatosaurus couldn’t have evolved if they didn’t have a reliable source of food. In fact, the landmasses of the Jurassic period were blanketed with thick, tasty coats of vegetation, including ferns, conifers, cycads, club mosses, and horsetails. Flowering plants continued their slow and steady evolution, culminating in the explosion that helped fuel dinosaur diversity during the ensuing Cretaceous period.

The Cretaceous Period

The Cretaceous period is when dinosaurs attained their maximum diversity, as ornithischian and saurischian families branched off into a bewildering array of armored, raptor-clawed, thick-skulled, and/or long-toothed and long-tailed meat- and plant-eaters. The longest period of the Mesozoic Era, it was also during the Cretaceous that the Earth began to assume something resembling its modern form. At that time,  life was dominated not by mammals but by terrestrial, marine and avian reptiles.

Geography and Climate During the Cretaceous Period

During the early Cretaceous period, the inexorable breakup of the Pangaean supercontinent continued, with the first outlines of modern North and South America, Europe, Asia and Africa taking shape. North America was bisected by the Western Interior Sea (which has yielded countless fossils of marine reptiles), and India was a giant, floating island in the Tethys Ocean. Conditions were generally as hot and muggy as in the preceding Jurassic period, albeit with intervals of cooling. The era also saw rising sea levels and the spread of endless swamps—yet another ecological niche in which dinosaurs (and other prehistoric animals) could prosper.

Terrestrial Life During the Cretaceous Period

Dinosaurs: Dinosaurs really came into their own during the Cretaceous Period. Over the course of 80 million years, thousands of meat-eating genera roamed the slowly separating continents. These included raptorstyrannosaurs and other varieties of theropods, including the fleet-footed ornithomimids ("bird mimics"), the strange, feathered therizinosaurs, and an uncountable profusion of small, feathered dinosaurs, among them the uncommonly intelligent Troodon.

The classic herbivorous sauropods of the Jurassic period had pretty much died out, but their descendants, the lightly armored titanosaurs, spread to every continent on earth and attained even more massive sizes.  Ceratopsians (horned, frilled dinosaurs) like Styracosaurus and Triceratops became abundant, as did hadrosaurs (duck-billed dinosaurs), which were especially common at this time, roaming the plains of North America and Eurasia in vast herds. Among the last dinosaurs standing by the time of the K/T Extinction were the plant-eating ankylosaurs and pachycephalosaurs ("thick-headed lizards").

Mammals: During most of the Mesozoic Era, including the Cretaceous period, mammals were sufficiently intimidated by their dinosaur cousins that they spent most of their time high up in trees or huddling together in underground burrows. Even so, some mammals had enough breathing room, ecologically speaking, to allow them to evolve to respectable sizes. One example was the 20-pound Repenomamus, which actually ate baby dinosaurs.

Marine Life During the Cretaceous Period

Shortly after the beginning of the Cretaceous period, the ichthyosaurs ("fish lizards") disappeared.  They were replaced by vicious mosasaurs, gigantic pliosaurs like Kronosaurus, and slightly smaller plesiosaurs like Elasmosaurus. A new breed of bony fish, known as teleosts, roamed the seas in enormous schools. Finally, there was a wide assortment of ancestral sharks; both fish and sharks would benefit immensely from the extinction of their marine reptile antagonists.

Avian Life During the Cretaceous Period

By the end of the Cretaceous period, pterosaurs (flying reptiles) had finally attained the enormous sizes of their cousins on land and in the sea, the 35-foot-wingspan Quetzalcoatlus being the most spectacular example. This was the pterosaurs' last gasp, though, as they were gradually replaced by the first true prehistoric birds. These early birds evolved from land-dwelling feathered dinosaurs, not pterosaurs, and were better adapted for changing climatic conditions.

Plant Life During the Cretaceous Period

As far as plants are concerned, the most important evolutionary change of the Cretaceous period was the rapid diversification of flowering plants. These spread across the separating continents, along with thick forests and other varieties of dense, matted vegetation. All of this greenery not only sustained the dinosaurs, but it also allowed the co-evolution of a wide variety of insects, especially beetles.

The Cretaceous-Tertiary Extinction Event

At the end of the Cretaceous period, 65 million years ago, a meteor impact on the Yucatan Peninsula raised huge clouds of dust, blotting out the sun and causing most vegetation to die out. Conditions may have been aggravated by the collision of India and Asia, which fueled an immense amount of volcanic activity in the "Deccan Traps." The herbivorous dinosaurs that fed on these plants died, as did the carnivorous dinosaurs that fed on the herbivorous dinosaurs. The way was now clear for the evolution and adaptation of the dinosaurs' successors, the mammals, during the ensuing Tertiary period.

mla apa chicago
Your Citation
Strauss, Bob. "Learn About the Different Dinosaur Periods." ThoughtCo, Sep. 8, 2021, thoughtco.com/the-three-ages-of-dinosaurs-1091932. Strauss, Bob. (2021, September 8). Learn About the Different Dinosaur Periods. Retrieved from https://www.thoughtco.com/the-three-ages-of-dinosaurs-1091932 Strauss, Bob. "Learn About the Different Dinosaur Periods." ThoughtCo. https://www.thoughtco.com/the-three-ages-of-dinosaurs-1091932 (accessed May 30, 2023).