Learn About the Different Dinosaur Periods

Prehistoric Life During the Mesozoic Era

Velociraptor
Andrew Bret Wallis/The Image Bank/Getty Images

It may come as a shock to the average dinosaur buff, but the Triassic, Jurassic, and Cretaceous periods weren't originally designated as a way to keep track of long-extinct reptiles. Rather, these vast stretches of historical time were marked out by geologists to distinguish among various types of geologic strata (chalk, limestone, etc.) laid down tens of millions of years ago. Of course, 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, you should 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.

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 in 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 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 into the very 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 shrunk in size, and 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, 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.

(A new study has shown that flowering plants appeared toward the middle of the Triassic period, 100 million years earlier than previously believed.) 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--and certainly a fizzle compared to the earlier Permian/Triassic extinction and the later Cretaceous/Tertiary (K/T) extinction--the Triassic/Jurassic extinction 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 the 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, no matter how clunky "Cretaceous Park" sounds as a summer blockbuster!

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), as well as the formation of intra-continental lakes and rivers 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, which 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. As detailed above, prehistoric birds had yet to fully evolve, leaving the skies firmly under the sway of these avian reptiles (with the exception of some pesky, buzzing 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: hence 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, though life (of course) 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 and the added twist of 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, including 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, including the uncommonly intelligent Troodon.

As for the herbivorous dinosaurs of the Cretaceous period, the classic 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 still, some mammals had enough breathing room, ecologically speaking, to allow them to evolve to respectable sizes; witness 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") vacated the scene, to be 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, and there were the usual 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 crowded out of the skies by the first true prehistoric birds (which 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 main innovation of the Cretaceous period was the rapid diversification of flowering plants, which 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 of this 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.