Dinosaurs of the Triassic, Jurassic, and Cretaceous Periods (2025)

Key Words

• Archosaur: Sometimes called“ruling reptiles,” this group of ancient animals included dinosaurs and pterosaurs (flying reptiles)
• Therapsid: Agroup of ancient reptiles that later evolved to become mammals
• Sauropod: Huge long-necked, long-tailed vegetarian dinosaurs (such as the Apatosaurus)
• 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-longQuetzalcoatlus
• 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 thePermian/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 thefirst true dinosaurslikeEoraptorandHerrerasaurus. Some archosaurs, however, went in a different direction, branching out to become the first pterosaurs (Eudimorphodonbeing a good example) and a wide variety ofancestral crocodiles, some of them two-legged vegetarians. Therapsids, in the meantime, gradually shrank in size. Thefirst mammalsof 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 andNothosaurusbut also the very firstplesiosaursand a flourishing breed of "fish lizards," the ichthyosaurs. (Some ichthyosaurs attained truly gigantic sizes; for example,Shonisaurusmeasured 50 feet long and weighed in the vicinity of 30 tons!) The vastPanthalassanOcean soon found itself restocked with new species ofprehistoric 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 laterBrachiosaurus) 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 extinctionwas a fizzle compared to the earlier Permian/Triassic extinction and the laterCretaceous/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 movieJurassic Park, people identify the Jurassic period, more than any other geological timespan, as a dinosaur period. 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 ensuingCretaceousperiod.

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) andLaurasiain 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-eatingprosauropodsof the Triassic period gradually evolved into multi-tonsauropodslikeBrachiosaurusandDiplodocus. This period also saw the concurrent rise of medium- to large-sizedtheropoddinosaurs likeAllosaurusandMegalosaurus. This helps explain the evolution of the earliest, armor-bearingankylosaursand stegosaurs.

Mammals:The mouse-sizedearly mammalsof 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 ArchaeopteryxandEpidendrosaurus. The first trueprehistoric birdsmay have 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 too did themarine reptilesof the Jurassic period gradually attain shark- (or even whale-) sized proportions. The Jurassic seas were filled with fiercepliosaurslikeLiopleurodonand Cryptoclidus, as well as sleeker, less frightening plesiosaurs like Elasmosaurus. Ichthyosaurs, which dominated the Triassic period, had already begun their decline.Prehistoric fishwere abundant, as were squids andsharks, 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 advancedpterosaurslikePterodactylus,Pteranodon, andDimorphodon. Prehistoric birdshad yet to fully evolve, leaving the skies firmly under the sway of these avian reptiles (except for some prehistoric insects).

Plant Life During the Jurassic Period

Gigantic plant-eating sauropods likeBarosaurusandApatosauruscouldn’t have evolved if they didn’t have a reliable source of food. 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, asornithischianandsaurischianfamilies 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 precedingJurassicperiod, 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 includedraptors,tyrannosaurs,and other varieties of theropods, including the fleet-footedornithomimids("bird mimics"), the strange, featheredtherizinosaurs, and an uncountable profusion of small,feathered dinosaurs, among them the uncommonly intelligentTroodon.

The classic herbivorous sauropodsof 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) likeStyracosaurus andTriceratopsbecame abundant, as didhadrosaurs(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-eatingankylosaursandpachycephalosaurs("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, theichthyosaurs("fish lizards") disappeared. They were replaced by viciousmosasaurs, giganticpliosaurslikeKronosaurus, and slightly smallerplesiosaurslikeElasmosaurus. A new breed of bonyfish, known as teleosts, roamed the seas in enormous schools. Finally, there was a wide assortment ofancestral 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-wingspanQuetzalcoatlusbeing the most spectacular example. This was the pterosaurs' last gasp, though, as they were gradually replaced by the first trueprehistoric birds. These early birds evolved from land-dwelling feathered dinosaurs, not pterosaurs, and were better adapted to 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 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, ameteor impacton theYucatan Peninsularaised 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 in present-day west-central India. 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.