The Three Ages of Dinosaurs - The Triassic, Jurassic, and Cretaceous Periods
Prehistoric Life During the Mesozoic Era
By Bob Strauss, About.com Guide
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 the Cambrian, Devonian and Carboniferous. It's only then that we reach the Mesozoic Era (250-65 million
years ago), which embraces the Triassic, Jurassic and Cretaceous periods.
The Triassic Period (250-208 Million Years Ago)
Prehistoric Life During the Triassic Period
Climate and Geography During the Triassic period, all of the earth's continents were joined together into a vast landmass called
Pangaea (which was itself surrounded by an enormous ocean called Panthalassa). There were as yet no polar ice caps, and the climate
was hot and dry, punctuated by violent monsoons.
Terrestrial Life The start of the Triassic period followed the Permian Extinction, an event of unknown cause that wiped out over twothirds of land-dwelling vertebrates and 95 percent of ocean-dwelling species. Taking their place were the therapsids (mammal-like
reptiles), archosaurs (from which the first dinosaurs like Herrerasaurus and Eoraptor evolved, as well as the first prehistoric crocodiles
and pterosaurs), and primitive reptiles called pelycosaurs (the most famous of which was Dimetrodon). It was during the late Triassic
period that the mammal-like reptiles evolved into the first mammals.
Marine Life Because the Permian Extinction depopulated the world's oceans, the Triassic period was ripe for the rise of early marine
reptiles like Placodus and Nothosaurus. The vast Panthalassan Ocean was soon restocked with new species of prehistoric fish, as well
as simple animals like corals and cephalopods.
Avian Life Except for insects, the skies of the early Triassic period were relatively quiet. It was only toward the end of the Triassic
that the earliest pterosaurs, such as Eudimorphodon, appeared on the scene, and they probably weren't the most accomplished fliers.
Plant Life 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, and Gingko-like trees. Part of the reason there were no enormous dinosaurs
during the Triassic (along the lines of Brachiosaurus) is that there simply wasn’t enough vegetation to nourish them!
The Jurassic Period (208-144 Million Years Ago)
Prehistoric Life During the Jurassic Period
Climate and Geography The Jurassic period witnessed the breakup of the Pangaean supercontinent into two big pieces, Gondwana in
the south and Laurasia2 in the north, as well as the formation of intra-continental lakes and rivers that opened new evolutionary niches
for aquatic and terrestrial animals. 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, relatives of the small, quadrupedal, plant-eating prosauropods of the Triassic period
gradually evolved into gigantic sauropods like Brachiosaurus and Diplodocus. This period also saw the rise of medium- to large-sized
theropod dinosaurs like Allosaurus and Megalosaurus, which helps explain the evolution of the earliest ankylosaurs (armored
herbivorous dinosaurs). The Jurassic period was also the heyday of the stegosaurs, typified by Stegosaurus.
The first, mouse-sized early mammals of the Jurassic period kept a low profile, scurrying around at night so as not to get squashed
under the feet of bigger dinosaurs. Elsewhere, the first small theropod dinosaurs began to evolve feathers, culminating in
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.
Marine Life 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 swimmers like Plesiosaurus. Prehistoric fish were abundant, as were squids and prehistoric sharks,
providing a steady source of nourishment for these and other marine reptiles.
Avian Life By the end of the Jurassic period, the skies were filled with relatively advanced pterosaurs like Pterodactylus, Pteranodon
and Dimorphodon. As stated above, 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 Gigantic herbivorous dinosaurs like Barosaurus and Apatosaurus couldn’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.
The Cretaceous Period (144-65 Million Years Ago)
Prehistoric Life During the Cretaceous Period
Climate and Geography 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. Conditions were as hot and muggy as in
the Jurassic, with the added twist of rising sea levels and the spread of endless swamps--yet another ecological niche in which
dinosaurs (and other prehistoric life) could prosper.
Terrestrial Life It was during the Cretaceous period that dinosaurs really came into their own. Thousands of genera roamed the
slowly separating continents, including raptors and tyrannosaurs. There were also other varieties of theropods, including the fleetfooted 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. 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 left standing by the time of the K/T
Extinction were the plant-eating ankylosaurs, notably Ankylosaurus and Euoplocephalus, and the pachycephalosaurs ("thick-headed
lizards"). The early mammals still kept to themselves; they would only emerge from the shadows after the dinosaurs had gone extinct.
Marine Life Shortly after the beginning of the Cretaceous period, the ichthyosaurs ("fish lizards") vacated the scene, to be replaced by
mosasaurs, gigantic pliosaurs like Kronosaurus, and slightly smaller plesiosaurs like Elasmosaurus. A new breed of bony prehistoric
fish, known as teleosts, roamed the seas in enormous schools, and there were the usual assortment of prehistoric sharks; both fish and
sharks would benefit immensely from the extinction of the marine reptiles.
Avian Life By the end of the Cretaceous period, the pterosaurs (flying reptiles) had finally attained the enormous sizes of their cousins
on land and in the sea, Quetzalcoatlus being the most spectacular example. This was the pterosaurs' last gasp, though, as they were
gradually crowded out of the sky by the first true prehistoric birds (which evolved from land-dwelling feathered dinosaurs, not
pterosaurs).
Plant Life The main innovation of the Cretaceous period was the evolution of flowering plants, which spread across the separating
continents, along with thick forests and other kinds of dense, matted vegetation; this is also when grass first started to appear on the
scene. 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. The herbivorous dinosaurs that fed on the plants died, as did the
carnivorous dinosaurs that fed on the herbivorous dinosaurs. The way was now clear for the evolution of mammals.
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