The time of the dinosaurs was called the Mesozoic era and it lasted from about 252 to 66 million years ago. The word Mesozoic means “middle life”.

The Mesozoic era was subdivided into three major periods: the Triassic, Jurassic, and Cretaceous. The Triassic period saw the rise of the reptiles, the Jurassic was the domain of the large dinosaurs, and the Cretaceous saw the advancement of dinosaurs that would eventually lead to the dawn of modern birds.

Triassic Period

Large amphibians and reptiles ruled the Triassic

Large amphibians and reptiles ruled the Triassic

Before the Triassic started, around 252 million years ago, there was a mass extinction, called the Permian–Triassic extinction event, during which approximately 90% to 96% of marine species and 70% of land vertebrates became extinct. It is also known as the “Great Dying” because it is considered the largest mass extinction in the Earth’s history. It took the Earth nearly 10 million years to recover from this event, but it paved the way for a new world. A new world for dinosaurs.

The first dinosaurs appeared during the Triassic period, but they would not totally dominate the planet until the Jurassic period later. The first true mammals also evolved during the Triassic, as well as flying reptiles called pterosaurs.

The supercontinent Pangaea, where all land masses were joined together

Pangaea, where all land masses were joined together

At this time of the Earth’s history, all of the landmasses were connected in a supercontinent called Pangaea. With all of the land connected in this way land dwelling creatures were able to travel from one end of the Earth to another, without needing to cross any oceans. By the mid-Triassic, Pangaea began to gradually rift into two separate landmasses, Laurasia to the north and Gondwana to the south.

The global climate during the Triassic was mostly hot and dry, with deserts spanning much of Pangaea’s interior. For most of this time the weather conditions were most suitable for certain types of plants and animals that enjoy a humid, hot atmosphere. Reptiles were well adept for this environment and the crocodile-like Phytosaurs and Rauisuchians reined for millions of years. Large early amphibians called Temnospondyls also shared the planet during this time, most opting to live mostly in the water after the reptiles had become more common on land.

Coelophysis, one of the first dinosaurs

Coelophysis, one of the first dinosaurs

One of the first true dinosaurs to appear was Coelophysis, a theropod dinosaur that walked on two legs, like Tyrannosaurus and Velociraptor. They were only a few feet long and hunted small prey in the shadow of the giant Rauisuchians. Coelophysis did not appear until near the end of the Triassic period, around 200 million years ago.

The Triassic period ended with a mass extinction, which was particularly severe in the oceans. Though the end-Triassic extinction event was not equally devastating everywhere on land, several important groups of large reptiles disappeared, as did most of the large amphibians, and some groups of small reptiles. Some of the early, primitive dinosaurs also went extinct, but other more adaptive dinosaurs survived to evolve in the Jurassic. Surviving plants that went on to dominate the Mesozoic world included modern conifers.

What caused this Late Triassic extinction is not known for certain. It was accompanied by huge volcanic eruptions that occurred as the supercontinent Pangaea began to break apart about 202 to 191 million years ago. These extinctions allowed the dinosaurs to expand into many niches that had become unoccupied. Dinosaurs became increasingly dominant, abundant and diverse, and remained that way for the next 150 million years.

Jurassic Period

The Jurassic period, where dinosaurs ruled the Earth

The Jurassic period, where dinosaurs ruled the Earth

The Jurassic period, from 201 to 145 million years ago, makes up the middle period of the Mesozoic Era. By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses, Laurasia to the north and Gondwana to the south. This created more coastlines and shifted the continental climate from dry to humid, and many of the arid deserts of the Triassic were replaced by lush rainforests.

The arid, continental conditions characteristic of the Triassic steadily eased during the Jurassic period. The warm, humid climate allowed lush jungles to cover much of the landscape. The Conifers in particular dominated the plant-life, as during the Triassic; they were the most diverse group and constituted the majority of large trees.

Landmasses began breaking apart in the Jurassic

Landmasses began breaking apart in the Jurassic

Crocodylians moved to the water leaving the land to be dominated by dinosaurs alone. The first birds appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. The oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, and pterosaurs were now the dominant flying animals.

The Jurassic was a golden age for the large herbivorous dinosaurs known as the sauropods—Camarasaurus, Apatosaurus, Diplodocus, Brachiosaurus, and many others—that roamed the land late in the period. They grazed upon either the prairies of ferns, palm-like cycads, or the higher coniferous growth, according to their adaptations. They were preyed upon by large theropods, such as Ceratosaurus, Megalosaurus, Torvosaurus and Allosaurus. All these belong to the ‘lizard hipped’ or saurischian branch of the dinosaurs.

Saurischian and ornithischian dinosaurs, along with the first birds

Saurischian & ornithischian dinosaurs, along with the first birds

During the Late Jurassic, the first bitrds, like Archaeopteryx, evolved from small dinosaurs. Ornithischian dinosaurs were less predominant than saurischian dinosaurs, although some, like stegosaurs, played important roles as small and medium-to-large (but not sauropod-sized) herbivores. In the air, pterosaurs were common; they ruled the skies, filling many ecological roles now taken by birds. Within the undergrowth were various types of early mammals, as well as tritylodonts, lizard-like creatures. Smaller amphibians also evolved during this period such as salamanders.

Plesiosaurs like Muraenosaurus roamed Jurassic oceans

Plesiosaurs like Muraenosaurus roamed Jurassic oceans

In the oceans during the Jurassic period, the primary vertebrates were fish and marine reptiles. The latter include ichthyosaurs, who were at the peak of their diversity, plesiosaurs, pliosaurs, and marine crocodiles. Numerous turtles could be found in lakes and rivers

Sea levels began to rise during the Jurassic, which was probably caused by an increase in seafloor spreading. The formation of new crust beneath the surface displaced ocean waters by as much as 200 m (656 ft) more than today, which flooded coastal areas. Pangaea began to rift into smaller divisions, bringing more land area in contact with the ocean by forming the Tethys Sea. Temperatures continued to increase and began to stabilize.

Cretaceous Period

The Cretaceous was the most diverse time for dinosaur life

The Cretaceous was the most diverse time for dinosaur life

The Cretaceous period is the last period of the Mesozoic Era, spanning 79 million years from 145 to 66 million years years ago. During this time dinosaurs continued to dominate on land, but new groups of mammals and birds appeared. The Cretaceous ended with a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs and large marine reptiles died out.

During the Cretaceous the supercontinent of Pangaea completed its tectonic breakup into present day continents, although their positions were substantially different at the time. The Cretaceous was a period with a relatively warm climate, resulting in high sea levels and creating numerous shallow inland seas. This was partly due to intense volcanic activity which produced large quantities of carbon dioxide. The production of large quantities of magma also caused extensional tectonics, further pushing sea levels up, so that large areas of the continental crust were covered with shallow seas. These oceans and seas were populated with now-extinct marine reptiles, ammonites and rudists (types of extinct mollusks).

The continents as we know them today started to take shape

On land, mammals were a small and still relatively minor component of the animals that existed. Early marsupial (mammals with pouches) evolved in the Early Cretaceous. During this period dinosaurs were still the dominant creatures on land and this was their most diverse stage. Pterosaurs were common in the early and middle Cretaceous, but as the Cretaceous proceeded they declined in numbers, although it is not entirely known why. Some think the competition from early birds may have played a part.

In the seas, rays, modern sharks and teleosts became common. Marine reptiles included ichthyosaurs in the early and mid-Cretaceous (becoming extinct during the late Cretaceous), plesiosaurs throughout the entire period, and mosasaurs appearing in the Late Cretaceous.

An early mammal feeding on insects of the Cretaceous

An early mammal feeding on insects of the Cretaceous

Insects diversified during the Cretaceous, and the oldest known ants, termites, akin to butterflies and moths, appeared. Aphids, grasshoppers and gall wasps appeared. Flowering plants spread during this period, although they did not become predominant until much later. Their evolution was aided by the appearance of bees, a good example of coevolution, where multiple groups evolve together. The first representatives of many leafy trees, including figs, planes and magnolias, appeared in the Cretaceous.

There was a progressive decline in biodiversity during the last part of the Cretaceous period leading up to the major extinction event that ended the reign of the dinosaurs. The impact of a meteorite or comet is widely accepted as the main reason for the Cretaceous–Paleogene extinction event. Different species went extinct at different times due to changes in the food chain. Species which depended on photosynthesis declined or became extinct because of the reduction in solar energy reaching the Earth’s surface due to atmospheric particles blocking the sunlight. Evidence suggests that herbivorous animals, which depended on plants and plankton as their food, died out as their food sources became scarce; consequently, top predators such as Tyrannosaurus rex also perished.

The impact of a meteorite or comet may have been the last straw for the dinosaurs

This event may have been the last straw for the dinosaurs

Small sea creatures and mollusks, including ammonites, rudists, freshwater snails and mussels, as well as organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, it is thought that ammonites were the principal food of mosasaurs, a group of giant marine reptiles that became extinct around this time.

Omnivores, insectivores and carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. At the end of the Cretaceous there seem to have been no purely herbivorous or carnivorous mammals. Mammals and birds which survived the extinction fed on insects, larvae, worms and snails, which in turn fed on dead plant and animal matter.

Next learn how dinosaurs lived their lives. Some dinosaurs raised their young, while others might have abandoned them like the sea turtles of today. What kind of parents were your favorite dinosaurs and what would have growing up as a dinosaur been like?