Mesozoic Era: When Dinosaurs Ruled The World

The Mesozoic era was an exceptionally busy time in earth’s history. It was the era of many firsts. Flowers blossomed. Birds took flight. Dinosaurs dominated the land before a brutal die-off. How did life recover after the worst mass extinction the world has ever known?

Timeline and Major Evolutionary Events

Mesozoic or “middle life” is the era between the Paleozoic and the Cenozoic. This era represents the transition of life from the ancient world to the modern world. Spanning 185 million years, geologists divide this era into three periods: the Triassic, Jurassic, and Cretaceous.

Two of the largest mass extinctions in history marked the beginning and end of the Mesozoic era. These extinction events opened niches for the evolution and diversification of new groups of organisms.

Permian-Triassic Mass Extinction (Great Dying)

Other names:

  • End-Permian extinction
  • P-Tr mass extinction
  • P-T extinction

Based on fossil records, our planet underwent five major mass extinction events. The Great Dying was the most devastating of them all. In as little as 100,000 years, almost all life on earth met its demise. Which is rather abrupt on a geologic time scale. By contrast, it took roughly 30 million years for the earth to fully recover from the catastrophe.

What caused the extinction?

The exact cause of the End-Permian extinction is uncertain. The prevailing hypothesis is that a colossal volcanic eruption in what is now Siberia was the main cause. Some 720,000 cubic miles of lava flooded the Siberian Traps and a hundred billion tons of carbon were released—a million times greater than any eruption ever witnessed by humans.

Volcanic Eruption

Large amounts of methane, fluorine, and chlorine may have been released into the atmosphere, damaging the ozone layer. All of these likely triggered global warming, acid rain, and marine oxygen depletion. The perfect recipe for ecological collapse. 

Which organisms went extinct?

According to some estimates, 96% of all marine animals and 70% of all terrestrial animals became extinct. Nearly 33% of insects perished. 

The death of tabulose and rugose corals halted reef-building for 14 million years. All 20,000 species of trilobites (relatives of lobsters and crabs) were lost. Large groups of gastropods (snails and slugs) were also hard hit. 

Among vertebrates, most members of the group of giant amphibians were wiped out. From the once-abundant therapsids (mammal-like reptiles), only two lineages were spared. Many reptiles went extinct, but the ones that survived would eventually give rise to the dinosaurs.

Triassic Period (251–200 million years ago)

The Triassic period was a time for recovering and rebuilding the ecosystems ravaged by the end-Permian event. If you could travel back in time, you would find a world with unfamiliar geography.

All the continents were still grouped together, forming the giant landmass called Pangaea. The supercontinent straddled the equator from pole to pole and was surrounded by the Panthalassa ocean, which is now the Pacific.

Being far from the ocean, large parts of Pangaea’s interior was hot and dry. Those farther from the equator may have had a highly seasonal climate, with very hot summers and rather cold winters. There were no ice caps on either pole, causing global sea levels to reach extreme heights.

Life in the Triassic Period

The start of the Middle Triassic until the end of the period proved to be beneficial for life. Ecosystems began to thrive once again. New species of plants and animals appeared, many of which are still with us today. 

  • Gymnosperms (cycads, conifers, and ginkgos) and ferns were the dominant vegetation.
  • There were no grasses and flowering plants.
  • The abundant green and red algae of the Permian period were replaced by the blue-green algae (cyanobacteria).
  • Ammonites (shelled cephalopods related to nautilus) were reduced to a single lineage.
  • The first true corals appeared in the Middle Triassic.
  • Marine reptiles were the dominant predators in the sea.
  • The earliest known mammals known as morganucodonts evolved; they had large brains and shrew-like forms.
  • The first squamates emerged; the group that gave rise to modern lizards and snakes
  • Pterosaurs (the first flying reptiles) terrorized the skies.
  • The first dinosaurs appeared.

The Age of Reptiles

The synapsids, which later evolved into mammals, dominated the Permian period. As they began to decline following the Permian-Triassic extinction, the archosaurs or “ruling reptiles” took over and became the dominant land animals.

The Archosaurs are divided into two groups:

Pseudosuchia

Crocodiles, Alligators, and their extinct relatives

Avemetatarsalia

Non-Avian Dinosaurs, Pterosaurs, and birds

Pseudosuchians (false crocodiles) flourished during the Triassic period. This group included giant, four-footed apex predators such as Saurosuchus, Fasolasuchus, and Prestosuchus, 

Saurosuchus was a giant crocodilian that dominated the land in the Late Triassic and likely could have fed on dinosaurs and all other animals that crossed its path.

Triassic reptiles lived in various habitats and diversified rapidly. The pterosaurs or flying reptiles ruled the air and the shark-like ichthyosaurs the seas. 

Dinosaurs in the Triassic Period

The first dinosaurs appeared in the Late Triassic, but they were not the fearsome predators you see in the movies and other popular media. In fact, they were rather small, walked on two legs, and many were preyed on by giant reptiles. By the end of the Triassic, some herbivorous dinosaurs reached enormous sizes, ushering in the age of dinosaurs.

Dinosaurs in the triassic period,
Photo Credit: Wikimedia Commons

The earliest known dinosaurs

Eoraptor (228 million years ago)

Also referred to as the “dawn raptor,” Eoraptor was a small carnivore with many razor-sharp teeth. It had five-fingered hands, a feature consistent with the earliest dinosaur ancestors. 

Herrerasaurus (228 million years ago)

One of the most primitive dinosaurs, the Herrerasaurus had powerful hind limbs and a long tail held straight for balance. As an agile hunter, it had short front limbs designed to capture prey.

Plateosaurus (214-204 million years ago)

The first giant herbivore and was also one of the most common dinosaurs of the late Triassic period. This “broad lizard” ate leaves from high vegetation and may have roamed in herds.

Coelophysis (213-190 million years ago)

Stretching no more than 10 feet from nose to tail, this skilled carnivore was one of the earliest dinosaurs to be discovered. Many of its skeletons were excavated in New Mexico.

Triassic – Jurassic Extinction 

Other names:

  • End-Triassic extinction
  • T-J extinction

The Triassic ended the way it started, with a biological catastrophe. By this time, the huge continent Pangaea started to break into two separate land masses, forming Gondwana in the south and Laurasia in the north. Scientists believe that the end-Triassic extinction set the stage for dinosaurs to dominate the Jurassic and Cretaceous.

What caused the extinction?

Similar to the end-Permian event, several studies lay the blame at the feet of massive volcanism and the large-scale release of carbon dioxide into the atmosphere. Huge amounts of lava flowed in the Central Atlantic Magmatic Province (CAMP), the biggest continental large igneous province (LIP) in the world. The well-preserved lava flow fields can be seen today in Morocco, Brazil. 

Which organisms became extinct?

While not as deadly as the Great Dying, the end of the Triassic saw a significant reduction of marine and terrestrial organisms. Over 76% of life on earth was wiped out. 

Many types of ichthyosaurs, the top aquatic predator of the Triassic, went extinct. And the few surviving species never regain their former glory. Among the therapsids (mammal-like reptiles), only the cynodonts survived. Most of the groups under Archosauria went extinct.

Jurassic Period (200–145 million years ago)

We can think of the past extinctions as a sort of reset button. Although large swathes of life on the planet were destroyed, a second chance was given to the less dominant species. The death of the top predators at the end of the Triassic wiped the slate clean for many of these “underdogs.”

Jurassic Period
Photo Credit: Wikimedia Commons

The dry climate of the Triassic was replaced by a warm, moist subtropical climate that brought forth an explosion of new life forms. The world was once again ripe for the opportunists, such as the utterly remarkable dinosaurs. They became so diverse and well-adapted that they occupied all ecological niches on land.

Life in the Jurassic Period

During the Jurassic period, surviving organisms diversified almost immediately, establishing renewed and abundant ecosystems. The dinosaurs began to rule the land and some of the well-known prehistoric creatures first emerged.

  • Gymnosperms became more diverse, with conifers as the dominant land plant. 
  • The most diverse family (Polypodiceae) of modern ferns emerged.
  • Pterosaurs increased in number and became more diverse.
  • Crocodylomorphs were the only surviving group of pseudosuchians.
  • Plesiosaurus emerged in the Jurassic oceans, a marine reptile that gave birth to live young instead of laying eggs.
  • The first snakes, lizards, and turtles appeared.
  • The first birds evolved from theropod dinosaurs.
  • Mammals diversified but remained small.
  • Aquatic life saw the first shrimps, lobsters, and crabs.

Dinosaurs in the Jurassic Period

About 28% of the known dinosaur genera are from the Jurassic, and only 7% are from the Triassic. Many have grown into such monstrous sizes, feasting on both meat and vegetation. 

Stegosaurus (170-100 million years ago)

Large herbivorous dinosaur of the Middle Jurassic with a flexible, spiked-tipped tail as a defensive weapon. It also had bony back plates to ward off taller predators.

Brachiosaurus (150-145 million years ago)

One of the largest dinosaurs with an extremely long neck. An adult brachiosaurus was about 50 feet tall and 85 feet long. 

Allosaurus (156-144 million years ago)

A fearsome predator of North America and part of Europe. With its serrated teeth, massive jaws, curved claws, and powerful leg muscles, it could grab and kill almost anything.

Diplodocus (155-145 million years ago)

A plant-eating dinosaur with a 26-foot-long neck, which allowed it to feed on tree foliage. 

Apatosaurus (150 million years ago)

A large herbivore of the Late Jurassic. The biggest weighed as much as four elephants. It has a long neck typical of all sauropods.

Camarasaurus (150-140 million years ago)

A resident of the Jurassic woodland, this herbivore ate soft plants, twigs, and branches. Its neck was shorter than its sauropod cousins.

Cretaceous Period (145–66 million years ago)

Of the three Mesozoic periods, the Cretaceous was undoubtedly the golden age of evolution. It was a time of massive reorganization or “upgrading” of ecosystems, both on land and marine realms. 

Cretaceous Period
Photo Credit: Wikimedia Commons

The rise of angiosperms (flowering plants) contributed greatly to the dramatic transformation of the earth’s biodiversity and landscapes. As flowers evolved, so did insects, bees, birds, and other land-dwelling animals. 

Evolutionary biologists call this crucial period the Great Divergence, previously called the Cretaceous Terrestrial Revolution. Marine life was also hugely successful, diversifying and evolving spectacular adaptations. 

Life in the Cretaceous Period

The movement of continents and rising sea levels created a range of new environments, which accelerated the evolution of new species. Animal life developed a staggering diversity; thanks to new habitats and food sources.  

  • Angiosperms (flowering plants) first appeared while the gymnosperms declined.
  • Grasses appeared at the end of the Cretaceous.
  • Teleost (the most common type of fish today) displaced holosteans (primitive bony fishes).
  • Early relatives of modern mammals adopted different lifestyles and diversified into all three of their current groups: monotremes, marsupials, and placentals.
  • Modern types of sharks emerged.
  • Common marine invertebrates were sea urchins, sponges, ammonites, belemnites (relatives of squids), and rudist bivalves (horn-shaped clams).
  • Many snails and crabs became land-dwellers.

Dinosaurs in the Cretaceous Period

The Cretaceous period was the heyday of dinosaurs. Roughly 65 % of known dinosaur genera lived in this period. The king of the dinosaurs, Tyrannosaurus rex, evolved during the Late Cretaceous. 

Microraptor (130-125 million years ago)

One of the smallest known dinosaurs, this four-winged hunter can glide through the air like a bird. 

Deinonychus (115-108 million years ago)

An agile carnivore that lived in the subtropical swamps and forests of the Early Cretaceous. Its long, curved claws were best suited for capturing prey.

Argentinosaurus (90 million years ago)

The largest herbivore that weighed about 100 tons. This colossal dinosaur ate 235–500 lb every day.

Velociraptor (74-70 million years ago)

A small but fierce predator that evolved into a skilled pack-hunter. Research suggests that this carnivore may have had feathers.

Triceratops (70-66 million years ago)

The largest member of the group of horned dinosaurs called ceratopsians. This tough-looking dinosaur only ate forest plants. 

Tyrannosaurus (67-65 million years ago)

No other dinosaur species is more iconic than the T.rex. It was the largest, two-legged carnivore to have ever lived. It can gobble up to 500 pounds of meat in one bite.

The age of dinosaurs had been pretty much clear sailing for more than 160 million years. It seemed as if it would never end. And then something happened at the end of the Cretaceous, a sudden mass death.

Cretaceous-Tertiary Extinction

Other names:

  • Cretaceous–Paleogene extinction
  • End-Cretaceous mass extinction
  • K–Pg mass extinction 
  • K-T extinction

Possibly the most compelling of all mass extinction events in history is the one that brought an end to the dinosaurs, at least to the non-avian ones. Species emerge and go extinct all the time. But it’s hard to wrap your head around why an animal as diverse and sophisticated as dinosaurs would be wiped out from the face of the earth.

What caused the extinction?

Researchers from different fields have proposed a multitude of theories for the mysterious disappearance of the dinosaurs, from the entirely possible to the downright outrageous (just google dinosaur constipation). The general consensus is that it was caused by a 6-mile-wide asteroid that struck the earth around 65 million years ago. 

The impact caused a gigantic crater on what is now the Yucatan Peninsula in the Gulf of Mexico. The Chicxulub Crater is about 180 km in diameter and 20 km deep. Much of it is underwater and is buried under a thick layer of limestone. 

Which organisms became extinct?

The dramatic extinction ended the Age of Dinosaurs and set life in a new direction, but dinosaurs were not the only casualties. Around 50% of all species disappeared. The extinction was most severe among large animals. It was said that no land-dwelling animal larger than a dog survived the global crisis.

All non-bird dinosaurs, from the mighty Tyrannosaurus to the tiny Microraptor, had been driven to extinction. Even the flying reptiles disappeared. In the ocean, ammonites, belemnites, and the remaining giant marine reptiles went extinct. 

Tectonic Evolution Through the Mesozoic Era

You may not notice it but the ground beneath your feet is constantly moving. This movement is called continental drift. The solid sections of the earth’s surface are called plates. These plates float on the molten layer beneath them and carry the continents along with them.

When plates move toward each other, it can cause earthquakes and tsunamis. But these movements can also alter the earth’s structure by creating volcanoes, islands, and glorious mountain ranges. The Theory of Plate Tectonics explains this phenomenon. 

The Breakup of Pangaea

At the start of the Mesozoic Era, all land areas were connected into a single landmass that we call Pangaea. No one knows what exactly caused the separation that led to the current seven continents. One possible cause was the strong heat flow from the earth’s mantle. Pangaea’s large mass trapped the heat underneath, causing the magma to surface and create a volcanic rift.

Pangaea
Photo Credit: Wikimedia Commons

During the Late Triassic to Early Jurassic, Pangaea began to separate, first dividing into two landmasses. 

  • Gondwana: the southern landmass which included Africa, India, Australia, South America, and Antarctica.
  • Laurasia: the northern landmass which included Europe, Asia, and North America.

Further fragmentation of these landmasses began in the Middle Jurassic. The widening Tethys Sea pushed between the two halves of Pangaea. Once the division was complete, the land organisms were no longer able to move to the other, giving rise to the divergence of species.

During the Middle Jurassic, North America separated from Eurasia and Gondwana. Africa began to split from South America to form the South AtlanticEvol Ocean. India split off from Antarctica. Near the end of the Cretaceous, South America moved northwestward, and Madagascar separated from Africa. 

At the end of the Mesozoic, the continents had shifted closer to their present locations. The breakup of Pangaea created the early Atlantic, Arctic, and Indian Oceans.

Mesozoic Era Climate

Pangaea’s climate was generally dry and highly seasonal. At the heart of the giant landmass was a vast desert. Paleontologists observe that fossils are scarce in Pangaea’s interior. The high temperature made it nearly uninhabitable. But why was the supercontinent so hot? 

Pangaea was a huge continent and its center was far from the Panthalassa Ocean. Large bodies of water help stabilize the temperature in nearby areas. Water can do this because it has a relatively higher heat capacity than land and the atmosphere, meaning it can absorb more heat from the sun and distribute it evenly across the region. 

Not all of Pangaea was scorching. Areas closer to the coasts were likely to have had the same temperature in today’s tropics.

The Jurassic experienced a slightly milder climate than the Triassic. Temperature continued to increase before it stabilized. There was a shift toward more humid conditions. Owing to the warm and wet climate, lush forests unfurled and the deserts retreated. 

The last period of the Mesozoic era enjoyed a slightly cooler climate, although it was still warmer and more humid than today. The global sea level was higher, submerging the continents and creating shallow seas. The flooding of the coastlines shrank all continents. Only 18% of the Cretaceous world was covered by land, compared with 29% today.

Mesozoic Era Plants

Terrestrial vegetation during the early Mesozoic was dominated by ferns, cycads, and ginkgos. Spermatophytes also referred to as seed plants first emerged in the Devonian period. They are divided into two groups, gymnosperms (“naked seeds”) and angiosperms (“seed in a vessel”).

Gymnosperms, including conifers, first evolved in their current forms in the Early Triassic period, but their first appearance stretches back into the Permian. Glossopteris was a group of woody plants that became a dominant flora in Gondwana but went extinct by the end of the Triassic.  

Plant life changed dramatically throughout the entire span of the Cretaceous. The first angiosperms had appeared and began to diversify, competing for space everywhere. All flowering plants are angiosperms, but so are the popular hardwood trees such as oak, maple, and ivy. 

Angiosperms ultimately toppled the dominance of gymnosperms by the Late Cretaceous. Ferns and mosses remained the aristocrats of the undergrowth.

Other Animals of the Mesozoic Era

Although there were more than a thousand species of dinosaurs that lived in the Mesozoic, not every gigantic animal with weird names was a member of the “dinosaur club.” It is easy to forget that dinosaurs coexisted with other spectacular prehistoric animals. Some of the Mesozoic animals that are often confused with dinosaurs are not even reptiles. 

Pterodactyl

Once and for all, pterodactyls were not dinosaurs. They were a type of flying reptile from the group called pterosaurs. While dinosaurs and pterosaurs shared a common ancestor, they evolved into separate groups. The same reason why you don’t call yourself a camel even though you are technically a mammal. All pterosaurs went extinct. But we still have the modern-day descendants of dinosaurs–the birds! 

Quetzalcoatlus

Just like pterodactyls, quetzalcoatlus was a pterosaur. It is known as the largest flying reptile with a wingspan of about 36 ft long, or the length of a school bus. By comparison, the largest bird by wingspan today is the wandering albatross, with a maximum recorded wingspan of 12.1 ft.

Plesiosaurus

Plesiosaurus roamed the world between the Early and Middle Jurassic. This marine reptile had a long neck, a small head, as well as a short and wide body typical of plesiosaurs. Its anatomy was designed for flexibility rather than speed. Plesiosaurus had four paddle-shaped flippers adapted for limb-propelled swimming. They were not dinosaurs. The clue here is swimming. With the exception of the Spinosaurus, dinosaurs couldn’t swim.

Ichthyosaurs

When dinosaurs ruled the land, the ichthyosaurs were the apex predators of the ocean. Today, they are one of the most well-known prehistoric animals and are often confused with dinosaurs, which they weren’t! About a hundred species of ichthyosaurs evolved before they began to disappear and were eventually replaced by mosasaurs by the Late Cretaceous. 

Sarcosuchus

Look! It’s the SuperCroc. The ancient crocodilian was not a dinosaur but a distant relative of crocodiles. Sarcosuchus was ten times bigger than any crocodile alive today and had a bite force of eight tons. They hunted like modern crocodiles too, shaking the prey wildly before pulling it into the water to drown.

Survivors of Mass Extinctions

The reign of dinosaurs ended around 66 million years ago. This paved the way for other animals to flourish. Turtles, snakes, lizards, amphibians, and crocodiles were some of the fortunate survivors. 

Marine fishes suffered significant losses, but they did not become entirely extinct. Birds, the only living descendants of the dinosaurs, suffered great losses too but had rebounded rapidly after the event.

Marine fishes.
Saint Louis Art Museum, via Wikimedia Commons

Of all the survivors, the story of the mammals and their humble beginnings was perhaps the most intriguing.

The traditional view is that mammals only diversified after the extinction of dinosaurs. But mounting evidence from recent fossil finds challenges this notion. Early mammals were small, none grew bigger than a dog. How did they survive the asteroid impact that obliterated 75% of life on earth?

The Rise of Mammals in the Mesozoic Era

The success of mammals seems related to their size and their “hunt-everywhere,” “eat-everything” lifestyle. Mesozoic mammals kept a low profile, but a million years of evolution was already brewing in the background. 

For example, some mammals developed wing-like membranes to glide from tree to tree. Others had feet or claws designed for digging; hiding in burrows would have protected them from intense climatic changes. 

Most importantly, mammals became good at eating different types of food such as aquatic plants, worms, seeds, and insects. At night, they may even have stolen dinosaur eggs. 

Millions of years of playing chess with the giant predators had proved to be beneficial for these warm-blooded critters. By the time the dinosaurs finally died out, mammals were ready to take over the world. 

FAQs

What major events happened in the Mesozoic Era?

Two major events marked the exciting and terrifying times of the Mesozoic era. First was the Triassic-Jurassic mass extinction that paved the way for the evolution and reign of dinosaurs. The second was the Cretaceous-Tertiary mass extinction that killed 75% of life on earth, including the dinosaurs.

What animals evolved during the Mesozoic Era?

The dinosaurs and reptiles were the dominant animals in the Mesozoic. Other prehistoric creatures that evolved in the Mesozoic include the pterodactyls, archaeopteryx, plesiosaurs, and ichthyosaurs. The first birds, crocodiles, lizards, and turtles also appeared.

Why is the Mesozoic Era called the age of dinosaurs?

Dinosaurs dominated the Mesozoic Era for more than 160 million years, peaking during the late Jurassic to early Cretaceous periods. They are thought to be the most successful animals ever to walk the earth. 

Did people and dinosaurs live at the same time?

No. All dinosaurs (except birds) went extinct 66 million years ago. The earliest known humans evolved roughly 2.4 million years ago. Home sapiens (our species) did not show up until about 200,000 years ago. So, nearly 65 million years passed before the first modern humans appeared.