What Was the Mesozoic Era?

Imagine you've just found a magnifying glass and you're looking at the smallest ant you've ever seen. That ant might seem tiny to you, but compared to an atom—the tiny building block of everything around you—that ant is absolutely enormous. Now imagine shrinking down even smaller than an atom, smaller than you can possibly imagine, and looking at the grand story of our planet Earth. Our Earth has been around for so incredibly long that if you tried to count every second that has passed since it was born, you would still be counting today and for many, many lifetimes more! Scientists call this incredibly long journey through time "deep time," and it is one of the most amazing stories you will ever learn about.

The Mesozoic Era is a special chapter in Earth's very long story—a chapter that lasted for approximately 186 million years, which is so long that our entire human history would fit inside it like a tiny drop of water in a giant ocean. If Earth's 4.5-billion-year history were compressed into a single year, with January 1st representing the birth of our planet and December 31st representing today, the dinosaurs would first appear around early December and disappear around December 26th. Modern humans wouldn't appear until the very last minute of December 31st! That is how briefly we have been here compared to the dinosaurs and the world they lived in.

The word "Mesozoic" comes from Greek words that mean "middle life," and scientists chose this name because the Mesozoic sits between two other great eras of Earth's history. Before the Mesozoic came the Paleozoic Era, which means "ancient life," and after the Mesozoic came the Cenozoic Era, which means "new life." Each of these eras was home to completely different kinds of creatures and plants, and each era changed our planet in ways that made the next era possible. The Paleozoic was when simple sea creatures first developed shells and bones, when fish learned to swim in the oceans, and when the first animals crawled out of the water onto land. It was also when strange plants called coal swamps covered much of the ancient continents and when the first reptiles walked upright on their four legs. The end of the Paleozoic, about 252 million years ago, saw one of the biggest disasters our planet has ever experienced—the Permian Extinction, which wiped out about 90 percent of all life in the oceans and about 70 percent of all life on land. It was a devastating event that nearly ended life on Earth entirely.

From the ashes of this catastrophe, the Mesozoic Era emerged like a phoenix rising. Life slowly recovered and diversified, filling ecological niches that had been emptied by the extinction. New groups of organisms evolved to take advantage of opportunities that had never existed before. It was during the Mesozoic that dinosaurs first appeared, ultimately becoming the dominant land animals on Earth for the entire era. It was during the Mesozoic that flowering plants first evolved, eventually covering our planet with the colorful flowers and trees we see today. It was during the Mesozoic that birds evolved from small dinosaur relatives, taking to the skies on feathered wings. And it was during the Mesozoic that the continents slowly drifted apart, reshaping the face of our planet into the arrangement we recognize today.

To understand the Mesozoic better, scientists have divided this massive span of time into three smaller periods, just like a really long book might be divided into three volumes. Each of these periods had its own distinct climate, geography, and collection of creatures. The first period is called the Triassic, which lasted from about 252 million years ago to about 201 million years ago. The second is the Jurassic, spanning from about 201 million years ago to about 145 million years ago. The third and final period is the Cretaceous, running from about 145 million years ago until the mass extinction event that ended the Mesozoic about 66 million years ago. Each of these periods was millions of years long and saw dramatic changes in both the life on Earth and the planet itself.

The Triassic Period was a time of recovery and experimentation. After the devastating Permian Extinction, the few survivors that remained had to rebuild ecosystems from nearly nothing. The climate was hot and dry, and much of the land was covered in vast deserts. Strange and wonderful creatures emerged during this time. Reptiles of all sizes began to explore new ways of living. On land, the first dinosaurs appeared—small, quick creatures that would eventually grow into the giant titans we know so well. In the seas, ichthyosaurs swam like modern dolphins, while plesiosaurs stretched their long necks through the water searching for fish. In the skies, the first pterosaurs took flight, becoming the first vertebrates to ever fly under their own power. The Triassic ended with another extinction event, smaller than the Permian but still significant, which cleared the way for dinosaurs to become the dominant land animals they would be throughout the Jurassic.

The Jurassic Period is probably the most famous of the three Mesozoic periods, partly because of its association with the huge, long-necked sauropods that many people think of when they imagine dinosaurs. The name "Jurassic" might sound familiar because of the famous movie franchise, but the real Jurassic was even more impressive than any movie could show. During this time, the supercontinent Pangaea continued to break apart, and the Atlantic Ocean began to form. The climate was warm and humid, and giant ferns, cycads, and conifers covered much of the land. This was the perfect environment for the giant sauropods to thrive—creatures like Brachiosaurus, Diplodocus, and Brontosaurus that could reach their heads into the treetops to eat leaves that no other animals could reach. Meanwhile, theropod dinosaurs like Allosaurus hunted the great herbivores, using sharp teeth and claws to catch their prey. In the skies, pterosaurs with wingspans of up to 30 feet soared over the landscapes, while the first birds—small, feathered creatures like Archaeopteryx—began to appear. The oceans teemed with life, including massive marine reptiles called pliosaurs that could bite a small boat in half.

The Cretaceous Period was the longest of the three Mesozoic periods and saw the continued diversification of dinosaurs into countless different species. This was the time of Tyrannosaurus rex and Triceratops, of armored Ankylosaurus and the duck-billed Hadrosaurus, of the giant sea monster Mosasaurus and the flying Quetzalcoatlus with its 40-foot wingspan. Flowering plants, which had first appeared in the Jurassic, became increasingly common during the Cretaceous, eventually dominating many ecosystems and changing the way dinosaurs ate and lived. The continents continued to drift apart, approaching their modern positions. But the Cretaceous ended catastrophically, with an asteroid or comet striking the Earth near what is now Mexico and triggering a mass extinction that killed all non-avian dinosaurs, as well as pterosaurs, marine reptiles, and countless other species. This event, 66 million years ago, marked the end of the Mesozoic Era and the beginning of a new chapter in Earth's story.

Now, you might be wondering how we know about all of this. How can scientists possibly know what happened millions and millions of years ago? The answer lies in one of the most amazing treasures our planet has to offer: fossils. Fossils are the preserved remains or traces of ancient organisms, and they are like nature's way of leaving notes about the past. When an animal or plant dies, its body usually decays completely or gets eaten by other creatures. But sometimes, under special circumstances, some part of the organism is preserved. This might happen if the creature is buried quickly by sediment, like mud or sand, before scavengers or decay can destroy it. Over millions of years, the original mater