The Rise and Fall of Dinosaurs, As Explained by Extinction Events

Dinosaurs ruled the Earth for over 150 million years. Compared to the mere 4–6 million years that scientists believe humans and their earliest ancestors have been on the planet, it wouldn’t be surprising if ancient alien maps labeled Earth “Dinotopia.” Dinosaurs finally met their end, but only partially. Many of their descendants—modern birds—are currently threatened by extinction, with hundreds of species at risk due to human activity. This article explores the major extinction events that influenced dinosaur survival and their eventual extinction, breaking down the causes, effects, and lasting legacy of these world-changing moments.

When Did Dinosaurs Get Started?

Arizonasaurus was an archosaur from the Middle Triassic.

Dinosaurs evolved from a group of reptiles called archosaurs (“ruling reptiles”) around 240–250 million years ago. This was during the Triassic period. This branch of the tree of life gave rise to various reptiles, including pterosaurs, and eventually to crocodiles and other modern species. Dinosaurs were a different branch of archosaurs that later evolved into birds. The specific ancestors of dinosaurs were small, fast, upright-walking reptiles called dinosauromorphs. These creatures already had a lot of the traits that would define true dinosaurs—like an upright stance, strong back legs, and lightweight bodies.

Is It Accurate to Say Dinosaurs Went Extinct?

It’s a common misconception that crocodiles and Komodo dragons are descendants of dinosaurs. Actually, they developed from different evolutionary branches than the dinosaurs and aren’t their descendants. Birds are, however, thought to be the evolutionary offspring of small theropod dinosaurs. So in one way, dinosaurs never went extinct; they just turned into birds. However, in the stricter sense of the word, dinosaurs had a whole combination of anatomical and behavioral characteristics that aren’t found in any single creature today, so it’s not wrong to say that (non-avian) dinosaurs went extinct.

Which Birds Today Are Most Like Dinosaurs?

Among today’s birds, the species most closely resembling their dinosaur ancestors are large, flightless ground-dwellers like ostriches, emus (pictured above), cassowaries, rheas, and kiwis. Some of their dinosaur-like traits are their bone structure, strong leg muscles for bipedal walking, sharp claws, and the ability to defend themselves by kicking.

The Triassic-Jurassic Extinction Event: How Dinosaurs Took Over

Roughly 201 million years ago, the Triassic-Jurassic extinction event wiped out about 76% of all marine and land species on Earth. This cleared the stage for dinosaurs to take over for the next 135 million years. This extinction was connected with massive volcanic eruptions caused by the breakup of the supercontinent Pangaea. The volcanism pumped greenhouse gases into the atmosphere, acidified the oceans, and made the temperature rise rapidly. All this happened over a span of about 40,000 years; too quickly for most species to adapt.

Paleontologists have found extensive fossil records showing the extent of this catastrophe. Many marine creatures like ammonoids and conodonts went extinct, as well as land animals like phytosaurs, aetosaurs, giant amphibians called metoposaurs, and lots of crocodile-like reptiles. However, early dinosaurs, pterosaurs, and small warm-blooded creatures like cynodonts survived.

Dinosaur Boom During the Jurassic Period

After surviving the Triassic shake-up, dinosaurs flourished in the Jurassic Period. One of the reasons for this was the breakup of the supercontinent Pangaea into separate landmasses with different environments. This created a lot of different ecological niches for species to adapt to. As a result, dinosaur diversity hit a high point during this time. Theropods like Megalosaurus (pictured) and sauropodomorphs kept evolving. Some grew larger and specialized in eating plants—a trend that later led to the huge, long-necked dinosaurs like Apatosaurus. Ornithischians—beaked, herbivorous dinosaurs—flourished, as did armored species like Stegosaurus. Flying reptiles (pterosaurs) and early birds began soaring the skies. By the end of the Jurassic, dinosaurs were the undisputed rulers of the land.

The Cretaceous Period: A Time of Big Changes

The Cretaceous Period, lasting from about 145 to 66 million years ago, was the end of the Mesozoic Era. The planet was warm, and sea levels were high. As a result, large landmasses were flooded with seawater. That is why we find marine fossils in places like Nebraska. On land, flowering plants were spreading.

Among the dinosaurs, herbivorous groups like hadrosaurs (duck-billed dinosaurs) and ceratopsians (horned dinosaurs like Triceratops) became especially dominant. Among predators, theropods like Tyrannosaurus rex ruled the late Cretaceous, while smaller raptor-like species such as Velociraptor showcased advanced hunting tactics and possible feathers. This era also saw the rise of more bird-like dinosaurs. They bridged the evolutionary gap between non-avian dinosaurs and modern birds.

The End-Cretaceous (K-Pg) Extinction: The Final Curtain

Around 66 million years ago, Earth endured a mass extinction event that marked the end of the Cretaceous and the start of the Paleogene period. Roughly 75% of all species vanished, including every non-avian dinosaur. It was caused by a giant asteroid, possibly 9 to 14.5 kilometers wide, that slammed into what is now the Yucatán Peninsula of Mexico. The impact caused massive earthquakes, tsunamis, and volcanic activity for thousands of miles around. It vaporized rock, blasted debris into the sky, and created continent-wide forest fires.

The smoke and debris blocked so much sunlight that the whole planet went into a global winter. Animals that survived the immediate explosion and its aftermath died out from exposure to the cold and starvation, as plants died from the weather and the lack of sunlight. This situation lasted for months to a few years, but the damage was irreversible for many species.

How Birds Survived

This was such an overwhelming catastrophe that it’s amazing any life survived at all. Yet somehow, a few resilient survivors made it through. Among them were small, ground-dwelling dinosaurs that were especially tough and adaptable. Their small size, burrowing habits, and possibly warm-blooded physiology helped them survive the new, harsher environment when larger species died out.

Scientists believe that some of these small theropods, likely feathered and already capable of gliding or limited flight, formed the evolutionary foundation of today’s birds. Over millions of years, their descendants diversified into more than 11,000 bird species, from hummingbirds to ostriches. In this way, the age of dinosaurs never fully ended—it simply took to the skies.

How Mammals Survived

The end-Cretaceous extinction created space in the ecosystem for birds, but also for mammals. Mammals first evolved from ancestors called therapsids during the late Triassic Period. These were tiny things, somewhat like shrews. They gradually developed warm-bloodedness, fur, and more advanced brains. Like the ancestors of birds, the mammals that survived the asteroid and its aftermath were small, ground-dwelling creatures that could keep themselves warm and had diversified diets.

Is the Final Extinction Event… Us?

The survival of mammals is good news for us, but maybe bad news for the rest of the ecosystem. In fact, the last great extinction event in the history of life on Earth could be our fault. Human activities such as habitat destruction, pollution, climate change, and overexploitation of species have led to drastic declines in the number of animals and a spike in extinctions. And of course, there is the ever-present danger that nuclear war or other destructive acts could create a sudden extinction event. As a result of these human-centered threats, some researchers call the possible future extinction of life the Anthropocene extinction.

The history of dinosaur extinctions is a powerful reminder of just how fragile—and how tough—life can be. Hopefully, we can learn to manage the planet in such a way that the Anthropocene extinction can be stopped. But if there is one thing we’ve learned from the history of past mass extinctions, it is (with a nod to “Jurassic Park”): “Life finds a way.”

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