4 reasons dinosaurs never really ruled the Earth

The 'terrible lizards' can reign supreme in the movies, but there's something seriously wrong about the way we've hyped up their history.
T. rex model, T. rex skull, and Triceratops skull at dinosaur display in the Museum of Natural History in Vienna
(Clockwise from top) A T. rex model, T. rex skull, and Triceratops skull on display at the Museum of Natural History in Vienna, Austria. DepositPhotos

We all know the line: For more than 150 million years, dinosaurs ruled the Earth. We imagine bloodthirsty tyrannosaurs ripping into screaming duckbills, gigantic sauropods shaking the ground with their thunderous footfalls, and spiky stegosaurs swinging their tails in a reign of reptiles so magnificent, it took the unexpected strike of a six-mile-wide asteroid to end it. The ensuing catastrophe handed the world to the mammals, our ancestors and relatives, so that 66 million years later we can claim to have taken over what the terrible lizards left behind. It’s a dramatic retelling of history that is fundamentally wrong on several counts. Let’s talk about some of the worst rumors and what really happened in the so-called “Age of Dinosaurs.”

Myth: Dinosaurs dominated the planet from their origin.

Fact: Dinosaurs started as cute pipsqueaks.

The oldest dinosaurs we know about are around 235 million years old, from the middle part of the Triassic Period. Those reptiles didn’t rule anything. From recent finds in Africa, South America, and Europe, we know that they were no bigger than a medium-sized dog and were lanky, omnivorous creatures that munched on leaves and beetles. Ancient relatives of crocodiles, by contrast, were much more abundant and diverse. Among the Triassic crocodile cousins were sharp-toothed carnivores that chased after large prey on two legs, “armadillodiles” covered in bony scutes and spikes, and beaked, almost ostrich-like creatures that gobbled up ferns.

Even as early dinosaurs began to evolve into the main lineages that would thrive during the rest of the Mesozoic, most were small and rare compared to the crocodile cousins. The first big herbivorous dinosaurs, which reached about 27 feet in length, didn’t evolve until near the end of the Triassic, around 214 million years ago. But everything changed at the end of the Triassic. Intense volcanic eruptions in the middle of Pangaea altered the global climate; the gases released into the air caused the world to swing between hot and cold phases. By then, dinosaurs had evolved warm-blooded metabolisms and insulating coats of feathers, leaving them relatively unfazed through the crisis, while many other forms of reptiles perished. Had this mass extinction not transpired, we might have had more of an “Age of Crocodiles”—or at least a very different history with a much broader cast of reptilian characters. The only reason the so-called Age of Dinosaurs came to be is because they got lucky in the face of global extinction.

Prehistoric predators fighting underwater. Illustration.
The biggest predators in the Cretaceous oceans were non-dinosaur reptiles and sharks. De Agostini via Getty Images

Myth: Dinosaurs spanned the entire planet.

Fact: Dinosaurs never evolved to live at sea.

It’s strange to talk about dinosaurs “dominating” an ocean world. While sea levels have risen and fallen over time, the seas make up about 71 percent of Earth’s surface and contain more than 330 million cubic miles of water. The claim that dinosaurs, as diverse as they were, were the dominant form of life on Earth only makes sense if we ignore that three-quarters of our planet is ocean.

Even though some dinosaurs swam, leaving scratches and swim tracks in ancient shallows, none have ever evolved to live their entire lives in the oceans. Even penguins—living dinosaurs—have not evolved the ability to remain at sea like many marine mammals have and must return to land to nest. If we were to emphasize prehistoric oceans, then there were marine reptiles of various shapes and sizes ruling over the watery kingdom. Fish-shaped ichthyosaurs, long-necked and four-flippered plesiosaurs, giant Komodo dragon relatives called mosasaurs, and many more non-dinosaur reptiles thrived in the seas for millions of years, many feeding on the even more abundant coil-shelled cephalopods called ammonites.

Of course, these ecosystems were built on a foundation of plankton. Without disc-shaped algae called coccoliths, the rest of the charismatic swimmers of the Triassic, Jurassic, and Cretaceous wouldn’t have thrived. It’s the abundant, small forms of life that let charismatic creatures like marine reptiles prosper—a further reminder that the animals that impress us on land or sea wouldn’t exist without various tiny organisms that set the foundations of food webs. What we might see as dominance, in any ecosystem, is really a consequence of many relationships and interactions that often go unnoticed.

Two mesonyx, a prehistoric mammal species, standing near a dead animal. Illustration.
Mammals flourished during and after the time of the dinosaurs. The wolf-life Mesonyx emerged in the Eocene, not long after the dinosaurs’ demise. De Agostini via Getty Images

Myth: Dinosaurs suppressed the evolution of mammals.

Fact: Mammals thrived throughout the Age of Dinosaurs.

The classic example of dinosaur dominance is a twitchy little mammal chasing an insect through the Cretaceous night. Dinosaurs would gobble up any beast that got too big or was foolish enough to wander out in the daylight, the argument went, so mammals evolved to be small and nocturnal until the asteroid allowed our ancestors and relatives to emerge from the shadows. The small size and insect-hunting adaptations of some Mesozoic mammals were taken as indicators that mammals were constrained by the success of the dinosaurs, preventing them from becoming larger or opening new niches.

In the past 20 years, however, paleontologists have rewritten the classic story to show that mammals and their relatives thrived alongside the dinosaurs. Throughout the Mesozoic there were furry beasts that swam, dug, glided between the trees, and even ate little dinosaurs. Ancient equivalents of squirrels, raccoons, otters, beavers, sugar gliders, aardvarks, and more evolved through the Jurassic and Cretaceous, including early primates that scampered through the trees over the heads of T. rexes. While it’s true that all the Mesozoic mammals we presently know of were small—the largest was about the size of an American badger— researchers have realized that the way our ancient ancestors interacted with each other was much more important to shaping their evolution than the dinosaurs were. In fact, even with the dinosaurs gone, most new mammal species stuck to being small. We get so hung up on size that we’ve missed the real story, closer to the ground.

Two pterosaurs fighting over prey in flight. Illustration.
Pterosaurs weren’t dinosaurs, but their aerial capabilities gave them an upper hand in the Late Triassic. De Agostini via Getty Images

Myth: Dinosaurs dominated the planet for millions of years.

Fact: No single species can dominate a planet.

Our fixation on a prehistoric hierarchy says more about us than the actual geological record. In our imaginations, we’ve turned dinosaurs into creatures that took over the planet and held on until a cosmic accident wiped them out. Dinosaurs of the Triassic, Jurassic, and Cretaceous lived on every major landmass for more than 150 million years. Often, their supposed reign is compared to what we think of as ours—a paltry 300,000 years that Homo sapiens has been around.  

But the comparison isn’t one-to-one. Dinosaurs were not a single species, but an entire group of organisms. More fundamentally, no species truly stands alone: Even the most long-lived and widespread organisms rely on others. Gigantic, plant-eating dinosaurs had to eat a Mesozoic salad bar of ginkgoes, horsetails, conifers, and other plants—food that required them to have specialized bacteria in their guts for digestion. Even the great T. rex was an ecosystem by itself, preying on herbivores that in turn, ate plants that fostered relationships with fungi and microorganisms in the soil. To look at such an image of life and focus on dominance is looking in the wrong place, dividing the history of life into winners and losers and missing the connections and community required for diverse creatures to thrive. Perhaps dinosaurs can reign supreme in the movies, where we have a perpetual fixation with putting ourselves in the way of their toothy maws. But the real lesson of Triceratops and kin is in how evolution flowers—not who rules the Earth.