‘We’re not dinosaurs’: book reveals how mammals came to rule the world

With a head like a horse, a body that resembles a giant bear and with huge knuckles with claws on which it walked like a gorilla, Anisodon resembles a figure from Greek mythology.

But it is not a beast of the underworld or a monster of nightmares. Instead, it’s one of a bizarre group of animals called Chalicotheres that roamed the Earth 46 million years ago, with the last of the creatures surviving long enough to have been encountered by human ancestors. In addition, Anisodon was a mammal. Just like us.

King Kong may have easily beaten a T-rex in the 1933 film, but since then our interest in dinosaurs has overcome any fascination with mammals. While the reptiles have been brought to the fore through movies like Jurassic Park, early mammals were the underdog—with mammoths and saber-toothed tigers being the few to rise to prominence.

Photo: Faenkova Elena/Shutterstock

Yet the mammal family tree is full of breathtaking creatures, from Anisodon to the largest creature that ever lived: the blue whale.

“I don’t think we appreciate this enough,” says Steve Brusattepaleontologist at the University of Edinburgh and author of The Rise and Reign of the Mammalswho wants to bridge the fascination gap. “Imagine if whales were extinct and we only had their bones. I mean, they would certainly be as famous and fascinating as dinosaurs.”

As a science advisor for the upcoming movie Jurassic World Dominion, Brusatte has nothing against dinosaurs, and the shelves of his office are teeming with sketches, plastic models and even origami creations of the beasts.

The lavish American even started out as a T-rex expert before he began studying mammal fossils. But there’s a simple reason why he’s so passionate about the latter. As he says in his new book, “Dinosaurs are great, but we’re not.”

Nothing short of a thriller, The Rise and Reign of the Mammals reveals the luck, evolutionary twists and near-apocalyptic catastrophes that led to today’s mammals, including us.

Fascinating revelations come thick and fast, from the discovery that ancient rodents and monkeys crossed the vast distance from Africa to South America on rafts, to the fact that whales have navels and elephants recognize themselves in the mirror.

Along the way, Brusatte brings readers face-to-face with our distant ancestors, including the last common ancestor of mammals and reptiles: a small, scaly, swampy creature that lived about 325 million years ago.

At some point, two populations of these lizard-like creatures became separated from each other. And the rest is history.

When natural selection began to work, a population amassed adaptations that would eventually lead to mammals. Chief among these was a single opening behind the eyes—allowing for larger, stronger jaw muscles—and teeth specialized for different purposes.

“A lot of our biological superpowers come from our teeth,” says Brusatte. “Something like a T-rex, or a lizard, basically all have the same type of teeth. They can just chew up and down. Mammals, we have all these different types of teeth, we actually have a Swiss Army knife in our jaws and the teeth do many things.”

Dimetrodone. Photo: Todd Marshall

The early ancestors of mammals are a long way from our furry pets. About 290 million years ago, the enormous sail-backed Dimetrodon, called “something of a Frankenstein creature” by Brusatte, prowled the landscape with its outstretched limbs and sharp teeth, and about 255 million years ago an intrepid time traveler might have encountered Inostrancevia, a group of monstrous saber-toothed beasts. “These were pesky meat eaters,” Brusatte says.

Soon hair started sprouting, the brain grew in size and higher metabolisms developed. “If you look at the fossil record, you see that there was a long story” [over] tens of millions of years, when mammals were essentially collected piece by piece through evolution,” says Brusatte.

Then, about 252 million years ago, volcanoes erupted in what is now Siberia. The result was runaway global warming and the death of about 90% of Earth’s species — an event called the end of the Permian extinction, or “dying big.”

Most mammalian precursors bite the dust. But against the odds, some survived, including a furry, feline creature called Thrinaxodon that could not only dig but also grow and reproduce quickly. It was the ultimate “disaster kind”.

Thrinaxodone. Photo: Todd Marshall

“It seems that only by the stupid luck of evolution most” [mammal ancestors] died, but a small number turned out to be particularly suitable for a world of chaos,” says Brusatte.

These survivors received new adaptations: Their lower jaw changed from a collection of bones to just one, and a new type of joint emerged — long thought to be the hallmark of real mammals. The vestigial bones were repurposed to become tiny bones in the middle ear, commonly known as the hammer and anvil — a radical development that made hearing superior. At some point they started to give their young milk and became really warm-blooded.

But there was also another kind of creature on the rise: dinosaurs. And as these beasts grew large — a diplodocus was about the length of a basketball court — mammals grew small. Brusatte wants to emphasize that the pressure went both ways. “You’ve never seen a triceratops the size of a mouse. And that’s because the mammals kept the dinosaurs big,” he says.

Their diminutive form is said to be the trump card of mammals when, about 66 million years ago, a space rock six miles wide collided with Earth. The dinosaurs, with the exception of the ancestors of birds, died out. So did a large number of mammals, perhaps as many as 90%.

But some lived. “Those who survived were those who were smaller, those who could burrow or hide more easily, and those on a very general diet that could eat a lot of things,” Brusatte says.

Mammals soon got bigger. And while some laid eggs, as platypus do today, others gave birth to live young — nursed them through a complex placenta in the womb, or in a pouch.

In the hallway of the University of Edinburgh, Dr Sarah Shelley, a paleontologist who illustrated Brusatte’s book, reveals the jaw of a creature that lived several hundred thousand years after the space rock impacted.

3D top view of contents in a cave: thrinaxodon and broomstega.
3D top view of contents in a cave: Thrinaxodon and Broomistega. Photo: Creative Commons Attribution License Todd Marshall

Pepriptychus was about the size of a border collie, but thicker, with a large head, huge cheek muscles, a small brain, and teeth like citrus juicers. And it was hairy and had five fingers and fingernails. “His hands look bizarrely human,” adds Shelley. “They’re not hooves yet, but they’re more than claws.”

But Brusatte isn’t just excited about displaying bizarre mammals from the past. He wants more appreciation for what is now. To illustrate his point, he notes that besides birds and pterodactyls, only one creature developed the ability to fly by flapping its wings: bats.

“Imagine if they were gone and we only had fossils. I mean, we’d marvel at something like a bat,” he says.

Humans also have a lot to offer: as Brusatte points out, we are sentient monkeys who have changed the world. But we are just one chapter in a much bigger story.

“I want people to appreciate our evolutionary history — where we come from, why we look the way we do, why we behave the way we do, why we have hair and milk our babies and we have the teeth that we do and we have big brains and sharp senses, and all these things,” says Brusatte. “It all comes from evolution.”

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