‘Dinosaurs Aren’t Us’: Book Reveals How Mammals Ruled the World

With a head like a horse, a body resembling a giant bear, and possessing huge clawed knuckles on which it walked like a gorilla, Anisodon looks like a character from Greek mythology.

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

King Kong may have easily bested a T rex in the 1933 film, but since then our interest in dinosaurs has conquered any fascination with mammals. While reptiles were thrust into the public eye by films such as Jurassic Park, the first mammals were the underdogs – with mammoths and saber-toothed tigers among the few famous.

Photography: Elena Faenkova/Shutterstock

Yet the mammalian family tree is full of jaw-dropping creatures, from Anisodon to the largest creature that ever lived – the blue whale.

“I don’t think we appreciate that enough,” says Steve Brusattepaleontologist at Edinburgh University and author of The Rise and Reign of the Mammals, which aims to bridge the gap of fascination. “Just imagine if whales were extinct and we only had their bones. I mean, surely they would be as famous, as fascinating as the dinosaurs.

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

The expansive American even started out as a T rex expert before branching out into the study of mammalian fossils. But there’s a simple reason why he’s so passionate about the latter. As he says in his new book, “Dinosaurs are awesome, but they’re not us.”

The Rise and Reign of the Mammals is nothing short of a thriller, revealing the luck, evolutionary twists and near-apocalyptic disasters that have led to today’s mammals, including us.

Fascinating revelations follow, 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 swamp-dwelling creature that lived around 325 million years ago.

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

As natural selection went to work, a population accumulated adaptations that would eventually give rise to mammals. Chief among them was a single opening behind the eyes – allowing for bigger and stronger jaw muscles – and specialized teeth for different purposes.

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

Dimetrodon. Photography: Todd Marshall

The earliest ancestors of mammals are a far cry from our fluffy pets. About 290 million years ago, the enormous sail-backed Dimetrodon, dubbed “something of a Frankenstein’s creature” by Brusatte, stalked the landscape with its sprawling limbs and razor-sharp teeth, and there are around 255 million years ago, an intrepid time traveler might have encountered Inostrancevia, a group of monstrous saber-toothed beasts. “These things were nasty flesh eaters,” Brusatte says.

Soon hair started to grow, brains got bigger, and higher metabolisms developed. “When you look in the fossil record, you see there was this long history [over] tens of millions of years ago, when mammals were essentially put together by evolution, piece by piece,” 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 around 90% of the planet’s species – an event called the end-Permian extinction, or “great death”.

Most mammalian precursors have bitten the dust. But, against all odds, some survived, including a hairy, cat-sized creature called Thrinaxodon that could not only burrow but also grow and reproduce rapidly. It was the ultimate “sort of disaster.”

Thrinaxodon. Photography: Todd Marshall

“It seems that just by the stupid luck of evolution the most [mammal ancestors] died, but a small number of them turned out to be particularly adapted to a chaotic world,” explains Brusatte.

These survivors gained new adaptations: their lower jaws changed from a collection of bones to a single one, and a new type of joint appeared – long considered the hallmark of true mammals. Residual bones were repurposed, becoming tiny middle ear bones commonly known as the hammer and anvil – a radical development that supercharged hearing. At some point they started giving milk to their young and got really hot blooded.

But another type of creature was also on the rise: dinosaurs. And as these beasts grew large – a diplodocus was about the length of a basketball court – the mammals grew small. Brusatte is keen to point out that the pressure went both ways. “You’ve never seen a mouse-sized triceratops. And that’s because the mammals kept the dinosaurs big,” he says.

Their small form must have been the mammals’ trump card when, about 66 million years ago, a six-mile-wide space rock collided with Earth. The dinosaurs, with the exception of the ancestors of the birds, disappeared. The same was true for a wide range of mammals, perhaps as high as 90%.

But some have lived. “Those who survived happen to be those who were smaller, those who could dig or hide more easily, and those who had very general diets who could eat a lot of things,” says Brusatte.

Mammals quickly grew in size. And while some laid eggs, like platypus today, others gave birth to live young, either by feeding them via a complex placenta in the womb or in a pouch.

In the hallway of Edinburgh University, Dr Sarah Shelley, a paleontologist who illustrated Brusatte’s book, unveils the jawbone of a creature that lived a few hundred thousand years after the rock impact spatial.

3D rendering of the top of the contents inside a burrow: thrinaxodon and broomistega.
3D rendering of the top of the contents inside a burrow: Thrinaxodon and Broomistega. Photo: Creative Commons Attribution LicenseTodd Marshall

Periptychus was about the size of a border collie, but stockier, with a large head, massive cheek muscles, a small brain, and teeth like juicers. And he was hairy, and had five fingers and nails. “His hands look oddly human,” Shelley adds. “They’re not hooves yet, but they’re more than claws.”

But Brusatte is not only enthusiastic about presenting bizarre mammals from the past. He wants a greater appreciation of what is here now. To illustrate his point, he notes that apart from birds and pterodactyls, only one creature has developed the ability to fly by flapping its wings: bats.

“Imagine if they weren’t around anymore and we just had fossils. I mean, we’d be amazed by something like a bat,” he says.

Humans also offer something to marvel at: as Brusatte points out, we are sensitive apes who have changed the world. But we are only one chapter in a much larger story.

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

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