The fall of Tyrannosaurus rex and the rise of mammals on Earth

Tyrannosaurus bones help piece together the story of dinosaurs’ demise, while mammals emerged and adapted, in two outstanding natural history books. 

Sixty-six million years ago, one of the most famous creatures in history roamed Earth: the Tyrannosaurus rex. It grew to 40 feet long, could weigh more than 7 tons, and had terrifyingly powerful jaws. It was the ruler of the late Cretaceous world, and it’s the subject of journalist David Randall’s captivating new book, “The Monster’s Bones: The Discovery of T. Rex and How It Shook Our World.”

The T. rex is long gone before the main action of Randall’s book begins. That action plays out in the Montana badlands of the early 20th century, where dinosaur-hunter Barnum Brown first discovered Tyrannosaurus bones in 1902. Brown is one of the foremost heroes of Randall’s book, an intrepid explorer with an almost infallible bone-prospecting instinct. “During his life, Brown was widely recognized as the best dinosaur collector who ever lived,” Randall writes. “He went out into the unknown and came back with new puzzle pieces that told the story of life on Earth, and he did it again and again and again, a run of success and discovery as impressive as navigating the stars.” 

And if we’ve got a hero, we need a villain, yes? In Randall’s telling, that would be Henry Fairfield Osborn, the foppish and dilettantish director of the American Museum of Natural History in New York (and Brown’s boss), who’s portrayed here as a windy theorizer sitting in his comfortable laboratory while people like Brown are out braving the elements.  

Amid plenty of tossed-off bits of fascinating trivia (because of the construction of their throats, “though many of them were likely feathered, dinosaurs did not sing”), Randall keeps his eye on the larger picture. He tells how the story of the T. rex “is the story of how we came to understand our planet and were offered the first glimpses into the power of a changing climate to eradicate the dominant forms of life.”  

This is an odd comment, since, famously, it wasn’t climate change that ended the reign of the Tyrannosaurus rex 66 million years ago – it was an asteroid that struck what is now the Yucatán Peninsula in Mexico and triggered a world-wide catastrophe that ended up killing approximately 75 percent of all species on Earth, including virtually all the dinosaurs. (The avian ones survived as birds). 

The relatively sudden elimination of the dinosaurs created a vast ecological niche, and that niche was filled by tiny, furry creatures who’d fled and hidden from their saurian overlords. These were the mammals, and they’re the subject of “The Rise and Reign of the Mammals: A New History, from the Shadow of the Dinosaurs to Us,” the terrific new book by Steve Brusatte (author of “The Rise and Fall of the Dinosaurs” from 2018). 

Those little mammals almost died out as well, as Brusatte points out – killer asteroids don’t play favorites, and if the mammals had gone extinct with the dinosaurs, their whole subsequent legacy wouldn’t have existed. “All they would accomplish – woolly mammoths, whales the size of submarines, the Renaissance, you reading this page – was almost rendered a nonstarter,” Brusatte writes.

Brusatte wrote his Ph.D. while working at the American Museum of Natural History, home to some of the most famous T. rex skeletons in the world – some of them Barnum Brown’s handiwork from a century earlier. In one engaging chapter after another, Brusatte takes readers through the long story of the little mammals that took over the world from those tyrannosaurs. 

It’s a fascinating story, and Brusatte fills it out with plenty of digressions about some of the people who dedicated their time to learning it. And Brusatte is always on hand with ruminations of his own on every aspect of mammalian evolution – like, for instance, the question of why no land mammals ever grew as big as some of the dinosaurs. (He suggests it was a question of lungs: Dinosaur lungs were much more efficient, giving them much greater oxygen with every breath than their mammalian counterparts). 

And like Randall, Brusatte keeps his eye on the big picture – the potential for human extinction. Just as the reign of the dinosaurs came to an end after millions of years, so too the reign of mammals could come to an end despite the fact that one species of mammal is currently in control of the entire world. “We could go extinct, or we could adapt,” Brusatte writes. “That’s the choice, and because we are sentient beings with big brains and tools and technology and global reach, it really is a choice.”

There’s always hope that humans will do what it takes to avoid extinction. Unless an asteroid has other ideas.