The Rise and Reign of the Mammals: A New History, from the Shadow of the Dinosaurs to Us

Maps of Earth Over Time

320 million years ago, Carboniferous (Pennsylvanian)

200 million years ago, Triassic-Jurassic Boundary

66 million years ago, end of the Cretaceous, time of asteroid impact

50 million years ago, Eocene

20 million years ago, Miocene

21,000 years ago, last advance of the Ice Age

Dedication

For Anthony, my favorite little mammal

Contents

Cover

Title Page

Maps of Earth Over Time

Dedication

Timeline of Mammals

Mammal Family Tree

Introduction: Our Mammalian Family

1. Mammal Ancestors

2. Making a Mammal

3. Mammals and Dinosaurs

4. The Mammalian Revolution

5. Dinosaurs Die, Mammals Survive

6. Mammals Modernize

7. Extreme Mammals

8. Mammals and Changing Climates

9. Ice Age Mammals

10. Human Mammals

Epilogue: Future Mammals

Acknowledgments

Notes on Sources

Index

About the Author

Also by Steve Brusatte

Copyright

About the Publisher

Timeline of Mammals

Mammal Family Tree

Introduction

Our Mammalian Family

FOR THE FIRST TIME IN years, the sun broke through the darkness. There was still a whiff of smoke wafting from the gray clouds, which blanketed the ground in shadow. Down below, the land was wrecked. It was all dirt and mud, a wasteland absent any greenery or color whatsoever. Silence hung in the wind, punctured only by the churn of a river, its currents clogged with sticks and stones and the residue of decay.

The skeleton of a beast lay upon the riverbank. Its flesh and sinew were long gone, its bones a moldy beige. Its jaws were agape in a scream, its teeth busted and scattered in front of its face. Each one the size of a banana, with the sharp edges of a knife, the murder weapons this monster had used to dismember and crush the bones of its prey.

It was, once, a Tyrannosaurus rex, the tyrant lizard, the King of the Dinosaurs, the oppressor of a continent. Now its entire species was no more. And little else seemed to be alive.

Then, from somewhere within the behemoth, a soft sound. A clicking chatter, a flutter of footsteps. A tiny nose poked out between a couple of T. rex ribs, haltingly, as if afraid to go any farther. Its whiskers trembled, in expectation of danger, but it found none.

Time to come out of hiding. It leapt upward, into the light, and scurried onto the bones.

Clothed in fur, with bulging eyes and a snout full of teeth that looked like mountain peaks and a whiplash tail, this critter couldn’t have been more different than a T. rex.

It paused for a moment to scratch the hair on its neck, turned its ear to the air, and scampered forward on all fours. Hands and feet planted firmly underneath its body, it moved fast, with purpose. Up the rib, across the backbone, and onto the dinosaur’s skull.

There, on the side of the head, where the eye of this T. rex once glared at herds of Triceratops, the furball stopped. It looked back in the direction of the rib cage, and let out a high-pitched squeak. From the bowels of the beast, out bounded a dozen smaller furballs. They raced toward their mother, and latched onto her belly, lapping up a breakfast of milk as they experienced their first minutes aboveground.

As she nursed her babies, the mother stared into the sunlight. The world now belonged to her, and her family. The Age of Dinosaurs was over, put to rest with the fiery destruction of an asteroid and a long, dark, global nuclear winter. Now the Earth was healing. The Age of Mammals had begun.

SOME 66 MILLION years later, more or less, another mammal stood in the same spot, swinging a pickaxe. Sarah Shelley was my first PhD student after I started my job as a paleontologist at the University of Edinburgh in Scotland. We were in New Mexico, on a fossil hunt, searching for the bones and teeth and skeletons that would help us understand how mammals survived the asteroid, outlasted the dinosaurs, and made the world their own, becoming the furry animals that we know, love, and sometimes fear today.

Mammals are the most charismatic and beloved creatures on the planet—with all due respect to reptiles and birds and the other eight-million-plus animal species that are not mammals. Perhaps this is because many mammals are simply cute and fluffy, but in part, I think it’s because, on a deeper level, we can relate to them, and see ourselves in them. The cheetahs and gazelles locked in chase on the television screen, as David Attenborough’s dulcet tones narrate the drama. The mother otter playing with her pups on the cover of a nature magazine. The elephants and hippos that make every child beg their parents to take them to the zoo, and the endangered pandas and rhinos that pull at our heartstrings when so many other appeals for charity might annoy us. The foxes and squirrels that tolerate our cities, the deer that encroach on our suburbs. Whales with bodies longer than basketball courts, emerging from the abyss to spray blowhole geysers several stories into the sky. Vampire bats that literally drink blood, lions and tigers that make our hair stand on end. Our cuddly pets, of the feline or canine or sometimes more exotic variety. For many of us, our food—beef burgers, pork sausages, lamb chops. And, of course, us. We are mammals, in the same way that a bear or a mouse is.

As a porcupine shaded itself from the New Mexico afternoon in the nook of a cottonwood, and a colony of prairie dogs chirped in the distance, Sarah brandished her pickaxe. Each strike into the rock released a haze of foul, sulfur-smelling dust. Each time she would wait for the dust to clear, to see if anything interesting had loosened from the Earth. For at least an hour, each strike brought only more rock. Until, with one thwack, something with a shape, a different texture, a different color poked out. She knelt down to take a look. And then hollered a victory cheer so loud, and so happily profane, that I can’t repeat it here.

Sarah Shelley and me in New Mexico, collecting teeth from mammals that lived soon after the dinosaurs went extinct. (photo by Tom Williamson)

Sarah had found a fossil, her first major discovery as a student.

I rushed over to see her prize, and she handed me a set of jaws, fused together at the tip. The teeth were coated in gypsum, and as they sparkled in the desert sun, I could see that there were sharp canines near the front and big, grinding molars at the back. Mammal! And not just any mammal, but one of the very species that assumed the crown from the dinosaurs.

We exchanged high fives, and got back to work.

Sarah’s jaws belonged to a species called Pantolambda, and it was big, about the size of a Shetland pony. It lived just a couple million years after the dinosaur extinction, generations after that little mother peered out from the T. rex rib cage, in my fictional but credible story. Already, Pantolambda was considerably larger than any mammal that ever saw a T. rex or Brontosaurus. Some of those meek creatures—none bigger than a badger—endured the asteroid, by virtue of their smallness and adaptability, and suddenly found themselves in a dinosaur-free world. They grew in stature, and migrated, and diversified, and soon began forming complex ecosystems, replacing the dinosaurs—which had ruled the Earth for over 100 million years.

This particular Pantolambda lived in a jungle, on the edge of a swamp (hence the nasty odor of its entombing rocks). It was the largest herbivore of this environment. As it waded into the cooling waters after a lunch of leaves and beans, it would have seen or heard an abundance of other mammals. Overhead, kitty-size acrobats negotiated the tree branches with their grasping hands. At the swamp’s edge, mutts with gargoyle faces burrowed into the mud, their claws seeking roots and tubers packed with nutrition. In the patchier parts of the forest, daintier ballerinas raced through the meadows on their hoofed toes. All the while, camouflaged in the thickest subtropical weeds of this Paleocene-aged jungle, a terror lurked: the apex predators, built like a stocky dog, with flesh-slicing teeth.

The death of the dinosaurs allowed these mammals—in ancient New Mexico and all around the world—to become ascendant. But mammals have a much deeper history. They—or rather, we—actually originated around the same time as the dinosaurs, over 200 million years ago, when all land was gathered together as one supercontinent, scorched by vast deserts. Those first mammals had an even deeper legacy, tracing back to about 325 million years ago, to a humid realm of coal swamps, when the ancestral mammal lineage split from the reptile line on the great family tree of life. Over these immense stretches of geological time, mammals developed their trademark features: hair, keen senses of smell and hearing, big brains and sharp intelligence, fast growth and warm-blooded metabolism, a distinctive lineup of teeth (canines, incisors, premolars, molars), mammary glands that mothers use to nourish their babies with milk.

From this long and rich evolutionary history came today’s mammals. Right now, there are over six thousand mammal species sharing our world, our closest cousins among the millions of species that have ever lived. All modern-day mammals belong to one of three groups: the egg-laying monotremes like the platypus, marsupials like kangaroos and koalas that raise their tiny babies in pouches, and placentals like us, which give birth to well-developed young. These three types of mammals, though, are simply the few survivors of a once-verdant family tree, which has been pruned by time and mass extinctions.

At various points in the past, there have been legions of saber-toothed carnivores (not only the famous tigers, but also marsupials that turned their canines into spears), dire wolves, giant woolly elephants, and deer with ridiculously enormous antlers. There were supersized rhinos that lacked horns but sported long necks to guzzle leaves high in the treetops to sustain their nearly twenty-ton girth—mammals mimicking Brontosaurus, setting the record for largest hairy beasts to ever live on land. Many of these fossil mammals are familiar: they are icons of prehistory, stars of animated movies, and exhibits at any reputable natural history museum.

But even more fascinating are some of the extinct mammals that have never quite made it into pop culture stardom. There were once wee mammals that glided over the heads of dinosaurs and others that ate baby dinosaurs for breakfast, armadillos the size of Volkswagens, sloths so tall they could dunk a basketball, and thunder beasts with three-foot-long battering-ram horns. There were oddballs called chalicotheres that looked like an unholy horse-gorilla hybrid, which walked on their knuckles and pulled down tree branches with their stretched claws. Before it docked with North America, South America was an island continent for tens of millions of years, and hosted a whole family of wacky hoofed species whose Frankenstein mashup of anatomical features flummoxed Charles Darwin—and whose true relationships to other mammals has only just been revealed by the shocking discovery of ancient DNA. Elephants were once the size of miniature poodles, camels and horses and rhinos once galloped across an American savanna, and whales once had legs and could walk.

Clearly, the history of mammals is far bigger than the mammals we can see today, and it’s about so much more than our own human origins and migrations over the last few million years. All of these fantastic mammals I’ve just mentioned, you’ll meet in these pages.

I started my scientific career by studying dinosaurs. Growing up in the midwestern United States, it was T. rex that fascinated me most, and I went to college and did a PhD and carved out a niche as a dinosaur specialist. A few years ago, I told the story of dinosaur evolution, from their humble origins to apocalyptic extinction, in my book The Rise and Fall of the Dinosaurs. I’ll always love dinosaurs, and will continue to study them. But since I moved to Edinburgh and became a professor, I’ve started to drift. Perhaps it’s logical: having studied the dinosaur extinction, I’ve become obsessed with what happened afterward. I’ve become obsessed with mammals.

Sometimes people ask me why. Children everywhere dream of growing up and digging up dinosaurs, so why do anything else? And why mammals? My retort is simple: dinosaurs are awesome, but they are not us. The history of mammals is our history, and by studying our ancestors, we can understand the deepest nature of ourselves. Why we look the way we do, grow the way we do, raise our babies the way we do, why we have back pain and need expensive dental work if we chip a tooth, why we are able to contemplate the world around us, and affect it so.

And if that’s not enough, consider this. Some dinosaurs were huge, as big as Boeing 737 airplanes. The biggest mammals—blue whales and kin—are even larger. Imagine a world where mammals were extinct, and all we had were their fossil bones. No doubt they would be as famous, as iconic, as dinosaurs.

We’re learning more about the history of mammals at breathtaking speed. More mammal fossils are being found than ever before, and we can study them with an array of technologies—CAT scanners, high-powered microscopes, computer animation software—to reveal what they were like as living, breathing, moving, feeding, reproducing, evolving animals. We can even get DNA from some mammal fossils—like those strange South American mammals that infatuated Darwin—which like a paternity test tells us how they are related to modern species. The field of mammal paleontology was founded by Victorian men, but is now increasingly diverse, and international. I’ve been privileged to have mentors who welcomed me—just another dinosaur guy—into their territory of mammal research, and now I find my greatest joy in mentoring the next generation, like Sarah Shelley (whose illustrations grace these pages!), and many other excellent students who will continue to write mammal history with their discoveries.

In this book, I will tell the story of mammal evolution, as we know it now. Roughly the first half of the book covers the early stages of the mammal lineage, from the time they split from the reptiles until the extinction of the dinosaurs. This was when mammals acquired nearly all of their signatures—hair, mammary glands, and so on—and morphed, piece by piece, from an ancestor that looked like a lizard into something we would recognize as a mammal. The book’s second part lays out what happened after the dinosaurs died: how mammals seized the opportunity and became dominant, adapted to constantly changing climates, rode along on drifting continents, and developed into the incredible richness of species today: runners, diggers, flyers, swimmers, and big-brained book readers. In telling the tale of mammals, I want to convey how we’ve pieced together this story using fossil clues, and give you a sense of what it’s like to be a paleontologist. I’ll introduce you to my mentors, my students, and the people who have inspired me, and whose discoveries have provided the evidence that allows me to chronicle the narrative of mammalhood.

This book does not focus obsessively on humans; there are plenty of others that do. I will discuss human origins: how we emerged from primate antecedents, reared up on two legs, inflated our brains, and colonized the world—after living alongside many other early human species. But I’ll do so in one chapter, and give humans about the same attention as horses and whales and elephants. After all, we are but one of many amazing feats of mammalian evolution.

Our story is one that needs to be told, however, because although we’re only a single mammalian species and we’ve only been around for a fraction of mammalian history, we are impacting the planet like no mammal before. Our phenomenal success in building cities and growing crops and connecting the globe with highways and flight routes is having an adverse effect on our closest kin. More than 350 mammals have gone extinct since Homo sapiens came out of the forests and swept across the world, and many species are at high risk of extinction today (think tigers, pandas, black rhinos, blue whales). If things continue at the current pace, half of all mammals might succumb to the same fate as woolly mammoths and saber-toothed tigers: dead and gone, with only ghostly fossils to remind us of their majesty.

Mammals are at a crossroads, at the most precarious point in their—our—history since they—we—stared down the asteroid that killed the dinosaurs. And what a history it has been. During our long evolutionary run, there have been times when mammals cowered in the shadows and times when they were dominant. Periods when they flourished and others when they were knocked back, and nearly knocked out completely, by mass extinctions. Eras when they were held in check by dinosaurs and eras when they were the ones doing the checking; times when none were bigger than a mouse and times when they were the biggest things that ever lived on Earth; times they weathered heat spikes and times they faced mile-thick glaciers, during the Ice Age. Times when they could occupy only the lower rungs of the food chain, and times when some of them—us—got conscious and became able to shape the entire Earth, in good and bad.

All of this history laid the foundation for today’s world, for us, and our future.

Steve Brusatte

Edinburgh, Scotland

JANUARY 18, 2022

1

Mammal Ancestors

Dimetrodon

SOMETIME AROUND 325 MILLION YEARS ago—give or take a few million years—a group of scaly creatures clung to a tangled raft of ferns and broken logs. They were usually solitary and preferred to lie camouflaged in the dense greenery of the jungle, occasionally emerging to snatch an insect before returning to anonymity. But desperate times had steered them together. Their world was changing, fast. Their swampy paradise, perched at the boundary between water and land, was becoming engulfed by the sea.

The small critters—the largest were barely a foot long—looked about nervously. They had the manner of a gecko or an iguana, in the way that their arms and legs stuck out to the sides and their long, thin tail dragged behind. Some of the smaller ones ambled across the rotting vegetation, holding on with their skinny fingers and toes. The older animals just stared out at the vastness of the sea, their tongues flickering as they bobbed in the waves, water lapping up against them.

A few weeks earlier, all had seemed normal. From their well-hidden dens, they would have peered into a forest dripping with humidity. Greens of every imaginable shade surrounded them. Ferns choked the forest floor, their spores dancing through the sticky air with each welcome gust of wind. Larger seed-bearing shrubs, some of which were distant ancestors of today’s evergreen trees, formed the midstory. Whenever it rained—which was much of the time—their marble-size seeds poured down with the showers, covering the ground with a layer of ball bearings that made walking treacherous.

With their tiny eyes, the scaly critters could not have seen the top of the forest, which seemed to stretch infinitely toward the heavens. Two types of trees made up the bulk of the canopy, both of which grew to around a hundred feet (thirty meters) high. One was called Calamites, and it looked like an emaciated Christmas tree, with a straight bamboo-esque trunk that spit out intermittent bunches of branches with whorls of needle-shaped leaves. The other was Lepidodendron, whose six-foot-thick trunks were naked except for a thicket of branches and leaves only at the very tip-top—a mop of foliage on a giant stalk. They grew remarkably fast, going from spore to sapling to the pinnacle of the canopy in only ten or fifteen years, before dying, being buried and turned to coal, and replaced by another generation.

The scaly critters were one of hundreds of animal species that, at least until today, had called the swamp forest home. These ranged from the mundane to the fantastic. Insects were common, making them a perfect food source. Spiders and scorpions clambered through the leaf litter and up the tree trunks. Primitive amphibians gathered along streams, stocked with fish and patrolled by eurypterids: armor-covered brutes that resembled giant scorpions, some the size of humans, which snapped their prey with nutcracker claws. Back during those calmer times, the streams trickled into a river, which fanned out into a delta that led into the quiet tidal waters of a brackish bay.

Occasionally, a creepy slither pierced the stillness. This was Arthropleura, a monstrous millipede more than six and a half feet (two meters) long, slurping up spores and seeds. Sometimes an even more terrifying sound echoed through the swamp: the wingbeats of Meganeura, a pigeon-size dragonfly with four enormous, translucent wings that buzzed as it searched for bugs. If it was hungry enough, it might even attack one of the scaly critters—another reason why they preferred to stay hidden.

As the group of scaly critters latched on to their makeshift boat of leaves and twigs, the fear of a Meganeura attack seemed quaint. The danger now was much greater. They were encircled by water, and the currents were growing stronger. Far to the south, a massive ice cap was melting, shedding water into the oceans and raising the sea level. All around the world, coastlines were flooding, drowning the mangrove bayous of Calamites and Lepidodendron and their animal inhabitants. The scaly critters had no way of knowing this. All they could sense—as frothy eddies of dead shrimp and jellyfish rocked up against them—was that their forest was no more.

Then, a flash of lightning. As thunder crashed overhead, a storm wind pushed a wall of water onto the raft, turning it over and breaking it in half. Some of the scaly critters were washed away by the surge, their limp bodies joining the rotting jellyfish and shrimp. Most of them, however, were able to scramble back onto one of the two remnants of the divided raft. As rain pelted the bay and the winds howled, the currents split: one sweeping to the east and the other to the west. The two rafts—and their scaly cargo—headed in opposite directions.

A few days later, as the storm subsided, the rafts washed up on different shores. As the two bands of critters ventured out into their new homes, they were faced with different challenges—different habitats, climates, and predators. Over the course of many generations, both groups became well adapted to their new environments, to the point where each became a new species. Both species then begat other species, and two major lineages were born. One of them developed two windowlike openings behind the eye socket, to provide room for bigger and stronger jaw muscles. The other developed a single, expansive opening.

The first group, with their two skull openings, were the diapsids. They would eventually evolve into lizards, snakes, crocodiles, dinosaurs, birds, and turtles (which closed up their holes). The second group, with their single skull opening, were the synapsids. They would diversify into a dazzling array of species, including—more than a hundred million years in the future—the mammals.

THIS IS A story, and this exact sequence of events probably did not happen. But it is true that around 325 million years ago—during a time in Earth history called the Pennsylvanian Period (also known as the Late Carboniferous Period)—there was an ancestral stock of small, scale-covered critters that lived in lush swamp forests, which were frequently inundated by rising seas. They split apart, with one side of the family tree leading to reptiles and the other toward mammals.

How do we know this? Paleontologists—scientists like me who study ancient life—have two key lines of evidence. It’s this evidence that I will marshal throughout this book to tell the story of mammal evolution.

First, there are fossils, and the rocks that entomb them. Fossils are direct evidence of species that used to be alive; they are the clues that paleontologists travel around the world searching for, often braving heat, cold, humidity, rain, lack of money, mosquitoes, war zones, or other obstacles. Many of us fancy ourselves as deep-time detectives, and in this analogy, fossils are the equivalent of hair or fingerprints left at a crime scene. They tell us what lived where, and when, and in some cases fossils can reveal prehistoric dramas of predators slashing prey, victims swept up in floods, survivors enduring grim extinctions. The most familiar fossils are body fossils: actual parts of a once-living organism, like a bone, tooth, shell, or leaf. Others are trace fossils: records of an organism’s behavior or something that it left behind, like footprints, burrows, eggs, bite marks, or coprolites (fossilized dung).

We don’t find fossils lying on the street or in the soil of our backyards, but rather inside of rocks like sandstone and mudstone. Different rocks formed in different environments, and some rocks can be dated using chemical techniques, which count the amount of radioactive parent and daughter isotopes to calculate an age, based on reference to known rates of radioactive decay from lab experiments. This all provides critical context for understanding when, and in which habitats, the fossilized creatures lived.

The second type of evidence is all around us, and doesn’t require any special skill (or luck) to find. It’s DNA, which we and all other organisms carry inside of our cells. DNA is the blueprint that makes us what we are, the genetic code that controls what our bodies look like, our physiology and growth, and how we produce future generations. DNA is also an archive; evolutionary history is written into the billions of base pairs that make up our genome. As species change over time, their DNA changes. Genes mutate, move around, and are switched on and off. Stretches of DNA are duplicated or erased. New bits of DNA are inserted. Consequently, as two species diverge from a common ancestor, their DNA will become progressively more different over time as each species goes its own way and adapts to its own changing situations. Thus, you can take the DNA sequences of modern-day species, line them up and compare them, and make a family tree by grouping together species with the most similar DNA. There’s also another nifty trick. You can take any two species, count the number of DNA differences, and then knowing something about the rate DNA changes in lab experiments, back-calculate to figure out when those species separated from each other.

I used both types of evidence in creating this story of the flood-ravaged swamp. DNA studies predict that the reptile and mammal lines separated from each other around 325 million years ago. Fossils and rocks tell us what this lost world was like, a landscape that was much different than today.

A map of the Pennsylvanian-aged Earth would have been scarcely recognizable. There were only two big landmasses, one called Gondwana centered on the South Pole and another named Laurasia that hugged the equator, fringed by a series of smaller islands to the east. Over many millions of years, Gondwana drifted northward, at about the rate that our fingernails grow, before colliding with Laurasia. This was the beginning of the birth of Pangea, the supercontinent on which the early stages of mammal and dinosaur evolution would eventually unfold. As the two slabs of crust slammed into each other, they deformed into a long stretch of mountains paralleling the equator, similar in scale to today’s Himalayas. Today’s modest Appalachian Mountains are a remnant of this once-towering chain.

The tropical and subtropical regions on both sides of the equatorial mountain range were havens for life. These were the coal swamps, so-called because much of the coal that fueled the Industrial Revolution—particularly that mined in Europe and the midwestern and eastern United States—formed in these very swamps. It was made from the dead, buried, and compressed remnants of those gigantic, fast-growing Lepidodendron and Calamites trees. These were not at all like the palms, magnolias, and oaks that are so common in similar lush environments today. In fact, these ancient trees didn’t have flowers, or produce any fruits or nuts. They were close relatives of club mosses and horsetails, primitive plants that persist today as rare parts of the understory, a sad remnant of what they once were. The Pennsylvanian trees—and the supersize dragonflies that buzzed around their branches and millipedes that scurried around their trunks—were able to grow so large because there was much more oxygen in the air then, some 70 percent more than today.

The trees formed vast rain forests, which clutched the shores of the shallow seas that lapped far onto the growing supercontinent, and the many streams, rivers, deltas, and estuaries that fed into them. An aerial survey of these swamps probably would have looked something like the Mississippi River bayous of modern Louisiana: a dense blanket of trees and smaller plants, twisted together, some perched on islands of mud between tangled stream networks, others with spidery roots extending into the water, with all sorts of creatures climbing and jumping and flying about. But no birds, no mosquitoes, no beavers or otters or other fur-covered mammals. All of them would evolve much later, in a very different world, although their ancestors did inhabit the coal swamps.

Why were so many trees getting buried and turned to coal? It’s because the swamps were constantly flooded. Sea level was always rising and falling, in a pulsating rhythm. The Pennsylvanian was a glacial world—in fact, it was the last major ice age before the most recent one, when mammoths and saber-toothed tigers reigned (a story we will get to later). The entire planet was not frigid; certainly the coal swamps weren’t. But over the South Pole of Gondwana, and then southern Pangea, there was an enormous ice cap. It owed its very existence to the coal swamps: the growth of so many giant trees drew carbon dioxide out of the atmosphere, and with less of this greenhouse gas to insulate the planet, temperatures plummeted. Over tens of millions of years, the ice cap waxed and waned in size, a conductor controlling the global sea level. Ice would melt, seas would rise, swamps would drown, trees would die and get buried. Then the ice would grow, sucking up water from the seas, lowering the sea level, making room for swamps to thrive. Back and forth it went. We know this because Pennsylvanian rocks often form barcode sequences called cyclothems, repeated series of thin layers formed on land and in the water, with coal seams tucked in between.

Fossils from this time are plentiful, especially where I grew up in northern Illinois. They are embedded in the cyclothems, above and below the coal. The best ones are found along the banks of Mazon Creek, a gentle tributary of the Illinois River, and the strip mines to the east. During the Pennsylvanian this was where sea met swamp, where denizens of the rain forests were flushed into the water, sunk to the bottom, and encased in ironstone tombs—oval, flattened, rust-colored nodules that you can pluck from the creek bed or mine tailings. As a teenager I searched for these nodules, near the tiny Route 66 town of Wilmington, where my mother was raised. I scoured the spoil piles of the long-closed mines, which more than a century earlier had beckoned my Italian great-grandparents to a new life in Middle America. I put the nodules in a bucket, took them home, put them outside in the brutal Chicagoland winter, and let them repeatedly freeze and thaw as the temperature fluctuated. When it looked as if one was starting to crack, I would finish the job with a hammer.

If I was lucky, the nodule would break open and a treasure would be revealed: a fossil on one side, its impression on the other. Each time it was an otherworldly experience, knowing that you were the first person to see this thing—which was once alive!—that had died over 300 million years ago. Many of the cracked nodules had plants inside: fern leaves, pieces of Calamites bark, chunks of Lepidodendron roots. I was particularly fond of jellyfish—what veteran Mazon Creek fossil hunters dismissively called blobs—and always enjoyed catching sight of a shrimp or worm.

What I really wanted—and never had the fortune of finding—was a tetrapod, a land-living animal with bones. I knew from the textbooks I was devouring, after school and on quiet weekend afternoons, that tetrapods had evolved from fishes and crawled onto land about 390 million years ago, before the Pennsylvanian Period. These first tetrapods were amphibians, which still needed to return to the water to lay their eggs. Some primitive amphibian skeletons—remote relatives of frogs and salamanders—had even been found at Mazon Creek.

Sometime during the Pennsylvanian a new group spun off from these amphibians. They were the amniotes, more specialized tetrapods named for their amniotic eggs, whose internal membranes surround the embryo to protect it and prevent it from drying out. These new eggs unlocked a great new potential: the amniotes no longer were handcuffed to the water but could lay their eggs inland, giving them access to new frontiers. Treetops, burrows, plains, mountains, deserts. Only with the advent of the amniotic egg did tetrapods truly divorce themselves from the seas, and properly conquer the land.

It was from the amniotes that the reptile and mammal lines—the diapsids and synapsids—arose, by splitting from each other like two siblings from their parents. This is not simply an analogy; it is how evolution produces new species, new families, new dynasties. Species are always changing as their environments change—this is Darwin’s evolution by natural selection. Sometimes populations of a single species become separated from each other, maybe by a flood, fire, or a rising mountain range. Each population will continue to change through natural selection, and if they are separated long enough, they will change in their own idiosyncratic ways, to become adapted to their different circumstances, so much so that they no longer look the same, or behave the same, or can mate with each other. At this point one species has become two. The two new species might then split again, two becoming four, and so on. Life is always diversifying in this way, branching like a tree that has been growing for over 4 billion years. This is why we use family trees to visualize genealogy—both for extinct species and our own human families—rather than family nets, or road maps, or triangles, or some other type of graphic aid.

The diapsid-synapsid split—which really would have begun inconspicuously with one small, scaly ancestral species dividing into two—was one of the keystone moments in vertebrate evolution. And I knew that the diapsids and synapsids—each characterized by their own unique system of skull holes and jaw muscles—were setting off right around the time the Mazon Creek nodules were forming. With each whack of the hammer, I hoped to find a Holy Grail fossil that would help tell this story, but alas, it never came.

Other fossil hunters in other parts of North America, however, were more successful. One important discovery was made in 1956, when a Harvard field team led by the legendary paleontologist Alfred Romer surveyed an abandoned coal mine in Florence, Nova Scotia, near the Atlantic coast. One of his technicians, Arnie Lewis, noticed several fossilized stumps of a tree called Sigillaria, a close relative of Lepidodendron, whose crown of leaves forked at the top, giving it the impression of a giant brush. The stumps were in life position, as if they had been drowned by rising seas only yesterday, not around 310 million years ago, their true age. Wading through the narrow shafts of the flooded mine, the team was able to collect five of the stumps. When they looked inside, they found quite the surprise: dozens of fossil skeletons! The poor creatures may have sought shelter in the trees as the seas encroached, not realizing they were entering their graves. One particular tree had more than twenty animals inside, including amphibians, diapsids, and synapsids: the trifecta of early land-living tetrapods.

Archaeothyris (illustrated by Todd Marshall)

The synapsids were later described as two new species, Archaeothyris and Echinerpeton, by a master’s student, Robert Reisz, who had just emigrated from Romania to Canada. Now one of the world’s leading paleontologists, Reisz cut his teeth on these early synapsids. He chose the name Archaeothyris—meaning ancient window—to highlight the most important feature of this animal: its large, portholelike opening behind its eye, which housed larger and more powerful jaw-closing muscles than its ancestors. It is this single opening, technically called the lateral temporal fenestra, which defines what it means to be a synapsid. All synapsids—from the coal swamp pioneers to today’s bats and shrews and elephants—have this fenestra, or a modified version of it. So do we, and we can feel it every time we close our jaws. Put your hand on your cheekbone, take a big bite, and feel the muscles of your cheek contracting. Those muscles are passing through the remnant of the fenestra, which in modern mammals has more or less merged with the eye socket, but still anchors the temporal muscles that stretch from the side of our head to the top of our lower jaw, powering our bites. This single opening developed early in synapsid history, right after they split from the diapsids, which went on to evolve two such openings behind their eyes.

The two main skull types of land-living vertebrates: diapsids with two openings for jaw muscles behind the eye and synapsids—including humans—with a single opening. Arrows denote the jaw openings. (illustrated by Sarah Shelley)

If you saw it scampering through the coal swamps, Archaeothyris would have looked unexceptional. It was about one and a half feet (fifty centimeters) long from snout to tail, with a small head perched on a long, slender body. Its limbs are not well known, but the preserved bones leave no doubt that the arms and legs sprawled out sideways, akin to a lizard or crocodile. Clearly, it was not built for speed. On closer inspection, however, it was exceptional in other ways. Not only were those bigger jaw muscles hiding inside its skull, but its snout bore a series of curved, pointed teeth. One of the front ones was noticeably larger than the others, such that it looked like a minicanine. Amphibians, lizards, and crocodiles do not have canines. All these animals have uniform teeth, which basically look the same across the entire jaw. Mammals, though, have a much more varied dentition, split into incisors, canines, premolars, and molars—a division of labor that allows us to grab, bite, and crush all at the same time. The full mammalian dentition would assemble later, over many evolutionary steps, but the little canines of Archaeothyris are the whispers of a dental revolution.

Taken together, the big jaw muscles, sharp teeth, and canines of Archaeothyris were an arsenal of weapons for feeding on big insects, and maybe other tetrapods, like Echinerpeton. This second Nova Scotian synapsid could have easily curled up and fit in between the pages of this book. But its scrappy fossils do show one peculiar feature, which gives it its name: spiny reptile. The spines on the neck and back vertebrae—the individual bones that make up the backbone—expand upward as elongate tabs. When lined up together they would have formed a small sail along the back, which might have been used for display, or as a solar panel to heat up the body on cool days, or a fan to shed heat on warm days, or something else entirely.

There is another, much more famous extinct animal with an even bigger sail on its back: Dimetrodon, which lived during the next interval of time after the Pennsylvanian, the Permian Period. All too often Dimetrodon is mistaken for a dinosaur, sharing space with T. rex on dinosaur posters, jostling with Brontosaurus and Stegosaurus in dinosaur toy sets. But it is not a dinosaur; it is a synapsid. More specifically, it is a type of primitive synapsid called a pelycosaur.

The pelycosaurs were the first big evolutionary wave of the synapsid lineage; they were the first to diversify and spread around the growing supercontinent of Pangea, and the first ones to start developing some of the trademark features that, some 300+ million years later, are still the things that make mammals stand out from amphibians, reptiles, and birds. Features like the temporal muscle opening, and the canine teeth. Features that we’ve already seen, in Archaeothyris and Echinerpeton. That’s because these two Nova Scotia species are the very oldest pelycosaurs, the founders of the first great dynasty on the journey toward Dimetrodon, and ultimately mammals.

AS THE PENNSYLVANIAN Period drew to a close, there were pelycosaur synapsids living across the equatorial regions of Pangea, on both sides of the still-rising mountain range. Some ate insects, others preyed on small tetrapods and fish, and a few started to experiment with a new food type that thus far had been ignored: leaves and stems. They were diversifying, but they remained minor components of their ecosystems, which were overrun with amphibians that could easily reproduce, and thus prosper, in the damp coal forests.

Then, between about 303 and 307 million years ago, the world dramatically changed during a spasm called the Carboniferous Rainforest Collapse. The climate became drier, temperatures swung cold and hot, and the ice caps melted, eventually disappearing for good in the ensuing Permian Period. The coal swamps were devastated, as the soaring Calamites, Lepidodendron, and Sigillaria trees found it harder to grow in the more arid conditions. They were replaced by conifers, cycads, and other seed-bearing plants, which were more drought resistant. The ever-wet rain forests gave way to more seasonal, semiarid drylands in the tropics, and other parts of Pangea became parched deserts. This is reflected in the rock record by the sudden shift from coals and cyclothems to red beds, full of rusty iron formed in dry climates.

These changes had a striking impact on biodiversity. Plants were hit especially hard. Not only was there a shift from Pennsylvanian coal swamp vegetation to the more dry-adapted seed plants,