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Other scientists had collected a wealth of fossils from similar-age rock units in New Mexico, Arizona, and Texas, which seemed to belong to more than a dozen species of dinosaurs, ranging from stocky apex predators and smaller meat-eaters to many different types of plant-munching ornithischians, the ancestors of Triceratops and the duckbills. It seemed that dinosaurs were everywhere. But that wasn’t the case in the Hayden Quarry. There were monster amphibians closely related to our Portuguese Metoposaurus, primitive crocodiles and some of their long-snouted and armored relatives, skinny reptiles with short legs called Vancleavea, which looked like scaly dachshunds, and even funny little reptiles that hung from the trees like chameleons, called drepanosaurs. Those are the common animals in the quarry. Dinosaurs were anything but. The Rat Pack found only three types of dinosaurs: a fleet-footed predator very similar to Baldwin’s Coelophysis, another swift carnivore called Tawa, and a somewhat larger and stockier meat-eater called Chindesaurus, which was closely related to the Argentine Herrerasaurus. Each is represented by only a few fossils. It was a great surprise to the team. Dinosaurs were rare in the tropical deserts of the Late Triassic, and it was only the meat-eaters that seemed to be hanging about. There were no plant-eating dinosaurs, none of the ancestral long-necked species that were so common in the humid zones, none of the ornithischian forebears of Triceratops. It’s a meager bunch of dinosaurs surrounded by all sorts of bigger, meaner, more common, more diverse animals. What, then, to make of the dozens of Triassic dinosaur species that other scientists had identified from all over the American Southwest? Irmis, Nesbitt, Smith, and Turner scrutinized all of the evidence they could find, traveling to every small-town museum where researchers had deposited their fossils. They saw that most of these specimens were isolated teeth and scraps of bone, not the best foundation for naming new species. But that wasn’t the shocker. The more they found at Hayden Quarry, the better search image the crew developed in their heads. They became able to tell a dinosaur from a crocodile from an amphibian almost by instinct. In a series of eureka moments, they realized that most of those supposed dinosaur fossils collected by others weren’t dinosaurs at all, but primitive dinosauromorph cousins of dinosaurs or, in some cases, early crocodiles and their kin that just so happened to look like dinosaurs. So not only were dinosaurs rare in the Late Triassic deserts, but they were still living alongside their archaic relatives, the same types of animals that were leaving their tiny footprints in Poland nearly 40 million years earlier. It was a jarring realization. Up until then, almost everyone thought that the primitive dinosauromorphs were an uninteresting ancestral stock whose only destiny was to give birth to the mighty dinosaurs. Once that job was done, they could quietly fade away to extinction.


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But here they were, all over Late Triassic North America, even a new poodle-size species called Dromomeron in the Hayden Quarry, living alongside proper dinosaurs for some 20 million years. Probably the only person not surprised by the findings was another student, an Argentine named Martín Ezcurra. Independently of the four American grad students, Martín was starting to doubt the identifications of some of the supposed North American “dinosaurs” collected by the older generation of paleontologists, but he didn’t have the resources to go study them, because he was from South America and still learning English. That, and he was a teenager. One thing he did have, however, was access to the tremendous collections of Ischigualasto dinosaurs from his home country, thanks to the generosity of Ricardo Martínez and other curators who responded positively to the unusual request of a high schooler wanting to visit their museums. Martín gathered photos of many of the mysterious North American specimens and carefully compared them to the Argentine dinosaurs, and recognized that there were key differences. One North American species in particular, a skinny carnivore called Eucoelophysis, which was supposed to be a theropod, was actually a primitive dinosauromorph. He published this result in a scientific journal in 2006, the year before Irmis, Nesbitt, Smith, and Turner published their first findings. Martín was seventeen years old when he wrote his paper. It’s hard to fathom why dinosaurs were doing so poorly in the deserts while so many other animals, including their dinosauromorph precursors, were having a better go at it. To get to the bottom of the question, Chinle’s Rat Pack collaborated with the skilled geologist Jessica Whiteside, who was also part of our excavation teams in Portugal. Jessica is a maestro at reading the rocks. Better than anyone I’ve ever known, she can look at a sequence of rocks and tell you how old they are, what the environments were like when they formed, how hot it was, even how much rain there was. Set her loose at a fossil site, and she’ll come back with a story from the distant past of changing climates, shifting weather, evolutionary explosions, and great extinctions. Jessica put her sixth sense to use at Ghost Ranch and determined that the animals of the Hayden Quarry did not have an easy life. They lived in an environment that wasn’t always a desert, but one in which seasonal climates dramatically fluctuated. It was bone-dry for much of the year, but wetter and cooler during other times—hyperseasonality, as Jessica and the Rat Pack call it. The culprit was carbon dioxide. Jessica’s measurements show that there were somewhere around 2,500 molecules of carbon dioxide per every million molecules of air in the tropical regions of Pangea back when the Hayden Quarry animals were alive. That’s more than six times the amount of carbon dioxide today.


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Let that sink in for a minute—just think about how quickly temperatures are rising now and how anxious we are about future climate change, even though there is much less carbon dioxide in today’s atmosphere. The high concentration of carbon dioxide in the Late Triassic started a chain reaction: huge fluctuations in temperature and precipitation, raging wildfires during parts of the year but humid spells in others. Stable plant communities had a difficult time establishing themselves. It was a chaotic, unpredictable, unstable part of Pangea. Some animals could deal with that better than others. Dinosaurs seem to have been able to cope a little bit, but not able to truly thrive. The smaller meat-eating theropods were able to manage, but the larger, fast-growing plant-eaters, which required a steadier diet, could not. Even some 20 million years after they had originated, even after they had taken over the large-herbivore niche in humid ecosystems and started to colonize the hotter tropics, dinosaurs were still having trouble with the weather. IF YOU WERE standing on safe ground during a Late Triassic flood, watching the animals eventually buried at Hayden Quarry get swept up by the seasonal river that drowned them, you might have had a hard time telling some of the corpses apart as they floated by. Sure, it would be easy to recognize one of the giant supersalamanders or some of those weird chameleon-mimic reptiles. But you might not be able to distinguish dinosaurs like Coelophysis and Chindesaurus from some of the crocodiles and their kin. Even if you were able to watch these animals alive, going about their business of eating and moving and interacting with each other, you still might have trouble. Why the confusion? It’s the same reason that the previous generation of paleontologists working in the American Southwest so often misidentified crocodile fossils as dinosaurs, and why other scientists in Europe and South America made the same mistakes. During the Late Triassic, there were many other animals that really, really looked and behaved like dinosaurs. In evolutionary biology speak, this is called convergence: different types of creatures resembling each other because of similarities in lifestyle and environment. It’s why birds and bats, which both fly, each have wings. It’s why snakes and worms, which both squirm through underground burrows, are both long, skinny, and legless. The convergence between dinosaurs and crocodiles is surprising, shocking even. The alligators that prowl the Mississippi delta and the crocodiles that lurk in the Nile may appear vaguely prehistoric, but they don’t look anything like a T. rex or a Brontosaurus. During the Late Triassic, however, crocodiles were very different. Recall that dinosaurs and crocodiles are both archosaurs—members of that large group of upright-walking reptiles that started to blossom after the end-Permian mass extinction, which proliferated because they could move much faster and more efficiently than the sprawling animals of the time.


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Early in the Triassic, archosaurs split into two major clans: the avemetatarsalians, which led to dinosauromorphs and dinosaurs, and the pseudosuchians, which gave rise to crocodiles. During the dizzying splurge of postextinction evolution, the pseudosuchian tribe also produced a number of other subgroups that diversified in the Triassic but then went extinct. Because they don’t survive today—unlike the crocodiles and dinosaurs (in the guise of birds) —these groups have largely been forgotten about, considered oddities from a distant past, evolutionary dead ends that never rose to the top. That stereotype is wrong, though, because for much of the Triassic these crocodile-line archosaurs were thriving. Most of the major types of Late Triassic pseudosuchians can be found at Hayden Quarry. There is a phytosaur called Machaeroprosopus, a member of that group of long-snouted, semiaquatic ambush predators whose bones we also found in Portugal. They were bigger than a motorboat and snatched fish—and the occasional passing dinosaur—with the hundreds of spiky teeth in their stretched jaws. It was neighbors with Typothorax, a plant-eater built like a tank with armor covering its body and spikes sticking out from its neck. It belongs to a group called the aetosaurs, a hugely successful family of mid-tier herbivores that closely resembled the armored ankylosaur dinosaurs that evolved millions of years later. They were good diggers and may have even cared for their young by building and guarding nests. Then there are proper crocodiles, but nothing like the ones we’re familiar with today. These primitive Triassic species—the ancestral breed that modern crocs evolved from—looked like greyhounds: they were about the same size, stood on four legs, had the emaciated build of a supermodel, and could sprint like champions. They fed on bugs and lizards and were most certainly not top predators. That title went to the rauisuchians, a ferocious bunch that grew up to twenty-five feet long, bigger than the largest saltwater crocodiles today. We met one of them previously, Saurosuchus, the top gun in the Ischigualasto ecosystem that would have haunted the nightmares of the very first dinosaurs. Imagine a slightly smaller version of a T. rex walking around on four legs, with a muscular skull and neck, railroad-spike teeth, and a bone-breaking bite. There’s also another type of crocodile-line archosaur found at Ghost Ranch—not in the Hayden Quarry itself, but in the nearby Coelophysis graveyard. It was found in 1947, not long after Whitaker discovered the bone bed, during those first few weeks of excavation. The American Museum team was digging up so many Coelophysis skeletons that, after a while, the excitement wore off and they got a little bored. Everything they saw started to look like Coelophysis. So they didn’t notice that one of the skeletons they collected was similar in size to Coelophysis, and had the same long legs and light build, but was a little different in other ways—notably, it had a beak instead of an arsenal of sharp teeth. The technicians back in New York didn’t notice either.


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They started to remove the specimen from the block of rock it was embedded in, but were all too keen to stop once they determined it was just another Coelophysis. It could go in the storehouse with the rest of them. The fossil stayed in the bowels of the museum, unconserved and unloved, until 2004. That’s when one of the Ghost Ranch quartet, Sterling Nesbitt, started his PhD at Columbia University in New York. Because he was planning a project on Triassic dinosaurs, he went back through all of the fossils collected by Colbert, Whitaker, and their teams in the 1940s. Many were still encased in plaster, so they would have to remain on the shelves. But that one block from 1947 had been opened and partially prepared by the conservators, so Sterling could study it. With an excited pair of eyes and an enthusiasm that escaped the weary field hands a half century earlier, Sterling recognized that he wasn’t looking at any old Coelophysis. He saw that it had a beak; he realized that its body proportions were different, that its arms were tiny. And then he noticed features of the ankle that were nearly identical to those of crocodiles. He wasn’t looking at a dinosaur at all; he was looking at a pseudosuchian that was heavily convergent on dinosaurs. The fierce predator Batrachotomus, one of the crocodile-line archosaurs (rauisuchians) that preyed on early dinosaurs. Photo courtesy of the author This was the sort of discovery that young scientists dream about when secluded, alone with their thoughts, trawling through the drawers of museum collections. Since Sterling discovered it, he got to name it, and he chose the evocative moniker Effigia okeeffeae: the first name being the Latin word for ghost, in reference to Ghost Ranch, and the second paying homage to the ranch’s most famous resident. Effigia made international headlines: the media loved this awkward-looking, toothless, stub-armed ancient crocodilian creature trying to pretend that it was a dinosaur. Stephen Colbert even devoted a segment of his show to the new discovery, complaining in jest that it should have been named after Edwin Colbert (who coincidentally shared a surname with the comedian) and not the feminist artist. I remember seeing that segment during the last year of my undergraduate studies, right around the time I was starting to plot out my own graduate-school future, and being in awe that a young grad student’s work could make such an impact. It also motivated me. Up until that point, I had been studying only dinosaurs, but I started to grasp that Effigia and the other dino-imitating pseudosuchians were critical in understanding how dinosaurs ascended to power. I started to read many of the classic studies in dinosaur paleontology, works by giants like Robert Bakker and Alan Charig, which were effusive in arguing that dinosaurs were special.


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They were so well endowed with superior speed, agility, metabolism, and intelligence that they outcompeted all of the other Triassic animals—the giant salamanders, the early mammal-like synapsids, and the crocodile-line pseudosuchians. Dinosaurs were the chosen ones. It was their manifest destiny to take on the weaker species, best them, and establish a global empire. There was almost a religious feel to some of these writings, perhaps not a surprise, given that Bakker also dabbles as an ecumenical Christian preacher and is renowned for his high-energy lectures, delivered in the style of an evangelist testifying to his congregation. Dinosaurs outmaneuvering their foes on the Late Triassic battlefield. It was a good story, but it didn’t sit well with me. New discoveries seemed to be upending the narrative, and a lot of that had to do with the pseudosuchians. So many of these crocodile-line archosaurs were dead ringers for dinosaurs. Or maybe it was the other way around: maybe Triassic dinosaurs were trying to be pseudosuchians. Regardless, if the two groups were similar in so many ways, then how could you argue that dinosaurs were a superior race? And it wasn’t only the convergence between dinosaurs and pseudosuchians that threw up a red flag. There were more pseudosuchians than dinosaurs in the Late Triassic: more species and a greater abundance of these species in individual ecosystems. The menagerie of croc cousins from Ghost Ranch—phytosaurs, aetosaurs, rauisuchians, Effigia-like animals, true crocodiles—was not a local phenomenon. These were diverse groups that prospered throughout much of the world. But, as scientists often like to say when trying to critique each other with subtlety, this all sounded a little arm-wavy. Could we somehow compare explicitly how dinosaurs and pseudosuchians were evolving in the Late Triassic? Was there a way to test whether one group was more successful than the other and whether that was changing over time? I buried myself in literature on statistics, unfamiliar territory for somebody who was consumed by dinosaurs but not yet very aware of other fields and techniques. I was a bit embarrassed to realize that invertebrate paleontologists—our redheaded stepsiblings, who study fossils like clams and corals, which don’t have bones—had come up with a method two decades earlier, one that had been ignored by dinosaur workers. It was something called morphological disparity. Morphological disparity sounds like a fancy term, but it is simply a measure of diversity. You can measure diversity in many ways. Counting up the number of species is one tack: you can say that South America is more diverse than Europe because there are more animal species there. Or you can compute diversity based on abundance: insects are more diverse than mammals because there are more insects in any given ecosystem. What morphological disparity does is measure diversity based on features of the anatomy. Thinking this way, you can consider birds to be more diverse than jellyfish, because birds have a much more complex body with lots of different parts, whereas jellyfish are just sacs of goo.


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This type of diversity measure can give great insight into evolution, because so many aspects of animal biology, behavior, diet, growth, and metabolism are controlled by anatomy. If you really want to know how a group is changing over time or how two groups compare in diversity, I would argue that morphological disparity is the most powerful way to do so. Counting the number of species or the abundance of individuals is easy. All you need are a good set of eyes and a calculator. But how to measure morphological disparity? How to take all the complexity of the animal body and turn it into a statistic? I followed the approach pioneered by the invertebrate paleontologists. It went something like this. I first came up with a list of all of the Triassic dinosaurs and pseudosuchians, as these were the animals that I wanted to compare. I then spent months studying the fossils of these species and made a list of hundreds of features of the skeleton in which they vary. Some have five toes, others have three. Some walk on four legs, others on two. Some have teeth, others do not. I encoded these features in a spreadsheet as zeros and ones, just as a computer programmer would. Herrerasaurus walks on two legs, state 0. Saurosuchus walks on four legs, state 1. At the end of nearly a year of work, I had a database with seventy-six Triassic species, each assessed for 470 features of the skeleton. With the long slog of data collection done, it was time for the math. The next step was to make what is called a distance matrix. It quantifies how different each species is from every other species, based on the database of anatomical characteristics. If two species share all features, then their distance score is 0. They are identical. If two other species share no characteristics, then their distance score is 1. They are completely different. For the in-between cases, let’s say that Herrerasaurus and Saurosuchus share 100 characteristics but differ in the 370 others. Their distance score would be 0.79: the 370 features they differ in divided by the 470 total features in the data set. The best way to envision this is to think of those tables in a road atlas, which give distances between different cities. Chicago is 180 miles from Indianapolis. Indianapolis is 1,700 miles from Phoenix. Phoenix is 1,800 miles from Chicago. That table is a distance matrix. Here’s the neat trick about a distance matrix in an atlas. You can take that table of road distances between cities, stick it into a statistics software program, run what is called a multivariate analysis, and the program will spit out a plot. Each city will be a point on that plot, and the points will be separated by distance, in perfect proportion. In other words, the plot is a map—a geographically correct map with all of the cities in the right places and distances relative to each other.


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So what happens if we instead input the distance matrix that encapsulates the skeletal differences of Triassic dinosaurs and pseudosuchians? The statistics program will also produce a plot in which each species is represented by a point, a plot that scientists call a morphospace. But really it is just a map. It visually shows the spread of anatomical diversity among the animals in question. Two species close together have very similar skeletons, just as Chicago and Indianapolis are comparatively near geographically. Two species at far corners of the graph have very different anatomies, like the longer distance between Chicago and Phoenix. This map of Triassic dinosaurs and pseudosuchians allows us to measure morphological disparity. We can group the animals in the plot by which great tribe they belong to—dinosaurs or pseudosuchians—and calculate which is occupying a larger swath of that map and therefore more anatomically diverse. In the same vein, we can further group the animals by time—Middle Triassic versus Late Triassic, let’s say—and see if dinosaurs or pseudosuchians were becoming more or less anatomically diverse as the Triassic progressed. We did that and came up with a startling result that we published in 2008 in a study that helped launch my career. All throughout the Triassic, the pseudosuchians were significantly more morphologically diverse than dinosaurs. They filled a larger spread of that map, meaning they had a greater range of anatomical features, which indicated that they were experimenting with more diets, more behaviors, more ways of making a living. Both groups were becoming more diverse as the Triassic unfolded, but the pseudosuchians were always outpacing the dinosaurs. Far from being superior warriors slaying their competitors, dinosaurs were being overshadowed by their crocodile-line rivals during the 30 million years they coexisted in the Triassic. PUT YOURSELF BACK in the tiny furry feet of our Triassic mammalian ancestors, surveying the Pangean scene as the Triassic was drawing to a close 201 million years ago. You would be seeing dinosaurs, but you wouldn’t be surrounded by them. Depending on where you were, you might not have noticed them at all. They were relatively diverse in the humid regions, where protosauropods got as large as giraffes and were the most abundant plant-eaters, but there carnivorous theropods and herbivorous to omnivorous ornithischians were considerably smaller and less common. In the more arid zones, there were only small meat-eaters, the herbivores and larger species being unable to tolerate the hyperseasonal weather and megamonsoons. There were no dinosaurs remotely approaching a Brontosaurus or T. rex in size, and all across the supercontinent they were living under the thumb of their much more diverse, much more successful pseudosuchian adversaries. You would probably consider the dinosaurs a fairly marginal group. They were doing OK, but so were many other newly evolved types of animals.


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If you were of a gambling persuasion, you would probably have bet on one of these other groups, most likely those pesky crocodile-line archosaurs, as the ones that would eventually become dominant, grow to massive sizes, and conquer the world. Some 30 million years after they originated, the dinosaurs had yet to mount a global revolution.


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The Rise and Fall of the Dinosaurs 3 Dinosaurs Become Dominant Scottish sauropod Chapter Title art by Todd Marshall SOME TIME AROUND 240 MILLION YEARS ago, the Earth began to crack. True dinosaurs hadn’t quite evolved yet, but their cat-size dinosauromorph ancestors would have been there to experience the cracking—except there wasn’t much to experience, at least not yet. There may have been some minor earthquakes, but these probably wouldn’t have even registered with the dinosauromorphs, who were busy with more important things like fending off the supersalamanders and surviving the megamonsoons. As these dinosauromorphs gave rise to dinosaurs, the fracturing continued, many thousands of feet underground. Imperceptible on the surface, these fissures were slowly moving, growing, merging together, a hidden danger lurking under the feet of Herrerasaurus, Eoraptor, and the other first dinosaurs. The very foundation of Pangea was splitting, and with the blissful ignorance of homeowners who don’t realize there’s a creeping crack in their basement until their house comes tumbling down, the dinosaurs had no inkling that their world was going to dramatically change. As these earliest dinosaurs were evolving in fits and starts during the final 30 million years of the Triassic, great geological forces were tugging on Pangea from both the east and west. These forces—a planet-scale cocktail of gravity, heat, and pressure—are strong enough to make continents move over time. Because the pull was coming from two opposite directions, Pangea began to stretch and gradually become thinner, each small earthquake causing another tear. Imagine Pangea as a giant pizza, being torn apart by two hungry friends at opposite ends of the table: the crust becomes thinner until there is a rupture and it breaks into two. The same thing happened with the supercontinent. After a few tens of millions of years of slow and steady tug-of-war, east versus west, the cracks reached the surface, and the giant landmass began to unzip down its middle. It’s because of that ancient divorce between east and west Pangea that the seaboard of North America is separated from western Europe and South America sits apart from Africa. It’s why there is now an Atlantic Ocean, which didn’t exist until seawater rushed in to fill the gap between the separating tracts of land. Those forces and fractures over 200 million years ago shaped our modern geography. But there was more to it than that, because continents don’t just split up and call it a day. As with human relationships, things can get really nasty when a continent breaks up. And the dinosaurs and other animals growing up on Pangea were about to be changed forever by the aftereffects of their home being ripped in two. The problem boils down to this: as a continent tears, it bleeds lava. It’s nothing more than basic physics. The Earth’s outer crust is pulled apart and thins, decreasing pressure on the deeper parts of the Earth. As pressure lessens, magma from the deeper Earth rises to the surface and erupts through volcanoes.


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If there is only a little rip in the crust—two small bits of a continent separating from each other, let’s say—then the effects aren’t too bad. You might get a few volcanoes, some lava and ash, some local destruction, and then eventually it stops. That kind of thing is happening in eastern Africa today, and it’s far from catastrophic. But if you’re slashing apart an entire supercontinent, then you approach the realm of apocalypse. At the very end of the Triassic, 201 million years ago, the world was violently remade. For 40 million years, Pangea had been gradually splintering apart, and magma had been welling underground. Now that the supercontinent had finally cracked, the magma had somewhere to go. Like a hot-air balloon rising through the sky, the liquid-rock reservoir rushed upward, broke through the shattered surface of Pangea, and gushed out onto the land. As with the volcanoes that had erupted at the end of the Permian Period some 50 million years earlier, causing the extinction that allowed dinosaurs and their archosaur cousins to get their start, these end-Triassic eruptions were different from any that humans have ever witnessed. We’re not talking Pinatubo here, with hot clouds of ash bursting into the sky. Instead, over a period of some six hundred thousand years, there were four big pulses of drama, when enormous amounts of lava would surge out of the Pangean rift zone like tsunamis from hell. I’m hardly exaggerating: some of the flows were, added up together, up to three thousand feet thick; they could have buried the Empire State Building twice over. In all, some three million square miles of central Pangea were drowned in lava. It goes without saying that this was a bad time to be a dinosaur, or for that matter, any other type of animal. These were some of the largest volcanic eruptions in Earth history. Not only did lava smother the land, but noxious gases that rode up with the lava poisoned the atmosphere and caused runaway global warming. These things triggered one of the biggest mass extinctions in the history of life, a mass die-off that claimed over 30 percent of all species and maybe much more. Paradoxically, however, it was also a mass extinction that helped dinosaurs break out of their early-life slump and become the enormous, dominant animals that stoke our imaginations. IF YOU’RE WALKING down Broadway in New York City and happen to catch a gap between the skyscrapers, you can see straight across the Hudson River to New Jersey. You’ll notice that the Jersey side of the river is defined by a steep cliff of drab brown rock, about a hundred feet high, studded with vertical cracks. Locals refer to it as the Palisades. During the summer it can be almost unrecognizable, engulfed by a dense forest of trees and bushes that somehow cling to the sheer slopes.


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Commuter towns like Jersey City and Fort Lee are perched on top of the cliff, and the western end of the George Washington Bridge is built deep into it, an ideal anchor for the world’s busiest overwater crossing. If you wanted to, you could walk along the Palisades for about fifty miles, from where it begins in Staten Island and extends along the Hudson to where it juts into upstate New York. Millions of people look at this cliff every week. Hundreds of thousands of people live on it. Few realize that it is a remnant of those ancient volcanic eruptions that tore apart Pangea and ushered in the Age of Dinosaurs. The Palisades is what geologists refer to as a sill—an intrusion of magma that pokes its way in between two layers of rock far underground, but then hardens into stone before it can erupt as lava. Sills are part of the internal plumbing system of volcanoes. Before they harden into rock, they are pipes, which transport magma underground. Sometimes they are conduits that bring magma to the surface; other times they are dead-end extensions of the volcanic system, cul-de-sacs that magma can’t escape from. The Palisades sill formed at the end of the Triassic, as Pangea was rupturing along what would become the eastern coast of North America, just a few miles from what is now New York City. It formed from those very magmas that were coursing up from the deep earth as the supercontinent broke into two. The magma that became the Palisades sill never made it to the surface. It never got to be part of those three-thousand-foot-thick lava sheets flushed out of the Pangean rift, the ones that engulfed ecosystems and belched out the carbon dioxide that would doom much of the planet. About twenty miles to the west the magmas did erupt, however, and the basalt rocks that formed from them can be seen in a low range of hills called the Watchung Mountains in northern New Jersey. Calling them mountains is generous—they’re just a few hundred feet high, and they cover a tiny area about forty miles north to south—but they are a beloved oasis of natural beauty within one of the most urbanized parts of the world. In the middle of the mountains is Livingston, a bedroom community of about thirty thousand people. In 1968 some folks discovered dinosaur footprints a couple of miles north of the town, in an abandoned quarry where red shales, formed in rivers and lakes near the ancient volcanoes, were being mined. There was a blurb in the local newspaper, which caught the eye of a mother, who told her fourteen-year-old-son, Paul Olsen, who was gobsmacked to learn that dinosaurs once lived so close to his home. He rounded up his friend, Tony Lessa, and they hopped on their bicycles and sped to the old quarry.


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It was no more than an overgrown, rock-strewn hole in the ground, but the discovery had caused a local sensation and several amateur collectors were already there, on the hunt for more tracks. Olsen and Lessa befriended some of the amateurs, who taught them the basics of fossil collecting: how to identify dinosaur footprints, how to remove them from the rock, and how to study them. The two teenagers became obsessed. They kept coming back to the quarry, and before long they were working late into the night, removing slabs of dinosaur footprints by firelight, even in the dead of winter. They had to go to school during the day, so the night was their only option. For over a year they toiled, outlasting the other rockhounds, who began to trickle away once the excitement of the discovery died down. The boys collected hundreds of tracks left by all kinds of creatures, including meat-eating dinosaurs similar to Coelophysis from Ghost Ranch, plant-eating dinosaurs, and some of the scaly and furry creatures that lived alongside. But the more they collected, the more they became dismayed: during their nighttime excavations, they were constantly interrupted by trucks illegally dumping trash, and while they were at school, unscrupulous collectors would often sneak into the quarry and poach footprints the boys hadn’t yet been able to remove. So what’s a 1960s teenager to do when his favorite fossil site is being destroyed? Paul Olsen skipped the middlemen and went right to the top. He began writing letters to Richard Nixon, the newly elected president who had yet to disgrace himself. Lots of letters. He begged Nixon to use his presidential powers to get the quarry preserved as a protected park, and even sent a fiberglass cast of a theropod track to the White House. Olsen led a media campaign, too, and was profiled in an article in Life magazine. His brazen persistence paid off: in 1970 the company that owned the quarry donated the land to the county, which made it into a dinosaur park called the Riker Hill Fossil Site. The next year, the site was granted official national landmark status and Olsen received a presidential commendation for his work. Little did he know it, but he was also an inch away from a White House visit. Some of Nixon’s image-conscious aides thought a photo op with a young science enthusiast would be great PR for the jowly president, but it was killed at the last minute by Nixon’s advisor John Ehrlichman, later one of the key villains of Watergate. It was a great accomplishment for a kid—collecting a haul of dinosaur tracks, getting his site preserved for posterity, becoming pen pals with the president. But Paul Olsen didn’t stop. He went to college to study geology and paleontology, completed a PhD at Yale, and was hired as a professor at Columbia University, across the Hudson from Riker Hill.


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He became one of the leading academic paleontologists in the world and was elected to the National Academy of Sciences, one of the greatest honors for any American scientist. He also had the burden of being a member of my PhD committee, a far lesser honor, when I did my doctorate, in New York. During that time, he became one of my most trusted mentors, a brilliant sounding board for whatever crazy research ideas I had. For two years, I assisted him as he taught his popular undergraduate course on dinosaurs at Columbia, always oversubscribed by nonmajor students, who were seduced by the eminent scientist with a white Geraldo moustache prancing around with the enthusiasm that comes from several preclass energy drinks. Much of my ebullient, wildly animated lecturing style comes from watching Paul. Paul Olsen made his career by continuing what he started as a teenager. Much of his work has focused on those events that were occurring around the time dinosaurs were leaving footprints in New Jersey: the breakup of Pangea at the very end of the Triassic, the unimaginable volcanic eruptions, the mass extinction, and the rise of dinosaurs to global dominance as the Triassic transitioned into the subsequent Jurassic Period. Although he had no idea when he first cycled up to that quarry as a kid, Paul grew up in the best place in the world for studying the Late Triassic and Early Jurassic. His boyhood stomping grounds are within a geological structure called the Newark Basin, a bowl-like depression filled with Triassic and Jurassic rocks. It is one of many such structures—called rift basins, because they formed as Pangea rifted apart—extending for over a thousand miles down the eastern coast of North America. The Bay of Fundy, up north in Canada, laps onto one of these basins. Farther south is the Hartford Basin, which cuts through much of central Connecticut and Massachusetts. Then the Newark Basin, followed by the Gettysburg Basin, site of the famous Civil War battle, the topography of the rocks so instrumental in shaping military strategy that depended on securing bits of high ground. South of Gettysburg are many smaller basins that pepper the backcountry of Virginia and North Carolina, finally culminating in the huge Deep River Basin of the Carolina interior. These rift basins follow the fracture between east and west Pangea. They are the dividing line, the frontier, the place where the supercontinent tore up. As those east-west tugging forces started to pull Pangea apart, faults formed deep within the crust, cutting through what used to be solid rock. Each bit of tugging would cause an earthquake, which would cause the rocks on either side of the fault to move a little bit relative to each other. Over millions of years the faults reached the surface, and as one side continued to fall, a basin was formed: a depression on the downward side of the fault rimmed by a high mountain range on the upward side.


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Each of the eastern North American rift basins formed this way, the result of more than 30 million years of pressure, tension, and tremors. This is exactly what is occurring in eastern Africa today, as Africa is pulling away from the Middle East at the rate of about one centimeter per year. The two landmasses used to be connected about 35 million years ago, but now they are separated by the long and skinny Red Sea, which continues to get wider year by year and will one day turn into an ocean. To the south, on the African mainland, there is a north-south band of basins, each growing wider and deeper with every earthquake that is yanking Africa and Arabia farther apart. Some of the deepest lakes in the world, like the nearly mile-deep Lake Tanganyika, fill some of these basins. Others are crisscrossed by raging rivers, which rush down from the mountains above, irrigating great tropical ecosystems lush with some of Africa’s most familiar plants and animals. Sprinkled throughout, poking up in random places, are volcanoes like Mount Kilimanjaro, escape valves for the magma building up underground as the land fractures. Occasionally one of these goes off and buries the basins, and their inhabitants, in lava and ash. Paul Olsen’s Newark Basin, and the many others lining the eastern coast of North America, underwent a similar process of evolution. They were formed gradually by earthquakes, were flushed with rivers that supported diverse ecosystems, eventually became so deep and full of water that the rivers turned into lakes, and then, depending on the quirks of climate, the lakes would dry up, rivers would form again, and the whole process would start over. Cycle after cycle after cycle. Dinosaurs, pseudosuchian cousins of crocodiles, supersalamanders, and early relatives of mammals thrived along the rivers’ edges, and blooms of fish choked the lakes. These animals left their fossils—the footprints Paul Olsen started to collect as a teenager, as well as bones—in the thousands of feet of sandstones, mudstones, and other rocks deposited by the rivers and lakes. And then, when Pangea had been stretched to its limit, the crust burst and volcanoes started to erupt, burying the basins and the creatures that lived within them. The first eruptions didn’t occur in the Newark Basin area. They happened in what is now Morocco, which at that time was nudged up against what would become eastern North America, just a few hundred miles or so from modern New York City. Then lava began pouring out in other places where Pangea was splitting: in the Newark Basin, in what is now Brazil, in those same lake environments where we found the supersalamander graveyard in Portugal—all along that zipper line, which, many millions of years later, would transform into the Atlantic Ocean. The lava came in four waves, each scorching the once verdant rift basins, each spreading toxic fumes all over the planet, each making a bad situation worse and worse.


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In only about half a million years—a blink of an eye in geological terms—the eruptions stopped, but they transformed the Earth forever. The dinosaurs, pseudosuchian crocodile-line archosaurs, big amphibians, and early mammal relatives living in the rift basins were blissfully unaware of what was about to happen. Things went sour quickly. The initial eruptions in Morocco released clouds of carbon dioxide, a powerful greenhouse gas, which rapidly warmed the planet. It got so hot that strange ice formations buried within the seafloor, called clathrates, melted in unison all throughout the world’s oceans. Clathrates are unlike the solid blocks of ice we’re used to, the ones we put in our drinks or carve into fancy sculptures at parties. They are a more porous substance, a latticework of frozen water molecules that can trap other substances inside it. One of those substances is methane, a gas that seeps up constantly from the deep Earth and infiltrates the oceans but is caged in the clathrates before it can leak into the atmosphere. Methane is nasty: it’s an even more powerful greenhouse gas than carbon dioxide, packing an earth-warming punch over thirty-five times as great. So when that first torrent of volcanic carbon dioxide increased global temperatures and melted the clathrates, all of that once-trapped methane was suddenly released. This initiated a runaway train of global warming. The amount of greenhouse gas in the atmosphere approximately tripled within a few tens of thousands of years, and temperatures increased by 3 or 4 degrees Celsius. Ecosystems on land and in the oceans couldn’t cope with such rapid change. The much hotter temperatures made it impossible for many plants to grow, and indeed upwards of 95 percent of them went extinct. Animals that fed on the plants found themselves without food, and many reptiles, amphibians, and early mammal relatives died out, like dominoes falling up the food chain. Chemical chain reactions made the ocean more acidic, decimating the shelly organisms and collapsing food webs. Climate became dangerously variable, with episodes of intense heat followed by cooler periods. This enhanced the temperature differences between northern and southern Pangea, causing the megamonsoons to become more severe, the coastal regions to become even wetter, and the continental interiors to grow much drier. Pangea had never been a particularly hospitable place, but those early dinosaurs that already were constrained by the monsoons, the deserts, and their pseudosuchian rivals were now in even worse shape. So how did these dinosaurs, still at such a relatively young stage in their evolution, deal with a world that was changing so quickly? The clues are in the footprints that Paul Olsen has been studying now for nearly fifty years. The quarry that Paul explored in New Jersey is one of more than seventy places where dinosaur footprints have been found along the eastern seaboard of the US and Canada.


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These sites are positioned one on top of the other, in geological sequence, stretching over 30 million years, from around the time the first dinosaurs were originating in what is now South America (but still absent in modern-day North America), through the Late Triassic, across the volcanic extinction, and into the ensuing Jurassic Period. Generations of dinosaurs and other animals left their traces in those cyclical beds of sandstones and mudstones deposited in the rift basins, and by studying them in succession, you can see how these creatures were evolving. The rocks tell a remarkable story. During the Late Triassic, beginning about 225 million years ago, when the rift basins were just beginning to form, dinosaurs started to leave their marks in the form of rare footprints. There are three-toed tracks called Grallator, ranging from about two to six inches (five to fifteen centimeters) long, made by small, fast-running, meat-eating dinosaurs that stood on two legs like the Ghost Ranch Coelophysis. There’s a second type of track called Atreipus, which are about the same size as Grallator but include small handprints next to the three-toed footprints, a sign that the trackmaker was walking on all fours. They were probably made by primitive ornithischian dinosaurs—the oldest cousins of Triceratops and the duck-billed dinosaurs—or perhaps by close dinosauromorph cousins to dinosaurs. These dinosaur tracks are vastly outnumbered by the prints left by pseudosuchians, large amphibians, proto-mammals, and small lizards. Dinosaurs were there, but they still remained role players in the rift basin ecosystems, right up until the end of the Triassic. But then the volcanoes kicked into gear. Suddenly the diversity of non-dinosaur tracks drops dramatically in those first Jurassic rocks above the lava flows. Many non-dinosaur tracks abruptly disappear, including some of the most conspicuous prints left by crocodile-cousin pseudosuchians, which had previously been more abundant and diverse than the dinosaurs. Whereas dinosaurs made up only about 20 percent of all tracks before the volcanoes, right afterward half of all footprints belong to dinosaurs. A variety of totally new dinosaur tracks enter the record: a handprint-footprint duo called Anomoepus probably made by an ornithischian, a large four-toed print called Otozoum made by the very first long-necked proto-sauropods to live in the rift valleys, and a three-toed track called Eubrontes that belonged to another type of swift predator. These Eubrontes tracks are a little over a foot long (about thirty-five centimeters), a big size increase over the Grallator prints left by similar but much smaller carnivores during the prevolcano days of the Triassic. It’s probably not what you were expecting. After some of the largest volcanic eruptions in Earth history desecrated ecosystems, dinosaurs became more diverse, more abundant, and larger. Completely new dinosaur species were evolving and spreading into new environments, while other groups of animals went extinct. As the world was going to hell, dinosaurs were thriving, somehow taking advantage of the chaos around them.


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When the volcanoes ran out of lava and their six-hundred-thousand-year reign of terror was over, the world was a very different place than it had been in the Late Triassic. It was much warmer, storms were more intense, and wildfires ignited with ease; new types of ferns and ginkgos had replaced the once abundant broadleaf conifers; and many of the most charismatic Triassic animals were gone. The piglike mammal-relative dicynodonts and the beaked plant-munching rhynchosaurs were both extinct; the supersalamander amphibians, almost completely knocked out. What about the pseudosuchians, those crocodile-line archosaurs that were overshadowing, outmuscling, and seemingly outcompeting the dinosaurs during the final 30 million years of the Triassic? Nearly every species bit the dust. The long-snouted phytosaurs, the tanklike aetosaurs, the apex-predator rauisuchians, and the weird Effigia-like critters that resembled dinosaurs—none of them were ever to be heard from again. The only pseudosuchians that made it through the great Pangean breakup were a few types of primitive crocodiles, a handful of battle-worn stragglers that would eventually evolve into the modern alligators and crocodiles but would never enjoy the same success they had in the Late Triassic, when they seemed primed to take over the world. Somehow dinosaurs were the victors. They endured the Pangean split, the volcanism, and the wild climate swings and fires that vanquished their rivals. I wish I had a good answer for why. It’s a mystery that quite literally has kept me up at night. Was there something special about dinosaurs that gave them an edge over the pseudosuchians and other animals that went extinct? Did they grow faster, reproduce quicker, have a higher metabolism, or move more efficiently? Did they have better ways of breathing, hiding, or insulating their bodies during extreme heat and cold snaps? Maybe, but the fact that so many dinosaurs and pseudosuchians looked and behaved so similarly makes such ideas tenuous at best. Maybe dinosaurs were just lucky. Perhaps the normal rules of evolution are ripped up when such a sudden, devastating, global catastrophe happens. It could be that the dinosaurs simply were the ones that walked away from the plane crash unscathed, saved by good fortune, when so many others died. Whatever the answer, it’s a riddle waiting for the next generation of paleontologists to figure out. THE JURASSIC PERIOD marks the beginning of the Age of Dinosaurs proper. Yes, the first true dinosaurs entered the scene at least 30 million years before the Jurassic began. But as we’ve seen, these earlier Triassic dinosaurs had not even a remote claim to being dominant. Then Pangea began to split, and the dinosaurs emerged from the ashes and found themselves with a new, much emptier world, which they proceeded to conquer. Over the first few tens of millions of years of the Jurassic, dinosaurs diversified into a dizzying array of new species. Entirely new subgroups originated, some of which would persist for another 130-plus million years.


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They got larger and spread around the globe, colonizing humid areas, deserts, and everything in between. By the middle part of the Jurassic, the major types of dinosaurs could be found all over the world. That quintessential image, so often repeated in museum exhibits and kids’ books, was real life: dinosaurs thundering across the land, at the top of the food chain, ferocious meat-eaters comingling with long-necked giants and armored and plated plant-eaters, the little mammals and lizards and frogs and other non-dinosaurs cowering in fear. Here are some of the familiar dinosaurs that start to show up after the Pangean rift volcanoes ushered in the Jurassic. There were meat-eating theropods like Dilophosaurus, with a weird double-mohawk crest on its skull; at around twenty feet long, it was much larger than the mule-size Coelophysis and most other Triassic carnivores. Plant-eating ornithischians covered in armor plates, like Scelidosaurus and Scutellosaurus, would soon after give rise to the familiar tanklike ankylosaurs and back-plated stegosaurs. Small, fast-moving, probably omnivorous ornithischians like Heterodontosaurus and Lesothosaurus, were early members of that lineage that would eventually produce the horned and duck-billed dinosaurs. Other familiar dinosaurs that had been around in the Triassic but restricted to only a few environments, like the long-necked proto-sauropods and the most primitive ornithischians, finally began to migrate around the planet. Nothing in this inventory of growing diversity encapsulates the newfound dominance of dinosaurs quite like the sauropods. They are those unmistakable long-necked, column-limbed, potbellied, plant-devouring, small-brained behemoths. Some of the most famous dinosaurs of all are sauropods: Brontosaurus, Brachiosaurus, Diplodocus. They show up in almost all museum exhibits and are stars of Jurassic Park; Fred Flintstone used one to mine slate, and a green cartoon sauropod has been the logo of Standard Oil for decades. Along with T. rex, they are the iconic dinosaurs. Sauropods evolved from an ancestral stock, what I’ve been calling the proto-sauropods, in the latest Triassic. These protospecies were the dog-to-giraffe-size plant-eaters with fairly long necks that were among that first wave of dinosaurs to appear in Ischigualasto about 230 million years ago. They then became the main herbivores in the humid parts of Triassic Pangea but were kept from achieving their full potential by their inability to settle the deserts. That changed in the early part of the Jurassic, when sauropods were able to break free of their environmental restrictions and move about the globe, evolving their characteristic noodle-necked bodies and growing to monstrous sizes in the process. The skull of Plateosaurus, one of the proto-sauropods, the ancestral stock that gave rise to the sauropods. Photo courtesy of the author Fossils of some of the first truly gigantic sauropods—ones that weighed over ten tons, were over fifty feet long, and had necks that could stretch several stories into the sky—have started turning up in Scotland over the past few decades, on a beautiful island off the west coast called the Isle of Skye.


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The clues have been meager—a stocky limb bone here, a tooth or a tail vertebra there—but they hint at an animal of enormous size living about 170 million years ago, far enough into the Jurassic that the Pangean split and volcanic apocalypse were distant memories but still during that time when dinosaurs were putting the final flourishes on their rise to dominance. The sauropod fossils from Skye piqued my interest when I moved to Scotland in 2013 to take up my new position at the University of Edinburgh, fresh out of my PhD in New York and bounding with the excitement of starting my own research lab. During my first few weeks on the job, I began to hang out with two scientists in my department: Mark Wilkinson, a hardened field geologist whose ponytail and scruffy beard give him the look of a hippie, and Tom Challands, a redheaded packhorse of man who also had a doctorate in paleontology, albeit on microscopic fossils from over 400 million years ago. Tom had recently finished a stint in the real world, putting his geological skills to use working for an energy company on the search for oil. For part of that time, he lived in a custom-made camper van, fitted with a bed and small kitchen, which he would park near whatever sites he was surveying. His new bride put the kibosh on that lifestyle after their wedding, but the van still came in handy for fieldwork travel, and Tom would often spend his weekends driving along the misty coasts of Scotland looking for whatever fossils he could find. Both Tom and Mark had done some geological work on Skye and knew the terrain well, so we made a pact to hunt for better fossils of the mysterious giant sauropods. The more we read about Skye, the more one name kept popping up: Dugald Ross. It was a name I wasn’t familiar with. He wasn’t a paleontologist or a geologist or a scientist of any kind. Yet he had discovered and described many of the dinosaur fossils found on Skye. Dugald was a local boy who grew up in the tiny hamlet of Ellishadder on the far northeastern arm of the island, a rugged landscape of craggy peaks, green hills, peat-colored streams, and windswept shores that looks like something out of a fantasy novel—very Tolkienesque. He was raised in a household that spoke Gaelic, the native language of the Scottish Highlands, which is spoken by only about fifty thousand people today but which still has a presence on the road signs and in the schools on remote islands like Skye. When Dugald was fifteen years old, he found a cache of arrow points and Bronze Age artifacts near his family’s home, and this sparked an obsession with the history of his native island that continued into adulthood, as he carved out a career as a builder and crofter (a Scottish Highlands term for a small-scale farmer and sheepherder).


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I got in touch with Dugald and told him about our dreams of finding huge dinosaurs on his island. It was one of the most fortunate e-mails I’ve ever sent, because it struck up a friendship and a remarkable scientific collaboration. Dugald—or Dugie, as he prefers to be called—invited us to visit him when we came up to his island a few months later. He instructed us to drive up the main two-lane road that snakes along the coast of northeastern Skye and meet him at a long ranch-style building, made up of a collage of different-size gray stones and a black tile roof, with antique farming instruments strewn across the lawn outside. There was a sign out front that said TAIGH-TASGAIDH—the Gaelic word for museum. Dugie emerged from his big red work van with a set of oversize skeleton keys, made his introductions, and proudly led us inside. In his soft-spoken lyrical accent—a charming combination of Sean Connery–style Scots and an Irish brogue—he explained how he had taken the ruins of a one-room schoolhouse and built the structure we were standing in, the Staffin Museum. He founded the museum when he was nineteen. Today, this single room—without a café, a big gift shop, or other expensive trappings of big-city museums, or even electricity—contains many of the dinosaurs he’s found on Skye, along with artifacts that trace the history of the island’s human inhabitants. It’s a surreal experience: big dinosaur bones and footprints displayed next to old mill wheels, iron rods for picking turnips, and antique mole traps once used by Highland farmers. The enchanting landscape of the Isle of Skye, Scotland. Photo courtesy of the author For the rest of that week, Dugie led us to many of his favorite hunting spots. We found a lot of Jurassic fossils—the jaw of a dog-size crocodile, the teeth and backbones of reptiles called ichthyosaurs, which resembled dolphins and lived in the oceans when dinosaurs started to dominate the land—but no giant sauropods. Over the next few years, we kept coming back. Dugie Ross removing a dinosaur bone from a boulder on the Isle of Skye. Photo courtesy of the author The dinosaur dance floor of sauropod tracks that I discovered with Tom Challands on the Isle of Skye. Photo courtesy of the author Finally, in the spring of 2015, we found what we set out for, although we didn’t even realize it at first. We spent most of the day on our hands and knees, looking for tiny fish teeth and scales embedded in a platform of Jurassic rocks that stretched into the icy waters of the North Atlantic, right below the ruins of a fourteenth-century castle. This was Tom’s idea: he now studies fossil fish, and in exchange for his help finding dinosaurs, I promised to assist him in collecting fishy bits. We had been squinting at the rocks for hours, bundled up in three layers of waterproof clothing but still freezing.


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The tide was coming in, the late afternoon light was going down, and dinner was beckoning. So Tom and I packed up our gear and our bags of fish teeth and started to stroll back to his tricked-out van parked on the other side of the beach. That’s when something caught our eyes. It was a malformed depression in the rock, about the size of a car tire. We had missed it earlier because our eyes were focused on the much smaller fish bones, our search image totally unsuitable for noticing something so big. As we continued to walk, we started to notice many other similar depressions, now visible in the low-angle afternoon light. They were all about the same size, and the closer we looked, the more we saw that they stretched in every direction around us. They seemed to show a pattern. Individual holes were lined up in two long rows, in something of a zigzag arrangement: left-right, left-right, left-right. Ribbons of them were crisscrossing much of the rock platform that we had been working on all day. Tom and I looked at each other. It was the kind of knowing glance between brothers, a nonverbal connection based on years of shared experience. We had seen these types of things before, not in Scotland, but in places like Spain and western North America. We knew what they were. The holes in front of us were fossilized tracks, huge ones. Dinosaur tracks, no doubt. As we looked closer, we could see that there were both handprints and footprints, and some of them had finger and toe marks. They had the telltale shape of tracks left by sauropods. We had found a 170-million-year-old dinosaur dance floor, records left by colossal sauropods that were about fifty feet long and weighed as much as three elephants. The tracks were made in an ancient lagoon, an environment not commonly associated with sauropods. We usually envision these monstrous dinosaurs stampeding across the land, causing a small earthquake with each step. And they did. But by the middle part of the Jurassic, the sauropods had become so diverse that they started branching out into other ecosystems, always searching for the vast quantities of leafy food needed to fuel their giant bodies. Our trackway site in Skye has at least three different layers of footprints, made by different generations of sauropods wading through a salty lagoon, living with smaller plant-eating dinosaurs, the occasional pickup-truck-size carnivore, and many types of crocodiles, lizards, and swimming mammals with flat tails like beavers. Scotland was much warmer back then, a land of swamps and sandy beaches and rolling rivers on an island in the middle of the growing Atlantic Ocean, perched between North American and European landmasses that moved farther and farther apart as Pangea continued to split. Thoroughly ruling this land were the sauropods and other dinosaurs, which had now—finally—become a global phenomenon.


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THERE’S REALLY NO better way to say it: the sauropods that made their marks in that ancient Scottish lagoon were awesome creatures. Awesome in the literal sense of the word—impressive, daunting, inspiring awe. If I was handed a blank sheet of paper and a pen and told to create a mythical beast, my imagination could never match what evolution created in sauropods. But they were real: they were born, they grew, they moved and ate and breathed, they hid from predators, they slept, they left footprints, they died. And there’s absolutely nothing like sauropods around today—no animals with a similar long-necked and swollen-gut body type, no creatures on land that even remotely approach them in size. Sauropods are so mind-twistingly big that, when their first fossil bones were discovered in the 1820s, scientists found themselves in a bind. Some of the first dinosaurs were being found around the same time, like the meat-eating Megalosaurus and the beaked herbivore Iguanodon. These were big animals, no doubt, but nowhere near the size of the creatures that left the gigantic sauropod bones. So scientists didn’t make the connection with dinosaurs. Instead, they considered the sauropod bones to belong to the one type of thing they knew could get so huge: whales. It was a few decades before that mistake was corrected. Amazingly, later discoveries would show that many sauropods got even bigger than most whales. They were the largest animals that ever walked the land, and they push the limit for what evolution can achieve. This raises a question that has fascinated paleontologists for over a century: how did sauropods become so large? It’s one of the great puzzles of paleontology. But before trying to solve it, we first need to come to grips with a more fundamental issue: how big did sauropods get? How long were they, how high could they stretch their necks, and most important, how much did they weigh? These turn out to be difficult questions to answer, particularly when it comes to weight, because you can’t just stick a dinosaur on a scale and weigh it. A trade secret among paleontologists is that many of the fantastical numbers you see in books and museum exhibits—Brontosaurus weighed a hundred tons and was bigger than a plane!—are pretty much just made up. Educated guesses or, in some cases, barely that. Recently, however, paleontologists have come up with two different approaches to more accurately predict the weight of a dinosaur based on its fossil bones. The first is really quite simple and relies on basic physics: heavier animals require stronger limb bones to support their weight. This logical principle is reflected in how animals are built.


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Scientists have measured the limb bones of many living animals, and it turns out that the thickness of the main bone in each limb that supports the animal—the femur (thighbone) for those that walk on two legs only or the femur plus the humerus (upper arm bone) for those that stand on all fours—is strongly statistically correlated with the weight of the animal. In other words, there is a basic equation that works for almost all living animals: if you can measure limb-bone thickness, you can then calculate body weight with a small but recognized margin of error—simple algebra you can do with a basic calculator. The second method is more intensive but a lot more interesting. Scientists are starting to build three-dimensional digital models of dinosaur skeletons, add on the skin and muscles and internal organs in animation software, and use computer programs to calculate body weight. It’s a method pioneered by a number of young British paleontologists—Karl Bates, Charlotte Brassey, Peter Falkingham, and Susie Maidment—and their network of collaborators, who include everyone from biologists specializing in living animals to computer scientists and programmers. A few years ago, when I was finishing my PhD, Karl and Peter invited me to take part in a study of sauropod body size and proportions using digital models. It was an ambitious goal: make detailed computer animations of all sauropods with complete enough skeletons and figure out how big these animals were and how their bodies changed as they grew into truly titanic sizes. I was invited for purely practical reasons: some of the best sauropod skeletons in the world are on display at the American Museum of Natural History in New York City, where I was based at the time, and they needed data for one of them in particular, a Late Jurassic species called Barosaurus. They instructed me how to gather the information to build the model, and I was surprised that all it required was a normal digital camera, a tripod, and a scale bar. I took about a hundred photos of the Barosaurus skeletal mount from all possible angles, keeping my camera steady on the tripod and making sure to include a ruler in most of the images. Then Karl and Peter input the images into a computer program that matches equivalent points on the photographs, works out the distances between them based on the scale, and does this continuously until a three-dimensional model is built from the original 2-D images. Brontosaurus at the American Museum of Natural History in New York, with a human skeleton for scale. American Museum of Natural History Library A digital computer model of the skeleton of the sauropod Giraffatitan, which helps scientists calculate the weight of the animal. Courtesy of Peter Falkingham and Karl Bates. The technique is called photogrammetry, and it’s revolutionizing how we study dinosaurs. The super-accurate models it creates can be measured in precise detail.


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Or they can be loaded into animation software and made to run and jump, in order to determine what kinds of motions and behaviors dinosaurs were capable of. They can even be used to animate movies or television documentaries, ensuring that the most realistic dinosaurs appear on screen. These models are bringing dinosaurs to life. Our computer modeling study and more traditional studies based on limb-bone measurements come to the same conclusion: sauropod dinosaurs were really, really big. The primitive proto-sauropods like Plateosaurus began to experiment with relatively large sizes in the Triassic, as some of them got up to about two or three tons in weight. That’s roughly equivalent to a giraffe or two. But after Pangea started to split, the volcanoes erupted, and the Triassic turned into the Jurassic, the true sauropods got much larger. The ones that left tracks in the Scottish lagoon weighed about ten to twenty tons, and later in the Jurassic, famous beasties like Brontosaurus and Brachiosaurus expanded to more than thirty tons. But that was nothing compared to some supersize Cretaceous species like Dreadnoughtus, Patagotitan, Argentinosaurus—members of an aptly named subgroup called the titanosaurs—which weighed in excess of fifty tons, more than a Boeing 737. The biggest and heaviest land animals today are elephants. Their sizes vary, depending on where they live and which species they belong to, but most weigh about five or six tons. Apparently the largest one ever recorded was around eleven tons. They have nothing on sauropods. Which circles back to the money question: how were these dinosaurs able to attain sizes so completely out of scale with anything else evolution has ever produced? The first thing to consider is what animals require to become really big. Perhaps most obvious, they need to eat a lot of food. Based on their sizes and the nutritional quality of the most common Jurassic foodstuffs, it’s estimated that a big sauropod like Brontosaurus probably needed to eat around a hundred pounds of leaves, stems, and twigs every day, maybe more. So they needed a way to gather and digest such vast quantities of grub. Secondly, they need to grow fast. Growing bit by bit, year by year is all well and good, but if it takes you over a century to get big, that’s many opportunities for a predator to eat you, or a tree to fall on you during a storm, or a disease to take you out long before you grow into your full-size adult body. Third, they must be able to breathe very efficiently, so they can take in enough oxygen to power all of the metabolic reactions in their immense bodies. Fourth, they need to be constructed in a way that their skeleton is strong and sturdy, but also not so bulky that it can’t move. Finally, they need to shed excess body heat, because in hot weather it is very easy for a big creature to overheat and die.


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Sauropods must have been able to do all of these things. But how? Many scientists who started to ponder this riddle decades ago went for the easiest answer: maybe there was something different in the physical environment back in the Triassic, Jurassic, and Cretaceous. Perhaps gravity was weaker, so heftier animals could move and grow more easily back then. Or maybe there was more oxygen in the atmosphere, so the hulking sauropods could breathe, and therefore grow and metabolize, more efficiently. These speculations might sound convincing, but on closer scrutiny they don’t check out. There is no evidence gravity was substantially different during the Age of Dinosaurs, and oxygen levels back then were about the same as today, or maybe even slightly lower. That leaves only one plausible explanation: there was something intrinsic about sauropods that allowed them to break the shackles that constrained all other land animals—mammals, reptiles, amphibians, even other dinosaurs—to much smaller sizes. The key seems to be their unique body plan, which is a mixture of features that evolved piecemeal during the Triassic and earliest Jurassic, culminating in an animal perfectly adapted for thriving at large size. It all starts with the neck. The long, spindly, slinky-shaped neck is probably the single most distinctive feature of sauropods. A longer-than-normal neck started to evolve in the very oldest Triassic proto-sauropods, and it got proportionally longer over time, as sauropods both added more vertebrae—the individual bones in the neck—and stretched each individual vertebra ever further. Like Iron Man’s armor, the long necks conferred a kind of superpower: they allowed sauropods to reach higher in the trees than other plant-eating animals, giving them access to a whole new source of food. They could also park themselves in one area for several hours and extend their necks up and down and all around like a cherry picker, gobbling up plants while expending very little energy. That meant they were able to eat more food, and thus take in energy more efficiently, than their competitors. That’s adaptive advantage number one: their necks permitted them to eat the huge meals necessary to put on excessive weight. Then there’s the way that they grew. Recall that the dinosauromorph ancestors of dinosaurs developed higher metabolisms, faster growth rates, and a more active lifestyle than many of the amphibians and reptiles that were also diversifying in the earliest Triassic. They weren’t lethargic, and it didn’t take them aeons to grow into adults like an iguana or a crocodile. This was also true of all of their dinosaur descendants. Studies of bone growth indicate that most sauropods matured from guinea-pig-size hatchlings to airplane-size adults in only about thirty or forty years, an incredibly short period of time for such a remarkable metamorphosis. That’s advantage two: sauropods obtained the fast growth essential to reach large size from their distant, cat-size ancestors. Sauropods also retained something else from their Triassic ancestors: a highly efficient lung.


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The lungs of sauropods were very similar to those of birds and very different from ours. While mammals have a simple lung that breathes in oxygen and exhales carbon dioxide in a cycle, birds have what is called a unidirectional lung: air flows across it in one direction only, and oxygen is extracted during both inhalation and exhalation. The bird-style lung is extra efficient, sucking up oxygen with each breath in and each exhalation. It’s an astounding feature of biological engineering, made possible by a series of balloonlike air sacs connected to the lung, which store some of the oxygen-rich air taken in during inhalation, so that it can be passed across the lung during exhalation. Don’t worry if it sounds confusing: it is such a strange lung that it took biologists many decades to figure out how it works. We know that sauropods had such a birdlike lung because many bones of the chest cavity have big openings, called pneumatic fenestrae, where the air sacs extended deep inside. They are exactly the same structures in modern birds, and they can only be made by air sacs. So that’s adaptation three: sauropods had ultra-efficient lungs that could take in enough oxygen to stoke their metabolism at huge size. Theropod dinosaurs had the same bird-style lungs, which could have been one factor that allowed tyrannosaurs and other giant hunters to get so large, but the ornithischian dinosaurs did not. This is why duck-billed dinosaurs, stegosaurs, horned species, and armored dinosaurs were never able to grow as huge as sauropods. It turns out that air sacs also have another function. Aside from storing air in the breathing cycle, they also lighten the skeleton when they invade bone. In effect, they hollow out the bone, so that it still has a strong outer shell but is much more lightweight, the way an air-filled basketball is lighter than a rock of similar size. Want to know how sauropods could hold up their long necks without toppling over like an unbalanced seesaw? It’s because all of the vertebrae were so engulfed by air sacs that they were little more than honeycombs, featherweight but still strong. And that’s advantage four: the air sacs allowed sauropods to have a skeleton that was both sturdy and light enough to move around. Without air sacs, mammals, lizards, and ornithischian dinosaurs had no such luck. And what about the fifth special adaptation, being able to expel excess body heat? The lungs and air sacs helped with this too. There were so many air sacs, and they extended throughout so much of the body, snaking their way into bones and between internal organs, that they provided a large surface area for dissipating heat. Each hot breath would be cooled by this central air conditioning system. Putting it all together, that’s how you can build a supergiant dinosaur.


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If sauropods had lacked any one of these features—the long neck, the fast growth rates, the efficient lung, the system of skeleton-lightening and body-cooling air sacs—then they probably would not have been capable of becoming such behemoths. It wouldn’t have been biologically possible. But evolution assembled all of the pieces, put them together in the right order, and when the kit was finally assembled in the post-volcanic world of the Jurassic, sauropods suddenly found themselves able to do something no other animals, before or since, have been able to do. They became biblically huge and swept around the world; they became dominant in the most magnificent way—and they would remain so for another hundred million years.


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The Rise and Fall of the Dinosaurs 4 Dinosaurs and Drifting Continents Stegosaurus Chapter Title art by Todd Marshall NESTLED WITHIN THE LEAFY STREETS of New Haven, Connecticut, on the northern fringes of the Yale University campus, there is a shrine. The Great Hall of Dinosaurs at Yale’s Peabody Museum may not bill itself as a place of spiritual pilgrimage, but that’s sure what it feels like to me. I get a shiver, as when I walked into Catholic mass as a child. It’s not a normal shrine—no statues of deities, flickering candles, or the hint of incense. It’s also not particularly magnificent, at least from the outside, tucked away inside a fairly nondescript brick building that blends in with the rest of the university’s lecture halls. But it houses relics that, to me, are as sacred as those you’ll find in most any religious shrine: dinosaurs. To me, there is nowhere better, anywhere on the planet, to go and immerse yourself in the wonder of the prehistoric world. The Great Hall was originally built in the 1920s to house Yale’s incomparable dinosaur collection, assembled over many decades by roughnecks who fanned across the American West and, for the right fee, sent fossil treasures eastward to be studied by the Ivy League elite. Coming up on its centennial, the gallery retains all of its original charm. This isn’t some New Age exhibit space with flashing computer screens and dinosaur holograms and a roaring soundtrack in the background. It’s a temple of science, where skeletons of some of the most iconic dinosaurs stand in solemn vigil, lights down low, in the sort of silence you really do expect in a church. Covering the entire east wall is a mural that stretches more than a hundred feet long and sixteen feet high. Taking four and a half years to complete, it was painted by a man named Rudolph Zallinger, who was born in Siberia, moved to the United States, and took up illustration professionally during the Great Depression. If he were around today, Zallinger would probably be working for an animation studio as a storyboard artist. He was a master at setting scenes and incorporating diverse sets of characters, telling grandiose stories with the stroke of his brush. His most famous work is undoubtedly The March of Progress—that often satirized timeline of human evolution in which a knuckle-walking ape gradually morphs into a spear-carrying man. More people have probably come to understand, or misunderstand, the theory of evolution through that one image than through all of the textbooks, school lectures, and museum exhibits the world over. But before he was painting humans, Zallinger was obsessed with dinosaurs. His mural inside the Great Hall—called The Age of Reptiles—is the crowning achievement of that stage of his career. It’s been on US postage stamps, was featured in a Life magazine series, and is either reproduced or plagiarized on all sorts of dinosaur paraphernalia.


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It’s the Mona Lisa of paleontology, surely the single most talked-about piece of dinosaur artwork that has ever been created. But really, it’s more akin to the Bayeux Tapestry, because it tells an epic tale of conquest. It’s the saga of how fishy creatures first emerged onto land, colonized a new environment, and diversified into reptiles and amphibians; then, of how these reptiles split off into the mammal and lizard lines, the proto-mammals having their day and the lizards following, eventually producing the dinosaurs. As the mural nears its end, some sixty feet and 240 million years from where it started, after a long journey through alien landscapes of primeval scaly beasts, the painting finally becomes engulfed in dinosaurs. It kind of sneaks up on you, as the transition from the lizards and proto-mammals to the dinosaurs unfolds incrementally across the canvas. Now it’s dinosaurs everywhere, of all shapes and sizes, some enormous and others blending into the background. Suddenly, the mural has taken on the feel of something quite different—of a Soviet propaganda poster with Stalin gesticulating before a crowd of peasants, or one of those hilariously self-aggrandizing frescoes in Saddam’s palaces. One glance at the dinosaurs and I feel the power. Strength, control, dominance. The dinosaurs were in command, and this was their world. The theropod Deinonychus stands guard over the Zallinger mural at the Peabody Museum, Yale University. Photo courtesy of the author This part of Zallinger’s mural beautifully encapsulates what it was like when dinosaurs had ascended to the peak of their evolutionary success. A monstrous Brontosaurus lounges in a swamp in the foreground, munching away on the ferns and evergreen trees surrounding the water. Off to the side, a bus-size Allosaurus rips into a bloodied carcass with its teeth and claws, its massive feet stomping on its prey for a little extra insult. Keeping a safe distance is a peaceful grazing Stegosaurus, which displays its full arsenal of bony plates and spikes just in case the carnivore has other ideas. Far in the background, where the swamp disappears into a wall of snowcapped mountains, another sauropod uses its long neck to vacuum shrubs off of the ground. Meanwhile, two pterosaurs—those flying reptiles closely related to dinosaurs, often called pterodactyls—chase each other overhead, dipping and diving through the tranquil blue sky. Odds are, this is the type of image that many of us think of when we think of dinosaurs. These are dinosaurs at their pinnacle. ZALLINGER’S MURAL IS not fiction. Like any good art, it takes a few liberties here and there, but it is largely rooted in fact. It’s based on those very same dinosaurs that stand in front of it in the Great Hall: familiar names like Brontosaurus, Stegosaurus, and Allosaurus. These dinosaurs lived during the Late Jurassic Period, about 150 million years ago. By this time, dinosaurs had already become the dominant force on land.


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Their victory over the pseudosuchians was 50 million years in the rearview mirror, and it had been a good 20 million years since some of the first giant long-necked species were splashing through the lagoons of Scotland. Nothing was holding back the dinosaurs anymore. We know a lot about the dinosaurs of the Late Jurassic. That’s because there are abundant fossils from this time, in many parts of the world. It’s just one of those quirks of geology: some time periods are better represented in the fossil record than others. It’s usually because more rocks were being formed during that time, or rocks of that age have better survived the rigors of erosion, flooding, volcanic eruptions, and all of the other forces that conspire to make fossils difficult to find. When it comes to the Late Jurassic, we enjoy two lucky breaks. First, there were hugely diverse communities of dinosaurs living alongside rivers, lakes, and seas all around the world—the perfect places to bury fossils in sediments that later turned to rock. Second, these rocks are today exposed in places convenient for paleontologists—in sparsely populated and dry regions of the United States, China, Portugal, and Tanzania, where annoyances like buildings, highways, forests, lakes, rivers, and oceans don’t cover up the fossil booty. The most famous Late Jurassic dinosaurs—those in Zallinger’s mural—come from a thick rock deposit that pokes out all across the western United States. Its technical term is the Morrison Formation, named for a small town in Colorado where there are some beautiful exposures of its colorful mudstones and beige-tinged sandstones. The Morrison Formation is a monster: it can be found in thirteen states today, covering nearly four hundred thousand square miles (a million square kilometers) of the American scrublands. It is easily sculpted into low hills and undulating badlands, the sort of classic backdrop you see in Western films. It’s also the source rock for some of the country’s most important uranium ore deposits. And, yes, it’s a hotbed of dinosaurs, ones whose uranium-infused bones make Geiger counters sing. Paul Sereno in Wyoming. Photo courtesy of the author Excavating sauropod bones in the Morrison Formation near Shell, Wyoming. At the center back is Sara Burch, who later became an expert on T. rex arms (see Chapter 6). I worked in the Morrison Formation for two summers as an undergraduate. It’s where I cut my teeth excavating dinosaur skeletons. I was apprenticing in the lab of the University of Chicago’s Paul Sereno, whom we last met leading the expeditions to Argentina that turned up some of the world’s very oldest dinosaurs, the Triassic-age Herrerasaurus, Eoraptor, and Eodromaeus. But Paul seemed to study everything and do fieldwork everywhere: he had also found bizarre fish-eating and long-necked dinosaurs in Africa, he’d explored China and Australia, and he’d even described important fossils of crocodiles, mammals, and birds. In addition, like any academic paleontologist, Paul also had to spend time in the classroom.


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Each year he taught a popular undergraduate class called Dinosaur Science, which combined theory with practice. Because you can’t find dinosaurs anywhere near Chicago, the class would take a ten-day field trip each summer to Wyoming, where the students had the once-in-a-lifetime opportunity to dig dinosaurs with a celebrity scientist. Although at the time I had little prior experience, I was brought on as a teaching assistant, Paul’s right-hand man as we herded the students—a diverse lot, from premeds to philosophy majors—across the high desert. Paul’s field sites were located near the tiny town of Shell, secluded between the Bighorn Mountains to the east, and Yellowstone National Park a hundred miles to the west. Only eighty-three people were counted during the last census. When we were there in 2005 and 2006, the road signs boasted of merely fifty residents. But that’s a good thing for paleontologists. The fewer people in the way of the fossils, the better. And although Shell is a forgettable dot on the map, it can rightly stake its claim as one of the world’s dinosaur capitals. It is built on the Morrison Formation, surrounded by beautiful hills carved out of muted green, red, and gray rocks bursting with dinosaurs. So many dinosaurs have been found here that it’s hard to keep track, but the count is probably well over a hundred skeletons by now. As we drove west from Sheridan, on a surprisingly treacherous road across the rugged Bighorns, I felt I was on the trail of giants. Some of the biggest dinosaurs of all have been found in the Shell area: long-necked sauropods like Brontosaurus and Brachiosaurus, and the huge carnivores, like Allosaurus, that ate them. But I also felt I was walking in the footsteps of another type of giant: the explorers who found the first bones in this area in the late nineteenth century, the railwaymen and laborers who started a dinosaur rush and seized the moment to reinvent themselves as mercenary fossil collectors on the payrolls of gilded institutions like Yale University. They were a ragtag bunch, Wild West ruffians with cowboy hats, mustaches, and unkempt hair, who hacked giant bones out of the ground for months on end, and spent their free time raiding one another’s sites, constantly feuding and sabotaging and drinking and shooting. But these unlikely characters revealed a prehistoric world that nobody knew existed. The first Morrison fossils were surely noticed by the many Native American tribes scattered across the West, but the first recorded bones were collected by a surveying expedition in 1859. In March 1877 the real fun started. A railroad worker named William Reed was returning home from a successful hunt, rifle and pronghorn antelope carcass in tow, when he noticed some huge bones protruding out of a long ridge called Como Bluff, not too far from the railroad tracks in an anonymous expanse of southeastern Wyoming.


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He didn’t know it, but at the same time a college student, Oramel Lucas, was finding similar bones a few hundred miles to the south, in Garden Park, Colorado. That same month, a schoolteacher named Arthur Lakes had just found a cache of fossils near Denver. By the end of that March, the fever of discovery was spreading throughout the American West, to even the most remote villages and railway outposts. Like any prospecting rush, the dinosaur frenzy attracted a horde of questionable characters to the Wyoming and Colorado backcountry. Many of these men were grizzled opportunists on one mission: to convert dinosaur bones into cash. It didn’t take long for them to realize who was paying top dollar: two dapper East Coast academics, Edward Drinker Cope of Philadelphia and Othniel Charles Marsh of Yale University, the same men we briefly met two chapters ago, who studied some of the first Triassic dinosaurs found in western North America. Once chummy, these two scientists had let ego and pride metastasize into a full-on feud, which was so radioactive that they would do anything to one-up each other in an insane battle to see who could name the most new dinosaurs. Cope and Marsh were opportunists, too, and with each letter from a ranch hand or railway porter reporting more new dinosaur bones from the Morrison badlands, they saw the opportunity they had been craving but had been unable to yet fulfill: a chance to beat the other guy once and for all. And they both went for it. Cope and Marsh treated the West like a battlefield, employing rival teams that often acted more like armies, scooping up fossils wherever they went and sabotaging the other side whenever they could. Loyalties were fluid. Lucas worked for Cope, and Lakes teamed up with Marsh. Reed worked for Marsh, but members of his team defected to Cope. Pillaging, poaching, and bribing were the rules of the game. The madness continued for over a decade, and when it was over, it was hard to separate the winners from the losers. On the plus side, the so-called Bone Wars led to the discovery of some of the most celebrated dinosaurs, the ones that roll off the tongue of every schoolchild: Allosaurus, Apatosaurus, Brontosaurus, Ceratosaurus, Diplodocus, Stegosaurus, just to name a few. On the other hand, the mentality of constant warfare caused a lot of sloppiness: fossils haphazardly excavated and hastily studied, scraps of bone mistakenly christened as new species, different bits of the skeleton of the same dinosaur regarded as belonging to totally different animals. Edward Drinker Cope, the Bone Wars protagonist. AMNH Library. A page from Cope’s 1874 field notebook, depicting the fossil-rich rocks of New Mexico. AMNH Library. Cope’s sketch of a horned dinosaur (ceratopsian), from 1889, an insight into how he envisioned dinosaurs as living animals. (He was much better as a scientist than as an artist.) AMNH Library.