The Rise and Fall of the Dinosaurs

by Steve Brusatte

  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. 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. 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. 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 vari
ety 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.

  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. 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. 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).

  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.