The Rise and Fall of the Dinosaurs

by Steve Brusatte

  Brown was delighted. He called the site an “an absolute, knockout dinosaur treasure trove!” and gleefully took his haul of dinosaurs back to New York, where they became crown jewels in the collection of the American Museum of Natural History. And then for many decades the Howe Quarry lay dormant, until a fossil collector from Switzerland named Kirby Siber rolled into Wyoming in the late 1980s.

  Siber is a commercial paleontologist: he digs up dinosaurs and sells them. It’s a thorny issue for many academic paleontologists like me, who see fossils as irreplaceable natural heritage that should be protected in museums, where they can be studied by researchers and enjoyed by the public, not sold off to the highest bidder. But there is a whole spectrum of commercial paleontologists, ranging from gun-toting criminals who illegally export fossils to diligent, conscientious, well-trained collectors whose knowledge and experience rival that of academics. Siber is in this latter category. In fact, he’s the archetype of this kind of collector. He is well respected by researchers and even founded his own dinosaur museum east of Zurich, called the Saurier Museum, which has some of the most remarkable dinosaur exhibits in Europe.

  Siber arranged access to the old Howe Quarry but didn’t find many dinosaurs. Brown’s team had pretty much cleared them all out. So the Swiss collector began prospecting in the surrounding gullies and hills, looking for new sites. It wasn’t long before he found a good one, about a thousand feet north of the original quarry. His backhoe first revealed some sauropod bones, and then a string of vertebrae from the backbone of a big, meat-eating theropod. Siber followed the spool-shaped bones, one by one, and before long it was clear that he had something special: the nearly complete skeleton of an Allosaurus, the top predator of the Morrison Formation ecosystem. It looked to be the single best fossil of this well-known dinosaur that had ever been found, more than 120 years after Marsh first named it during the heat of the Bone Wars.

  Allosaurus was the Butcher of the Jurassic, both figuratively and literally. This fierce predator stalked the Morrison floodplains and riverbanks—think T. rex but a little smaller and lighter, about two to two and a half tons in weight and thirty feet (nine meters) long as an adult, and better equipped for running. But it truly earned the title of Butcher, because paleontologists think it used its head like a hatchet to hack its prey to death. Computer models find that the thin teeth of Allosaurus couldn’t bite very strongly on their own, but the skull could withstand massive amounts of impact force. We also know that Allosaurus could open its jaws obscenely wide, so we think a hungry Allosaurus would attack with mouth agape and slash down at its prey, slicing through the skin and muscle with its thin but sharp teeth, which were lined up along its jaws like the blades of scissors. Many a Stegosaurus and Brontosaurus probably breathed its last this way. If for some reason the blood-lusting Allosaurus couldn’t quite knock off its victim using only its jaws of death, it could always finish the job with a couple of swipes of its clawed, three-fingered arms, which were longer and more versatile than the stubby little forelimbs of T. rex.

  Finding such a complete and well-preserved Allosaurus was one of the highlights of Siber’s career, but emotions were about to turn. After the summer’s excavation had ended, while Siber was at a fossil show peddling his wares and the Allosaurus skeleton remained in the ground, an agent from the US Bureau of Land Management (BLM) happened to be flying over that dusty stretch of northern Wyoming near the Howe Quarry. The agent was checking for signs of fire, part of his job monitoring public lands administered by the US government. But as he glided high above the badlands, he noticed that the dirt roads around the Howe Quarry had been shredded by tire marks. Somebody had been doing some heavy work that summer. That’s no problem around the Howe Quarry itself—it’s on private land, and Siber had the permission of the landowner. But the BLM agent wasn’t quite sure what was private ground and what was public, which could only be worked on by accredited scientists with BLM permission. So he doubled-checked and found that Siber had strayed a few hundred feet into BLM territory. Because Siber had no right to work there, he could no longer excavate the Allosaurus skeleton. It was probably an honest mistake, but a costly one.

  The BLM now had a problem. A gorgeous dinosaur skeleton was sitting in the ground, and the people who had found it and begun to excavate it couldn’t finish the job. So the agency assembled a crack team led by legendary paleontologist Jack Horner’s crew at Montana’s Museum of the Rockies (Horner is best known for two things: discovering the first dinosaur nesting sites in the 1970s and being the science advisor for the Jurassic Park films). Under the eye of television cameras and a throng of newspaper reporters, the academics took out the skeleton and trucked it up to Montana to be carefully conserved in the safety of the laboratory. The dinosaur turned out to be more spectacular than Siber could even have imagined. About 95 percent of the bones were there, an almost unheard of number for a large predatory dinosaur. At about twenty-five feet (eight meters) long, this Allosaurus was only about 60 to 70 percent grown. It was still a teenager, but it had already lived a tough life. Its body was covered with all types of maladies: broken, infected, and deformed bones that testify to the rough-and-tumble world of the Late Jurassic, when even the biggest predators didn’t have an easy time hunting behemoths like Diplodocus and Brontosaurus, when the sharpest teeth and claws were no guarantee of surviving a whack from the spiky tail of a Stegosaurus.

  The Allosaurus was nicknamed Big Al, and it became a celebrity dinosaur. It even had its own television special broadcast internationally by the BBC. But once the buzz died down, there remained a huge hole in the ground that was still full of all kinds of fossils that were buried underneath Big Al. Paul Sereno received permission from the BLM to use the site as a field laboratory to teach excavation techniques to his students, and that’s why we were taking three big SUVs full of undergraduates there.

  During that first season in Wyoming, in the summer of 2005, I spent many days parked out on the high desert, carefully removing globs of popcorn-textured mudstone to help the team uncover the skeleton of a Camarasaurus. It may not be one of the brand-name dinosaurs, but Camarasaurus is one of the more common species in the Morrison Formation. It is yet another type of sauropod, a close cousin of Brontosaurus, Brachiosaurus, and Diplodocus. Camarasaurus had the usual sauropod body: long neck that could reach several stories into the trees, small head with chisel-shaped teeth for stripping leaves, a massive frame that was about fifty feet (fifteen meters) long and weighed around twenty tons. It was probably the type of tasty plant-guzzler that Big Al and the other Allosauruses liked to eat, although its freakish size would have afforded it quite a lot of protection from even the scariest flesh-eaters. Maybe it was a Camarasaurus like this one that gave Big Al some of those nasty injuries.

  Camarasaurus is one of many enormous sauropods that have been found in the Morrison Formation. It’s joined by its famous cousins, the big three of Brontosaurus, Brachiosaurus, and Diplodocus. Then there are the under-the-radar players known only to the cognoscenti (or, perhaps, your average dinosaur-obsessed kindergartner): Apatosaurus, Barosaurus, and further on down the roster, Galeamopus, Kaatedocus, Dyslocosaurus, Haplocanthosaurus, and Suuwassea. There are various other sauropods that have been named based on scrappy bones, which may belong to even more species. Now, the Morrison Formation covers a wide swath of time, and was deposited across a huge geographic area. Not all of these sauropods lived together. But many of them did—they have been found at the same sites, their skeletons mingled together. The normal situation in the Morrison world was numerous varieties of sauropods cohabiting in the river valleys, their heavy footsteps thundering as they trawled the land in search of the daily hundreds of pounds of leaves and stems that sustained them.

  The skulls of Diplodocus (left) and Camarasaurus (right), two sauropods that used their differently shaped skulls and teeth to feed on different types of plants.

  Courtesy of Larry Witmer.

  What a weird scene to conjure up!
It’s akin to imaging five or six different species of elephants crowded onto the African savannahs, all trying to find enough food to survive while lions and hyenas lurk in the background. The Morrison world was no less dangerous. If a sauropod was staggering around with an empty belly, then you could confidently bet an Allosaurus was hiding in the brush, ready to pounce at the long-neck’s moment of weakness.

  In addition to Allosaurus, there were many other predators below it on the food chain. There was Ceratosaurus, a twenty-foot-long mid-tier hunter with a frightening horn on its snout, a horse-size carnivore named Marshosaurus after the Bone Wars pugilist, and a donkey-size primitive cousin of T. rex called Stokesosaurus. Then you had the slashers: a number of lightly built, fast-running pests like Coelurus, Ornitholestes, and Tanycolagreus, the Morrison version of cheetahs. And all of these meat-gobblers, even Allosaurus, probably lived in fear of another monster that reigned near the top of the food chain. It’s called Torvosaurus, and we don’t know much about it, because its fossils are very rare. But the bones we have paint a terrifying picture: a knife-toothed apex predator that was thirty feet (ten meters) long and weighed about two and a half tons or perhaps more, not too far off from the proportions of some of the big tyrannosaurs that would evolve much later.

  It’s easy to understand why so many predators stalked the Morrison ecosystem: there were a lot of sauropods to eat. It’s much more difficult to explain how so many of these giant sauropods lived together. It’s an even greater puzzle because there were also plenty of other, smaller plant-eaters that feasted on shrubs closer to the ground: the plate-backed Stegosaurus and Hesperosaurus, the tanklike ankylosaurs Mymoorapelta and Gargoyleosaurus, the ornithischian Camptosaurus, and a whole zoo of small, fast-running fern-chewers like Drinker, Othnielia, Othnielosaurus, and Dryosaurus. The sauropods were also sharing space with all of these herbivores.

  So how did the sauropods do it? It turns out that their diversity was their key to success. There were many species of sauropods, yes, but they were all slightly different. Some were absolute colossuses: Brachiosaurus was around fifty-five tons, and Brontosaurus and Apatosaurus tipped the scales in the thirty-to-forty-ton range. But others were smaller: Diplodocus and Barosaurus were skinny little things, at least as far as sauropods go, weighing a mere ten to fifteen tons. So it goes without saying that some species would need more food than others. These sauropods also had different types of necks: that of Brachiosaurus arched proudly into the heavens with the erect profile of a giraffe, perfect for reaching the highest leaves, but Diplodocus may not have been able to lift its neck much past its shoulders and may have acted more like a vacuum cleaner sucking up shorter trees and shrubs. Finally, the heads and teeth of these sauropods differed as well. Brachiosaurus and Camarasaurus had deep, muscle-wrapped skulls and jaws lined with spatula-shaped teeth, so they could eat harder foods like thick stems and waxy leaves. But Diplodocus had a long head made up of delicate bones, with a row of tiny pencil-shaped teeth at the front of its snout. It would break its teeth if it tried to eat anything too hard. Instead, it spent its time stripping smaller leaves from the branches, its head rocking back and forth like a rake.

  Different species of sauropods were specialized for eating many different kinds of foods—and they had a lot to choose from, as the lush Jurassic forests were cluttered with towering conifers, with thickets of ferns, cycads, and other shrubs down below. The sauropods weren’t competing for the same plants, but dividing the resources among themselves. The scientific term for this is niche partitioning—when coexisting species avoid competing with each other by behaving or feeding in slightly different ways. The Morrison world was highly partitioned, which is a sign of just how successful these dinosaurs were. They were carving up almost every square inch of the ecosystem, a dizzying array of species flourishing alongside each other in the hot, humid, waterlogged forests and coastal plains of ancient North America.

  But what about Late Jurassic dinosaurs in other parts of the world? The story seems to be the same nearly everywhere we look. We also see a similar cast of diverse sauropods, smaller plant-eating stegosaurs, and small to large carnivores of the Ceratosaurus and Allosaurus mold in those other places with rich records of Late Jurassic fossils, like China, eastern Africa, and Portugal.

  It all comes down to geography. Pangea had started to break up many tens of millions of years before, but it takes a long time for a supercontinent to split. Landmasses can move apart from each other by only a few centimeters each year, about the same pace that our fingernails grow. Thus, there were still big land connections between most parts of the world persisting into the latest Jurassic. Europe and Asia were still globbed together, and they were linked to North America by a series of islands that could easily be traversed by a wayfaring dinosaur. These northern lands—called Laurasia—were beginning to split from southern Pangea, called Gondwana, which was a stuck-together mess of Australia, Antarctica, Africa, South America, India, and Madagascar. Laurasia and Gondwana were intermittently connected by land bridges when sea level was low, and even during times of higher water, other islands provided a convenient migratory route between north and south.

  The Late Jurassic, then, was a time of global uniformity. The same suite of dinosaurs ruled every corner of the globe. Majestic sauropods divided food among them, reaching a peak of diversity unmatched by any other large plant-eaters in Earth history. Smaller plant-chewers prospered in their shadows, and a motley crew of meat-eaters took advantage of all of that herbivore flesh. Some, like Allosaurus and Torvosaurus, were the first truly giant theropods. Others, like Ornitholestes, were the founding members of that dynasty that would eventually produce Velociraptor and birds. The planet was sweltering and the dinosaurs were able to move around wherever they wanted. This was the real Jurassic Park.

  145 MILLION YEARS ago, the Jurassic Period transitioned into the final stage of dinosaur evolution, the Cretaceous Period. Sometimes the switch between geological periods happens with a flourish, as when the megavolcanoes closed out the Triassic. Other times, it’s barely noticeable, and more a matter of scientific bookkeeping, a way for geologists to break up long stretches of time without any major changes or catastrophes. The changeover between the Jurassic and Cretaceous is that type of boundary. There was no calamity like an asteroid impact or a big eruption that ended the Jurassic, no sudden die-off of plants and animals, no brave new world as the Cretaceous dawned. Rather, the clock just ticked over, and the diverse Jurassic ecosystems of giant sauropods, plate-backed dinosaurs, and small to monstrous meat-eaters continued into the Cretaceous.

  That’s not to say, however, that nothing changed, for plenty was happening to the Earth around the Jurassic-Cretaceous boundary—no apocalyptic disasters but slower changes to the continents, oceans, and climate occurring over some 25 million years. The hothouse world of the Late Jurassic was interrupted by a cold snap, followed by a turn to more arid conditions, before things swung back to normal in the Early Cretaceous. Sea levels started to fall during the latest Jurassic and stayed low across the boundary, until the waters started to rise again some 10 million years into the Cretaceous. With low sea levels came a lot more exposed land, which allowed dinosaurs and other animals to move around even more easily than during the Late Jurassic. Pangea continued to rupture, the fragments of the supercontinent moving farther and farther apart from one another as time marched on. Gondwana, that huge expanse of southern lands, finally began to split, the cracks starting to define the shapes of today’s southern hemisphere continents. First the conjoined mass of Africa and South America detached from the section of Gondwana containing Antarctica and Australia, and then this latter chunk also began to fracture. Volcanoes welled up through the fissures, and although none were on the scale of the monster eruptions at the end of the Permian or Triassic, they would have brought with them the same nasty stew of environment-poisoning lava and gases.

  None of these changes were particularly deadly on their own, but together they we
re an insidious cocktail of dangers. The long-term shifts in temperature and sea level were probably unrecognizable to dinosaurs, the sort of thing that neither they nor any of us, had we been around, would ever have noticed in one lifetime. Plus, in the dinosaur-eat-dinosaur world of the Late Jurassic and Early Cretaceous, the Brontosauruses and Allosauruses had more important things to stress over than little changes in the tide line or slightly cooler winters. Given enough time, however, these changes built up and became silent killers.

  By about 125 million years ago, some 20 million years after the Jurassic ended, a new Cretaceous world had emerged, ruled by a very different suite of dinosaurs. The most obvious change had to do with the most prominent dinosaurs—the gargantuan sauropods. Once so diverse in the Late Jurassic Morrison ecosystems, the long-necks suffered a crash in the Early Cretaceous. Almost all of the familiar species like Brontosaurus, Diplodocus, and Brachiosaurus went extinct, while a new subgroup called the titanosaurs began to proliferate, eventually evolving into supergiants like the middle Cretaceous Argentinosaurus, which at more than a hundred feet (thirty meters) long and fifty tons in mass was the largest animal known to have ever lived on land. But despite the outlandish sizes of the new Cretaceous species, never again would sauropods be as dominant as they were in the Late Jurassic; never again would they boast such a variety of necks and skulls and teeth that allowed them to exploit so many ecological niches.