The Rise and Fall of the Dinosaurs

by Steve Brusatte

  Covering the body—the head, the wee arms, the stocky legs, all the way to tip of the tail—was a thick, scaly hide. In this way, T. rex resembled an overgrown crocodile or an iguana—lizardlike. But there was one key difference: Rex also had feathers sticking out from between its scales. As mentioned in the last chapter, these were not big branching ones like those on a bird wing, but were simpler filaments that looked and felt more like hair, the larger ones stiff like the quills of a porcupine. T. rex certainly couldn’t fly, and neither did its ancestors that first evolved these proto-feathers, way back in the early days of the dinosaurs. No, as we’ll learn later, feathers started out as simple wisps of integument, which creatures like T. rex used to keep warm, and as displays to attract mates and scare off rivals. Paleontologists have yet to find any fossilized feathers on a T. rex skeleton, but we’re confident that it must have had some fluff because primitive tyrannosaurs—Dilong and Yutyrannus, which we met last chapter—have been found coated in hairlike feathers, as have many other types of theropods preserved in those rare conditions that allow soft bits to turn into fossils. That means that the ancestors of T. rex had feathers, so it is highly likely that Rex did too.

  T. rex lived from about 68 to 66 million years ago, and its dominion was the forest-covered coastal plains and river valleys of western North America. There it lorded over diverse ecosystems that included a bounty of prey species: the horn-faced Triceratops, the duck-billed Edmontosaurus, the tanklike Ankylosaurus, the dome-headed Pachycephalosaurus, and many more. Its only competition for food was from the much smaller dromaeosaurs—raptor dinosaurs à la Velociraptor—which is to say it didn’t have much competition at all.

  Although several other tyrannosaurs had thrived in these same environments during the preceding 10 to 15 million years, they were not the ancestors of T. rex. Instead, Rex’s closest cousins were Asian species like Tarbosaurus and Zhuchengtyrannus. T. rex, as it turns out, was an immigrant. It got its start in China or Mongolia, hopped across the Bering Land Bridge, journeyed through Alaska and Canada, and made its way down into the heart of what’s now America. When the young Rex arrived at its new home, it found things ripe for the taking. It swept across western North America, an invasive pest that spread all the way from Canada down to New Mexico and Texas, elbowing out all of the other midsize to large predatory dinosaurs so that it alone controlled an entire continent.

  Then one day it all ended. T. rex was there when the asteroid fell down from the sky 66 million years ago, putting a violent end to the Cretaceous, exterminating all of the nonflying dinosaurs. That’s a story we’ll get to later. For the time being, only one fact really matters: the King went out on top, cut down at the peak of its power.

  WHAT FEAST BEFITS the King? We know T. rex was a carnivore of the highest order, a pure meat-eater. It’s one of the simplest inferences that we can make about any dinosaur, and it doesn’t require any fancy experiments or machines to figure out. T. rex had a mouth full of thick, serrated, razor-sharp teeth. Its hands and feet boasted big pointy claws. There’s really only one reason an animal would have these things: they’re weapons, used to procure and process flesh. If your teeth look like knives and your fingers and toes are hooks, then you’re not eating cabbages. For anybody who doubts that, there is plenty of other evidence: bones have been found preserved in the stomach area of tyrannosaur skeletons and in the coprolites (fossilized dung) dropped by tyrannosaurs, and western North America is peppered with skeletons of plant-eating dinosaurs—particularly Triceratops and Edmontosaurus—with bite marks that perfectly match the size and shape of T. rex teeth.

  Like so many monarchs, Rex was a glutton. It devoured meat. Scientists have predicted how much food an adult T. rex would need to survive, based on the food intake of living predators scaled up to an animal of Rex’s size. The estimates are nauseating. If T. rex had the metabolism of a reptile, then it would have required about 12 pounds (5.5 kilograms) of Triceratops chops per day. But that’s very likely a vast underestimate, because as we’ll see later, dinosaurs were much more birdlike than reptilian in their behaviors and physiology, and they (or at least many of them) may have even been warm-blooded like us. If that was the case, then Rex needed to gobble up some 250 pounds (about 111 kilograms) of grub each and every day. That’s many tens of thousands of calories, maybe even hundreds of thousands, depending on how fatty the King liked its steak. It’s roughly the same amount of food eaten by three or four large male lions, some of the most energetic, and hungriest, modern carnivores.

  Maybe you’ve heard the rumor that T. rex liked its meat dead and rotten, that Rex was a scavenger, a seven-ton carcass collector too slow, too stupid, or too big to hunt for its own fresh food. This accusation seems to make the rounds every few years, one of those stories that science reporters can’t get enough of. Don’t believe it. It defies common sense that an agile and energetic animal with a knife-toothed head nearly the size of a Smart car wouldn’t use its well-endowed anatomy to take down prey but would just walk around picking up leftovers. It also runs against what we know about modern carnivores: very few meat-eaters are pure scavengers, and the outliers that do it well—vultures, for instance—are fliers that can survey wide areas from above and swoop down whenever they see (or smell) a decaying body. Most carnivores, on the other hand, actively hunt but also scavenge whenever they have the chance. After all, who turns down a free meal? That’s true of lions, leopards, wolves, even hyenas, which are not the pure scavengers of legend but actually earn much of their food through the chase. Like these animals, T. rex was probably both a hunter and an opportunistic scavenger.

  Still doubt that Rex went out and got its own food? There’s fossil evidence that proves T. rex hunted, at least part of the time. Many of those Triceratops and Edmontosaurus bones pockmarked with T. rex tooth impressions show signs of healing and regrowth, so they must have been attacked while alive but survived. The most provocative of these specimens is a set of two fused Edmontosaurus tailbones with a T. rex tooth stuck between them, enveloped by the gnarly mass of scar tissue that merged the two bones together as they healed. The poor duck-billed dinosaur was viciously attacked by a tyrannosaur and left with a terrible injury, but it kept the predator’s tooth as a trophy from its near-death experience.

  Many of Rex’s bite marks are peculiar. Most theropods left simple feeding traces on the bones of their prey: long, parallel, shallow scratches, a sign that the teeth were just barely kissing the bone. That’s not surprising, because even though dinosaurs could replace their teeth throughout life (unlike us), no predator would want to break its chompers every time it ate. T. rex was different, though. Its bite marks are more complex: they start with a deep circular puncture, like a bullet hole, which grades into an elongate furrow. This is a sign that Rex bit deeply into its victim, often right through the bones, and then ripped back. Paleontologists have come up with a special term for this style of eating: puncture-pull feeding. During the puncture phase of its bite, Rex clamped down hard enough to literally break through the bones of its prey. This is why the fossilized dung heaps left by T. rex are chock full of bony chunks. Bone crunching is not normal—some mammals, like hyenas, do it, but most modern reptiles do not. As far we know, big tyrannosaurs like T. rex were the only dinosaurs capable of it. It was one of the powers that made the King an ultimate killing machine.

  How was T. rex able to do it? For starters, its teeth were perfectly adapted. The thick, peglike teeth were strong enough that they wouldn’t easily break when they hit bone. Next, consider the power behind those teeth: T. rex’s jaw muscles were massive, bulging mounds of sinew that provided enough energy to shatter the limbs, backs, and necks of Triceratopses, Edmontosauruses, and other prey. We can tell that Rex had some of the largest and most powerful jaw muscles of any dinosaur, based on the very broad and deep gullies on the skull bones where the muscles attached.

  Experiments can simulate the actions of these jaw muscles. One of my colleagues, Greg Erickson of Fl
orida State University, designed a particularly clever experiment in the mid-1990s, right after he finished graduate school. Greg is one of my favorite people to hang around with—he talks with the cadence of high school jock and often looks the part in his worn baseball cap, cold beer in hand. A few years back, Greg was a regular talking head on a cable TV program about weird animal incidents—alligators crawling through sewers and invading trailer parks, that kind of thing. As much fun as he is, I admire Greg deeply as a scientist, because he brings a different approach to paleontology—experimental, quantitative, rigorously grounded in comparisons to modern animals.

  Greg spends a lot of time with engineers, and one day they came up with a crazy idea: they would rig up a laboratory version of T. rex and determine how strong its bite was. They started with a Triceratops pelvis with a half-inch-deep puncture left by a Rex, and then asked a simple question: how much force would it take to make an indentation this deep? They couldn’t take a real T. rex and make it bite into a real Triceratops, but they found a way to simulate it by making a bronze and aluminum cast of a T. rex tooth, putting it into a hydraulic loading machine, and smashing it into the pelvis of a cow, which is very similar in shape and structure to the Triceratops bone. They pushed and pushed the tooth until it made a half-inch-deep hole, and then used their instruments to read out how much force it required: 13,400 newtons, equivalent to about 3,000 pounds.

  That’s a staggering number—about the weight of an old-school pickup truck. By comparison, humans exert a maximum force of about 175 pounds with our rear teeth, and African lions bite at about 940 pounds. The only modern land animals that come close to T. rex are alligators, which also bite at around 3,000 pounds. However, we need to remember that the 3,000-pound figure for T. rex is for only a single tooth—imagine how much power a mouth full of these railroad spikes would have delivered! And because it’s a measure of the force required to make one observed fossil bite mark, it’s likely that this is an underestimate of the maximal biting power. Rex probably had the strongest bite of any land animal that ever lived. It could crunch bones with ease and would have been strong enough to bite through a car.

  All of that strength came from the jaw muscles; they were the engine that powered the teeth to deliver the bone-breaking bite. But that’s not the entire story. If the muscles delivered enough force to bust the bones of prey, they could have also broken the skull bones of the T. rex itself. Basic physics: every action has an equal and opposite reaction. So it wasn’t enough for T. rex to have massive teeth and huge jaw muscles—it also needed a skull that could withstand the tremendous stresses that occurred each time it snapped its jaws shut.

  To figure out how, we needed to turn back to the engineers and to another paleontologist who has crossed over into the realm of hard-core numbers science. Emily Rayfield’s lab at the University of Bristol in England is a big bright room with a row of computers, its large windows and breezy open plan like something out of Silicon Valley. The shelves are lined with manuals for various software packages, but there’s nary a fossil in sight. Emily doesn’t often collect fossils; she’s not that kind of paleontologist. Instead, she builds computer models of fossils—say, the skull of T. rex—and uses a technique called finite element analysis (FEA) to study how they would have behaved in a mechanical sense.

  FEA was developed by engineers and calculates the stress and strain distributions in a digital model of a structure when it is subjected to various simulated loads. In plain English, it’s a way to predict what will happen to something when some kind of force is applied to it. This is very useful for engineers. Before a construction crew starts building a bridge, let’s say, the engineers better be pretty damn sure that the bridge isn’t going to collapse when heavy cars start driving over it. To check, they can build a digital model of the bridge and use the computer to imitate the stresses from real cars to see how the bridge reacts. Does it absorb the weight and force of the cars easily, or does it start to crack under pressure? If it does start to crack, the computer can identify the weak points and the engineers can go back to the plans for the actual bridge to make the necessary fixes.

  Emily does the same thing with dinosaurs, and T. rex has been one of her favorite muses. She built a digital model of Rex’s skull based on CAT scans of a well-preserved fossil, and then used the FEA program to simulate the forces of a bone-crunching bite and analyze how the skull reacted. The verdict: T. rex had a remarkably strong skull that was optimized to endure the extreme pushing and pulling forces of its three-thousand-pound-per-tooth chomp. It was built like an airplane fuselage: the individual bones tightly sutured together so that they wouldn’t come apart when the stress hit. The nasal bones above the snout were fused together into a long, vaulted tube, which acted as a stress sink. Thick bars of bone around the eye provided strength and rigidity, and the robust lower jaw was almost circular in cross section so that it could withstand high pressures from all directions. None of these things are present in other theropods, which had daintier skulls with looser connections among the various bones.

  The skull of Tyrannosaurus rex.

  Courtesy of Larry Witmer.

  The brain cavity (upper right-hand corner) and sinuses inside the skull of a Tyrannosaurus rex, revealed by CAT scans.

  Courtesy of Larry Witmer.

  That’s the final piece of the puzzle, the last component in the tool kit that allowed T. rex to bite so strongly that it punctured, and then pulled through, the bones of its supper. Thick peg-like teeth, huge jaw muscles, and a rigidly constructed skull: that was the winning combination. Without any of these things, T. rex would have been a normal theropod, slicing and dicing its prey with care. That’s how the other big boys did it—Allosaurus, Torvosaurus, and the carcharodontosaurs—because they didn’t have the arsenal necessary for bone-crunching. Once again, the King stands alone.

  T. REX WAS able to gnash through most anything that it wanted to eat, whether it was splurging on a forty-foot-long Edmontosaurus or snacking on smaller contemporaries like the donkey-size ornithischian Thescelosaurus. But how did it capture its food?

  Not, as it turns out, with exceptional speed. T. rex was a special dinosaur in many ways, but one thing it could not do is move very fast. There’s a famous scene in Jurassic Park where the bloodthirsty T. rex, convulsed by its insatiable appetite for human flesh, chases down a jeep driving at highway speeds. Don’t believe the movie magic—the real T. rex likely would have been left in the dust once the jeep got up to third gear. It’s not that Rex was a plodding slouch that waddled through the forest. Far from it—T. rex was agile and energetic, and it moved with purpose, its head and tail balancing each other as it tiptoed through the trees, stalking its prey. But its maximum speed was probably in the ballpark of ten to twenty-five miles per hour. That’s faster than we can run, but it’s not as quick as a racehorse or, certainly, a car on the open road.

  Once again, it’s high-tech computer modeling that has allowed paleontologists to study how T. rex moved. This work was pioneered in the early 2000s by John Hutchinson, an American transplant to England who is now a professor at the Royal Veterinary College near London. He spends his days working with animals: monitoring the livestock on his university’s research campus, making elephants run across scales to study their posture and locomotion, dissecting ostriches and giraffes and other exotic creatures. John chronicles his adventures on his popular blog, the wonderfully but somewhat disturbingly titled What’s in John’s Freezer? He also pops up frequently as a talking head on television documentaries, often adorned in his favorite purple shirt, which somehow doesn’t crack the cameras with its glare. Like Greg Erickson, John is a scientist whom I’ve long admired because of his unique angle on studying dinosaurs. For John, the present is very much the key to the past: find out as much as we can about the anatomy and behaviors of today’s animals, and that will help us understand dinosaurs.

  If you visit John’s lab, he really does have freezers stocked with the frozen cad
avers of animals of all shapes and sizes, from all over the world. Odds are, one or two of them will be out thawing, getting ready for the dissection table. But there is a more sterile side to John’s lab: the computers, which he uses to make digital models of dinosaurs, like those we saw in chapter 3 that we made to predict the weight and posture of long-necked sauropods. He starts with a three-dimensional model of a skeleton, captured through CAT scans, laser surface scans, or the photogrammetry method we learned about earlier. Then he uses his knowledge of modern animals to flesh it out: to add muscles (whose sizes and positions are based on the attachment sites visible on the fossil bones) and other soft tissues, wrap it up in skin, and position it in realistic postures. The computer does its magic, putting the model through all sorts of gymnastics routines, and calculates how fast the real animal was likely able to move. John’s modeling provides us with the range of ten to twenty-five miles per hour that I cited for T. rex’s speed.

  The computer models also make clear that Rex would have needed absurdly large leg muscles to run as fast as a horse: more than 85 percent of its total body mass in its thighs alone, which is obviously impossible. T. rex was simply too big to run exceptionally fast. Its sheer size also conferred another liability: the Tyrant King couldn’t turn very quickly, or otherwise it would topple over like a truck taking a corner too sharply. Thus, the reality is, Spielberg had it wrong, T. rex was no sprinter, and it would have ambushed its prey with a quick strike rather than chasing it down like a cheetah.