The Rise and Fall of the Dinosaurs Page 17
Ambushing prey can take a lot of energy—in bursts. Thankfully, T. rex had another trick up its sleeve, or more precisely, deep inside its chest. Remember those hyperefficient lungs of sauropods, which allowed them to reach such enormous sizes? T. rex had the same lungs. They are the lungs of today’s birds: rigid bellows anchored to the backbone, able to extract oxygen when the animal breaths in and also when it breathes out. They’re different from our lungs, which can take in oxygen only during inhalation, then spew out carbon dioxide during exhalation. They are a stunning feat of biological engineering. When today’s birds—and also T. rex—breathe in, oxygen-rich air courses through the lungs as you would expect. However, some of the inhaled air doesn’t go through the lungs right away but is shunted into a system of sacs connected to the lung. There it waits, until it is released when the animal exhales, passing through the lungs and delivering its oxygen-rich hit even as carbon dioxide waste is being expelled. Birds get twice the bang for the buck, a continuous supply of energy-sustaining oxygen. If you’ve ever wondered how some birds can fly at tens of thousands of feet, in rarefied air where we would have a hard time breathing (just ask anyone who has experienced the oxygen masks coming down midflight), their lungs are their secret weapon.
Paleontologists have yet to find a fossilized T. rex lung and probably never will. The thin tissues are too delicate to fossilize. But we know that Rex had a birdlike, ultra-efficient lung, because this kind of breathing system leaves impressions on the bones, which do fossilize. It all has to do with the air sacs, the air-storage compartments integral to the bird-style lung. These sacs are akin to balloons: they are soft, thin-walled, compliant bags that inflate and deflate during the ventilation cycle. Many air sacs are connected to the lung, nestled in between the many other organs of the chest, including the trachea and esophagus, the heart, the stomach, and intestines. Sometimes they run out of room and start wiggling their way into the only space still available: the bones themselves. As they do so, they invade the bone through large, smooth-walled holes and then expand into chambers once inside. These signatures are easy to identify on fossils. We see them on the backbones of T. rex, along with many other dinosaurs, including, as we learned about earlier, the humongous sauropods. We never see these things on mammals, or lizards, or frogs, or fish, or any other types of animals—only in modern birds and extinct dinosaurs and a few very close relatives, a telltale fingerprint of their unique lungs.
The drama of a T. rex ambush is coming into focus. The lungs delivered the energy, which was then transferred to the leg muscles, which propelled the Rex forward with a burst of speed to lunge at its startled victim. And then what happened? Just imagine T. rex as a giant land shark. Like a Great White, all of the action was with its head. Rex led with its noggin and used its clamp-strong jaws to grab its dinner, subdue it, kill it, and crunch through its flesh and guts and bones before swallowing. T. rex simply had to hunt headfirst, because its arms were pitifully tiny. The King evolved from smaller tyrannosaur ancestors, like Guanlong and Dilong, that used their much longer arms to grab their prey. But during the course of tyrannosaur evolution, the head got bigger, the arms got smaller, and the skull gradually took over all of the hunting functions that the arms used to perform.
Why, then, did T. rex still have arms? Why didn’t it lose them completely, the way whales ditched their no-longer-necessary hindlegs when they evolved from land mammals that colonized the water? That mystery has captivated scientists for a long time, and it’s kept cartoonists and comedians supplied with an endless source of material for bad puns. As it turns out, those little arms—as silly as they may look—were not useless. Although short, they were stocky and muscular, and they served a purpose.
Sara Burch figured it out. Sara and I both trained in Paul Sereno’s lab at the University of Chicago, where we became friends, but our paths diverged afterward: I went down the route of studying genealogy and evolution, and Sara became enthralled with bones and muscles. She did her PhD in an anatomy department, where she dissected a zoo’s worth of animals, and has since carved out a career that is common for paleontologists: teaching human anatomy to medical students. Sara knows more about the anatomical structure of dinosaurs than almost anyone alive—how their bones connected to each other, what kind of muscles they had. She reconstructed the forearm muscles of T. rex and many other theropods, determining which muscles were present, and how big they were, from the preserved attachment sites on the bones, helped along by comparisons to modern reptiles and birds for guidance. Rex’s seemingly sad arms actually turned out to have powerful shoulder extensors and elbow flexors—exactly those muscles needed to hold on to something that is trying to pull away, to keep it close to the chest. It seems that T. rex used its short but strong arms to hold down struggling prey while the jaws did their bone-crunching thing. The arms were accessories to murder.
Now there’s one final twist in the story of how T. rex hunted. We increasingly believe that Rex didn’t go on the prowl alone; it traveled in packs. The evidence comes from a Canadian fossil site located between Edmonton and Calgary, in what is now Dry Island Buffalo Jump Provincial Park. It was discovered back in 1910 by none other than Barnum Brown, who just a few years earlier found the first T. rex skeleton in Montana. Brown was traveling through the heart of the Canadian prairies, floating down the Red Deer River on a boat and dropping anchor wherever he saw dinosaur bones sticking out of the riverbank. When he came to Dry Island, he noticed a number of bones from a slightly older cousin of T. rex called Albertosaurus, one of the North American apex predators right before Rex migrated over from Asia. He had time to collect only a small sample before heading back to New York.
Those bones languished deep in the vaults of the American Museum for decades, until Phil Currie—Canada’s leading dinosaur hunter (and one of the nicest guys you’ll ever meet) — took notice of them in the 1990s. He retraced Brown’s steps, relocated the site, and began excavating. Over the next decade, his team collected more than a thousand bones, which belong to at least a dozen individuals, ranging from youngsters to adults, all of them Albertosaurus. There’s really only one way numerous individuals of the same species can be preserved together: they must have lived and died together. A few years later, Phil’s crew found a similar mass graveyard in Mongolia, packed with several Tarbosauruses, the very closest Asian cousin of T. rex. Albertosaurus and Tarbosaurus were evidently pack animals, and we reckon that Rex itself was as well. If a seven-ton, bone-crunching, ambush predator isn’t scary enough on its own, then just imagine a pack of them working together. Sweet dreams!
LET’S GET INTO the King’s head. What did it think? How did it sense its world? How did it locate its prey? These are, of course, very difficult questions to answer. Even with modern living animals, it’s almost impossible to put ourselves in their feet, or paws, or paddles and feel what their world is like. But we can study their brains and sense organs and start to put together a picture. With dinosaurs, however, we are usually out of luck: the brains, eyes, nerves, and tissues associated with the ears and nose are soft and decay easily, meaning they rarely make it through the rigors of fossilization. What can we do?
Technology, yet again, makes the impossible possible. The brains, ears, noses, and eyes of dinosaurs may be long gone, but these organs occupied spaces in the bones. The brain cavity, the eye socket, and so on. We can study these spaces to get a sense of the original sense organs that filled them, but there is another problem: many of these spaces are inside the bones, not observable from the outside. That’s where the technology comes in: we can use CAT scans (also known by the shorter abbreviation of CT) to visualize the inside of dinosaur bones. CAT scans are nothing more than high-powered X-rays. That’s why they’re popular in medicine: if you feel a pain in your gut or a creak in your bones, your doctor will probably stick you in a CAT scanner to see what’s going on inside your body without having to cut you open. Ditto with dinosaurs. We can use the X-rays to take an array of internal
images, which we can then stitch together into three-dimensional models using various software packages. This procedure has become practically routine in paleontology, such that many labs—including my own in Edinburgh—have a CAT scanner onsite. Ours was hand-built by one of my colleagues, Ian Butler, a geochemist by training who now finds himself scanning fossil after fossil, each one leading him deeper into the addiction that is paleontology.
Ian Butler CAT-scanning the skull of the primitive tyrannosaur Timurlengia at the University of Edinburgh.
Photo courtesy of the author
A CAT scan reconstruction of the brain, inner ear, and associated nerves and blood vessels of Tyrannosaurus rex.
Courtesy of Larry Witmer.
Ian and I are newcomers to the fossil-scanning game. We’re following in the footsteps of a few giants in the field: Larry Witmer of Ohio University, Chris Brochu of the University of Iowa, and the wife-and-husband team of Amy Balanoff and Gabe Bever, who started at the University of Texas, moved on to the American Museum in New York (where I met them when I was a PhD student), and are now ensconced at Johns Hopkins University in Baltimore. Balanoff and Bever are virtuosos who can read CAT scans the way a linguist deciphers ancient manuscripts. In the grayscale splotches of the X-rays, they can make out the internal structures that powered the intelligence and sensory prowess of long-dead dinosaurs. Tyrannosaurs like T. rex have been some of their favorite subjects—their favorite patients, if you will, whose behaviors and cognitive abilities are mysteries to be diagnosed.
The scans tell us quite a bit about our patient. First off, Rex had a distinctive brain. It didn’t look anything like our brain but was more of a long tube with a slight kink at its back, surrounded by an extensive network of sinuses. It’s also a relatively large brain, at least for a dinosaur, which hints that T. rex was fairly intelligent. Now, measuring intelligence is riddled with uncertainties, even for humans: just think of all of the IQ tests, exams, SAT scores, and other things that we use to try to assess how smart people are. However, there is a straightforward measure that scientists use to roughly compare the intelligence of different animals. It’s called the encephalization quotient (EQ). It’s basically a measure of the relative size of the brain compared to the size of the body (because, after all, bigger animals have bigger brains simply because of their body size: elephants have bigger brains than we do but are not more intelligent). The largest tyrannosaurs like T. rex had an EQ in the range of 2.0 to 2.4. By comparison, our EQ is about 7.5, dolphins come in around 4.0 to 4.5, chimps at about 2.2 to 2.5, dogs and cats are in the 1.0 to 1.2 range, and mice and rats languish around 0.5. Based on these numbers, we can say that Rex was roughly as smart as a chimp and more intelligent than dogs and cats. That’s a whole lot smarter than the dinosaurs of stereotype.
One part of the tyrannosaur brain was particularly enlarged: the olfactory bulbs. These are the lobes at the front of the brain that control the sense of smell. The two bulbs were each a little larger than a golf ball, much bigger in absolute size than in any other theropods. Of course, T. rex was one of the biggest theropods, so maybe it had whopping olfactory lobes simply by virtue of its extreme bulk. What is needed, then, is a relative measure of olfactory bulb size. My friend Darla Zelenitsky of the University of Calgary did just that. She compiled CAT scans of numerous theropods, calculated the size of their olfactory bulbs, and normalized them by dividing by body size. Even after all of this, she still found the big tyrannosaurs to be extreme outliers: they, along with the raptor dinosaurs, had proportionally enormous olfactory bulbs, and thus a sharp sense of smell, compared to other meat-eating dinosaurs.
It wasn’t only the nose. Other senses were heightened as well. The CAT scans allow us to see inside Rex’s inner ear: the pretzel-shaped network of tubes that control both hearing and balance. The semicircular canals at the top of the inner ear—which make the pretzel shape—were long and loopy. As we know from comparisons to modern animals, this means that T. rex was agile and capable of highly coordinated head and eye movements. Sticking downward from the pretzel is the cochlea, the part of the inner ear that regulates hearing. In T. rex the cochlea was elongated, more than in most other dinosaurs. There is a tight relationship in living animals: the longer the cochlea, the better sensitivity to lower-frequency sounds. In other words, Rex also had a keen sense of hearing. Vision, too: the huge eyeballs of T. rex faced partially to the side and partially to the front, meaning that they were capable of binocular vision. The King could see in three dimensions and perceive depth, just like us. There’s another scene in Jurassic Park where the freaked-out humans are told to stay still, because if they don’t move, then the T. rex can’t see them. Nonsense—because it could sense depth, a real Rex would have made an easy meal out of those sad, misinformed people.
Thus it wasn’t all brute strength. T. rex had brawn all right, but it also had brains. High intelligence, world-class sense of smell, keen hearing and vision. Add these things to the armory: they’re what Rex used to target its victims, to choose which poor dinosaurs would have to die.
WHEN I ENVISION T. rex as a real animal, what most amazes me is that it would have started life as a tiny hatchling. All dinosaurs, as far as we know, hatched from eggs. We have yet to find any T. rex eggs, but we do have eggs and nests of many closely related theropods. Most of these dinosaurs seemed to guard their nests and provide at least a bit of care for their young. Without some parental love, the baby dinosaurs would have been hopeless, because they were tiny: no dinosaur eggs that we know of are larger than a basketball, so even the mightiest species like T. rex would have been, at most, the size of a pigeon when they entered the world.
Back when my parents were learning about dinosaurs in school, the assumption was that T. rex and kin grew like iguanas: they kept growing throughout their life, gradually getting bigger and bigger and bigger. Rex was able to get so large because it lived for a long time: after about a century, it would reach its final size of forty-two feet and seven tons, then finally saunter off and die. This type of thinking even percolated into the dinosaur books I read as a child, but like many once cherished notions about dinosaurs, it turns out to be false. Dinosaurs like T. rex grew rapidly, a lot more like birds than lizards.
The evidence is buried deep inside the bones of dinosaurs, and paleontologists like Greg Erickson found a way to tease it out. Bones are not static rods and blobs stuck in our bodies; no, they’re dynamic, growing, living tissues that repair and remodel themselves constantly. This is why your bones heal if you break them. As most bones grow, they get wider in all directions, expanding outward from the center, but usually bones grow rapidly only during certain parts of the year: the summer or the wet season, when food is plentiful. Growth slows down during the winter or dry season. If you cut open a bone, you can see a record of each time growth transitions from rapid to slow: a ring. That’s right—just like trees, bones have rings inside, and because that summer-to-winter switch happens once a year, that means one ring is laid down each year. By counting the rings you can tell how old a dinosaur was when it died.
Tyrannosaurus rex skeleton on display at the Royal Tyrrell Museum in Alberta, Canada.
Photo courtesy of the author
Greg got permission to cut open the bones of several different T. rex skeletons, along with many other close tyrannosaur relatives like Albertosaurus and Gorgosaurus. Shockingly, not a single bone had more than thirty growth rings. That means tyrannosaurs matured, reached adult size, and died within three decades. Big dinosaurs like T. rex didn’t grow slowly for many decades (or centuries) but must have reached their huge sizes by growing rapidly for a much shorter period of time. But how quickly? To figure it out, Greg constructed growth curves: he plotted the age of each skeleton, determined from the number of bone rings against its body size, calculated from those equations we learned about earlier that estimate weight based on limb dimensions. This allowed Greg to compute how quickly T. rex grew each year. The number is almost too big to c
omprehend: during its teenage years, from about ages ten to twenty, Rex put on about 1,700 pounds (760 kilograms) per year. That’s close to 5 pounds per day! No wonder T. rex had to eat so much—all of that Edmontosaurus and Triceratops flesh fired the insane teenage growth spurt that turned a kitty-size hatchling into the King of the Dinosaurs.
You could call T. rex the James Dean of dinosaurs: it lived fast and died young. And all of that hard living put a tremendous strain on its body. The skeleton had to endure the daily addition of five pounds during the spurt years. Somehow the body had to morph from wee hatchling to monster, so it comes as no surprise that the skeleton of T. rex changed dramatically as it matured. As youngsters, they were sleek cheetahs, as teenagers gangly looking sprinters, and as adults pure-blooded terrors longer and heavier than a bus. The younger ones probably ran a lot faster than the adults and maybe could have chased down their prey, whereas the silverbacks were so huge that they could only ambush and relied much more on their strength than their speed. What’s particularly frightening is that juveniles and adults seemed to live together in packs, meaning they may have hunted in teams, complementing each other’s skills to make life hell on their prey.
One of my dearest paleontologist friends has made a career studying how T. rex changed as it grew. He’s a Canadian named Thomas Carr, now a professor at Wisconsin’s Carthage College. You can spot Thomas from a mile away. He has the fashion sense of a 1970s preacher and some of the mannerisms of Sheldon Cooper from The Big Bang Theory. Thomas always wears black velvet suits, usually with a black or dark red shirt underneath. He has long bushy sideburns and a mop of light hair. A silver skull ring adorns his hand. He’s easily consumed by things and has a long-running obsession with absinthe and the Doors. That and tyrannosaurs: he’ll talk a lot about T. rex, because it’s his favorite subject of all. Ever since he was young, he wanted to study the Tyrant King, and he eventually wrote a PhD dissertation on how the skull of T. rex changed as it matured. It was over 1,270 pages long; meticulous as Thomas always is, it’s one of his shorter scholarly works.