The Rise and Fall of the Dinosaurs

by Steve Brusatte

  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. 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.

  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. 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 wa
s 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. 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.

  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. 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.