The Rise and Fall of the Dinosaurs

by Steve Brusatte

  Alexander Averianov, my friend from the Zoological Institute of the Russian Academy of Sciences in Saint Petersburg, is one of those paleontologists. Sasha, as we all call him, is among the world’s experts on those puny mammals that lived alongside (or more correctly, underneath) the dinosaurs. He also studies the dinosaurs that were keeping his beloved mammals down. Sasha began his career as the Soviet Union was disintegrating, and through his numerous discoveries and meticulous descriptions of fossil anatomy, he has now become one of the leading paleontologists in the new Russia.

  A few years ago, Sasha showed me a new dinosaur fossil from Uzbekistan at a conference. He whisked me up to his room, ceremoniously opened an ornately colored orange-and-green cardboard box, and pulled out part of the skull of a meat-eater. He put the fossil back in the box and handed it to me so I could take it back to Edinburgh to CAT-scan it. But before he let go, he looked me in the eye and, in the Russian-accented drawl of movie bad guys, said, “Be careful with the fossil, but be even more careful with the box. This is Soviet box. They don’t make them like this anymore.” Grinning with mischief, he then pulled out a small bottle of dark-colored liquid. “And now we toast with Dagestan cognac,” he proclaimed, pouring two glasses, then another two, and then a third round. We toasted to his tyrannosaurs.

  Like Brown’s first fossil of Tyrannosaurus rex, Sasha’s dinosaur Kileskus was only a fraction of a skeleton. There was part of the snout and the side of the face, a tooth, a chunk of the lower jaw, and some random bones from the hand and feet. These bones were all found within a couple of square meters in a quarry that Sasha’s team had been working in for many years, in the Krasnoyarsk region of central Siberia. Krasnoyarsk is one of the more than eighty “federal subjects” of Russia, as the post-Soviet constitution calls the equivalents to American states or Canadian provinces. It’s no little Delaware, or even Texas, or incredibly, even Alaska. Krasnoyarsk stretches across nearly the entire midsection of Russia, from the Arctic Sea up north to almost touch the border with Mongolia down south. It’s a shade below one million square miles in area, much bigger than Alaska and even slightly larger than Greenland. A lot of space, but very few people: the entire population is about the same as Chicago’s. In this vast wilderness, Sasha was able to find the world’s oldest tyrannosaur. The name he gave it, Kileskus, is based on the word “lizard” in a local language that is spoken by only a few thousand people in this isolated part of the world.

  The discovery didn’t get much buzz in the press, and it escaped the attention of many scientists when Sasha described it in an obscure Russian journal that isn’t on the radar of most paleontologists. Kileskus didn’t get a funny nickname, and it surely won’t be appearing in any future Jurassic Park films. It’s one of those fifty-some new dinosaurs that are announced in a technical scientific paper every year and then mostly forgotten about, except by a handful of specialist paleontologists. To me, though, Kileskus is one of the most interesting discoveries of the last decade, because it is clear proof that tyrannosaurs had gotten an early evolutionary start. Kileskus was found in rocks formed during the middle part of the Jurassic Period, about 170 million years ago, more than 100 million years before T. rex and its colossal cousins were at the top of their game in North America and Asia.

  Kileskus may be important, but it is underwhelming to behold. I first scrutinized the bones in Sasha’s dark office, in a grand old building along the icy Neva River, which was still thawing in early April. Yes, Sasha’s fossil is only a few bones, but that’s not too surprising. The vast majority of new dinosaur discoveries are just a few jumbled pieces of bone, because it takes a whole lot of luck for even a tiny fraction of a skeleton to withstand millions of years buried in the ground. No, what struck me about Kileskus was how small it is. All of the bones can comfortably fit into a couple of shoeboxes. I could easily lift them up off the shelf. If I wanted to pick up the skull of T. rex back in New York, I would need a forklift.

  It’s hard to believe that a meek creature like Kileskus could have given rise to a giant like T. rex. Although an accurate measure of its size is difficult because of its scrappy bones, Kileskus was probably only seven or eight feet long, most of that being the skinny tail. It stood a couple of feet tall at most—it would have come up to your waist or chest like a big dog. And it wouldn’t have weighed more than a hundred pounds or so. If the forty-foot-long, ten-foot-tall, seven-ton T. rex was living in Russia during the Middle Jurassic, it could have brushed Kileskus aside with little effort, even with its pathetic little arms. Kileskus was not a brutish monster. It wasn’t a top predator. It was probably something like a wolf or jackal, a long-legged, lightweight hunter that used speed to chase down small prey. It’s surely no coincidence that the quarry in Krasnoyarsk where Kileskus was found is bursting with the fossils of small lizards, salamanders, turtles, and mammals. It was these things that the very first tyrannosaurs were eating, not long-necked sauropods or jeep-size stegosaurs.

  Because Kileskus is so different from T. rex in size and hunting habits, how do we know that it’s even a tyrannosaur? If Kileskus had been discovered at the same time as T. rex, scientists probably wouldn’t have made the connection. Even if Kileskus had been found a few decades ago, it likely wouldn’t have registered as a primitive tyrannosaur, a great-great-great-grandparent of T. rex. Now we know, and once again, it’s because of new fossils.

  Sasha had the great fortune of finding Kileskus just four years after a team in far western China, led by my colleague Xu Xing, came across a very similar small meat-eater from the middle part of the Jurassic. Thankfully, Xu’s team didn’t just find a couple of broken bones. They uncovered two nearly complete skeletons, one an adult and the other a teenager. The story of how these dinosaurs got there could be written into the script of a disaster movie. The teenager was found at the bottom of a pit several feet deep, trampled by the adult. They were both engulfed in mud and volcanic ash. Something terrible had clearly happened, but what was torture for these dinosaurs was a lucky break for paleontologists.

  Xu and his group named their new dinosaur Guanlong, meaning “crown dragon” in Chinese. The name refers to the gaudy Mohawk-like crest of bone that runs along the top of the skull. The crest is thinner than a dinner plate and pierced by a number of holes. It’s the type of absurdly impractical-looking thing that probably had only one function: a display ornament for attracting mates and intimidating rivals, kind of like the flamboyant tail of a male peacock, which is for nothing but show.

  I spent days poring over the bones of Guanlong in Beijing. The crest is what grabbed my attention first, but other features of the bones offer critical clues for placing Guanlong on the family tree and linking it to both Kileskus and T. rex. For a start, it is clearly very similar to Kileskus: both are about the same size, have huge windowlike nostrils at the front of the snout, and have long upper-jaw bones with a deep depression above the teeth that would have housed a huge sinus. On the other hand, Guanlong exhibits many characteristics that are only seen in T. rex and other big tyrannosaurs among all of the meat-eating dinosaurs. In other words, evolutionary novelties, which as we learned earlier, are the key to understanding genealogy. For example, it has heavily fused nasal bones at the top of the snout, a broad and rounded front of the snout, a small horn in front of the eye, and two massive muscle attachment scars on the front of the pelvis. There are many more similarities as well, anatomical minutiae that may seem boring but tell my scientific colleagues and me that Guanlong is definitively a primitive tyrannosaur. And because the complete skeletons of Guanlong share so many features with the much scrappier bones of Kileskus, the latter must be a primitive tyrannosaur as well.

  Along with helping to prove that Kileskus is a tyrannosaur, the complete skeletons of Guanlong also paint a clearer picture of what these earliest and most primitive tyrannosaurs would have looked like, how they behaved, and how they fit into their ecosystems. Based on its limb dimensions—which are known to correlate closely with body wei
ght in living animals—Guanlong would have weighed about 70 kilograms, or roughly 150 pounds. Guanlong was lithe and lean, with long skinny legs and a tail that stretched far beyond its body for balance. No doubt it was a speedy hunter. It had a mouth full of steak-knife-like teeth befitting a predator, but it also had fairly long arms with three claw-capped fingers capable of grabbing prey with extreme strength. They are totally different from the withered two-fingered arms of T. rex.

  Guanlong could hunt with its arsenal of speed, sharp teeth, and deadly claws, but it was not a top predator. It lived alongside much larger carnivores like Monolophosaurus, which was over fifteen feet long, and Sinraptor, a thirty-foot-long close cousin of Allosaurus that weighed more than a ton. Guanlong lived in the shade of these animals, and probably in fear of them too. At best, Guanlong was a second- or third-tier predator, an inconspicuous link in a food chain dominated by other dinosaurs. This would have been the same for Kileskus and for some of the other small and primitive tyrannosaurs that have been found recently, like the tiniest one of all, the greyhound-size Dilong from China, and Proceratosaurus, a dinosaur discovered over a century ago in England that was only recently recognized as an archaic tyrannosaur because it has a Mohawk-like crest similar to Guanlong.

  These petite tyrannosaurs may not have been much to look at and wouldn’t have haunted anyone’s nightmares, but they obviously were doing something right. The more fossils we find, the more successful we realize they were. There were a bunch of them, spread all over the world during the approximately 50 million years from the middle part of the Jurassic period well into the Cretaceous, from about 170 until 120 million years ago. They clearly survived that cocktail of environmental and climate changes that felled Allosaurus, the sauropods, and the stegosaurs around the Jurassic-Cretaceous boundary. We now have their fossils from throughout Asia, multiple sites in England, the western United States, and probably even Australia. They were able to disperse so widely because they lived when the supercontinent Pangea was still breaking apart, meaning they could easily hop across land bridges that linked the continents, which had yet to move very far away from one another. These early tyrannosaurs had carved out a niche as small to midsize predators living in the underbrush, and they were good at it.

  The skeleton of the dog-size primitive tyrannosaur Dilong.

  Photo courtesy of the author

  The skull of the human-size primitive tyrannosaur Guanlong, showing the gaudy crest of bone on top of its head.

  Photo courtesy of the author

  AT SOME POINT, however, tyrannosaurs changed from bit players to the celebrated apex predators that we all love. The first whispers of this transformation are seen in fossils from the early part of the Cretaceous, about 125 million years ago. Most tyrannosaurs living at this time were small. The pint-size Dilong is the most extreme example, barely registering on the scales at about twenty pounds. Some were a bit larger, like Eotyrannus from England and a few of its older cousins like Juratyrant and Stokesosaurus, which were bulkier than Dilong, Guanlong, and Kileskus, and maybe reached lengths of about ten to twelve feet and weights of a thousand pounds or so. If you were around back then, and these midsize tyrannosaurs cooperated, you could have ridden them like horses, but they still weren’t top-of-the-food-chain animals.

  Then in 2009, another piece of the puzzle: a team of Chinese scientists described a highly unusual fossil from the northeastern corner of the country, which they called Sinotyrannus. As is so often the case, the new dinosaur was fragmentary: only a small collection of bones was preserved, including the front of the snout and lower jaw, some portions of the backbone, and a few pieces of the hand and pelvis. These bones were really similar to Guanlong, and also to Kileskus, which would be described a few months later. The base of a tall bony crest was visible right where the snout region was broken, the nostril opening was huge, and there was a deep sinus depression above the teeth. But there was one major difference: Sinotyrannus was substantially bigger than Guanlong. Based on comparisons to the bones of other meat-eating dinosaurs, it was estimated that this new predator would have been around thirty feet long, and perhaps over a ton in weight. That’s the equivalent to at least ten Guanlongs. At about 125 million years in age, Sinotyrannus was the oldest example of a large-bodied tyrannosaur ever found.

  I read the announcement of the new species as a graduate student, about a year after I began my PhD project on carnivorous dinosaur evolution. It was clear to me that the new dinosaur was a tyrannosaur and that it was big, but I didn’t know what else to make of it. The fossils were too scrappy to be certain of how large it was or to place it accurately in the family tree. Was it a very close relative of T. rex, the first member of that group of really big, deep-skulled, tiny-armed carnivores—Tyrannosaurus, Tarbosaurus, Albertosaurus, Gorgosaurus—that dominated the very end of the Cretaceous, about 84 to 66 million years ago? If so, maybe it would tell us how these dinosaur icons became so huge, so dominant. But was it something else? Maybe it was merely a primitive tyrannosaur that outgrew its contemporaries. After all, Sinotyrannus lived about 60 million years before T. rex, a time when every other tyrannosaur we knew of could fit in the back of a pickup truck.

  Could this one find really rewrite tyrannosaur history? I had the sinking feeling that this fossil would remain a problem for a long time. This happens all too often in the field of dinosaur research: a single fossil emerges that hints at a major evolutionary story—the oldest member of a major group, or the first fossil to exhibit a really important behavior or feature of the skeleton—but it’s too broken or incomplete or poorly dated to be certain. Then another fossil is never found and it’s just left hanging, a cold case waiting to be solved.

  The skull of Gorgosaurus, a large-bodied, latest Cretaceous tyrannosaur closely related to T. rex.

  Photo courtesy of the author

  But I shouldn’t have been so pessimistic. Just three years later, Xu Xing in China—the man who described Guanlong and Dilong—published a sensational article in the journal Nature. Xu and his team announced yet another new dinosaur, which they called Yutyrannus. They had more than just a few bones at their disposal—they had skeletons, three of them. Their new dinosaur was obviously a tyrannosaur and was very similar to Sinotyrannus. There were similarities in size and also in the bones—Yutyrannus had a flashy head crest and huge nostrils, just like Sinotyrannus. Yutyrannus was big: the largest skeleton was about thirty feet long. This wasn’t an estimate, because Xu and his team could take out a tape measure and size up their new dinosaur, rather than using mathematical equations to guesstimate the size of a complete skeleton based on just a few broken bones, as was our only recourse with Sinotyrannus. So Yutyrannus sealed the deal: there really were large tyrannosaurs in the Early Cretaceous, at least in China.

  There was something else peculiar about Yutyrannus. The skeletons were so well preserved that details of the soft tissue were visible. Usually the skin, muscles, and organs decay away long before a fossil is entombed in stone, leaving only the hard parts like bones, teeth, and shells. With Yutyrannus we got lucky—these skeletons were buried so quickly, after a volcanic eruption, that some of their softer parts did not decay. Packed all around the bones were dense clusters of slender filaments, each about fifteen centimeters (six inches) long. Similar structures were preserved on the much smaller Dilong, which was found in the same rock unit in northeastern China.

  These are feathers. Not the quill-pen feathers that make up the wings of today’s birds but simpler ones that look more like strands of hair. These were the ancestral structures that bird feathers evolved from, and it is now known that many (and perhaps all) dinosaurs had them. Yutyrannus and Dilong establish beyond a doubt that tyrannosaurs were among these feathered dinosaurs. Unlike birds, tyrannosaurs certainly were not flying. Instead, they probably used their feathers for display or to keep warm. And because both a large tyrannosaur like Yutyrannus and a small tyrannosaur like Dilong have feathers, this implies that the common ance
stor of all tyrannosaurs had feathers, and therefore that the great T. rex itself was most likely feathered, too.

  The fluff-covered skeletons of Yutyrannus launched this new dinosaur to stardom in the international press, but feathers are a story that we’ll come back to later. For me, the real importance of Yutyrannus was that it could help us better understand how tyrannosaurs evolved into their huge sizes. Yutyrannus and Sinotyrannus were big—much larger than any other tyrannosaurs living before the very end of the Cretaceous, when T. rex and its brethren reigned supreme. However, these two Chinese tyrannosaurs weren’t truly colossal: they were about the same size as Allosaurus or the big predator Sinraptor that preyed on Guanlong, nowhere near the monstrous forty-foot-long, seven-ton body sizes of T. rex and its very close relatives. Not only that, but when the complete skeletons of Yutyrannus are compared bone by bone with the skeletons of T. rex, it becomes clear they are quite different. Yutyrannus looks like an overgrown version of Guanlong, with its ornamental head crest, big nostrils, and long, three-fingered hands. It doesn’t have the deep muscular skull, thick railroad-spike teeth, and pathetic arms of T. rex.

  This leads to an unexpected conclusion: despite their big bodies, Yutyrannus and Sinotyrannus weren’t very closely related to T. rex, and they didn’t have much to do with the evolution of colossal sizes in the latest Cretaceous tyrannosaurs. Instead, they were primitive tyrannosaurs experimenting with large body sizes independent of their later cousins. Put another way, they were evolutionary dead ends that, as far as we know, didn’t exist outside of one corner of China during the early Cretaceous. (This assertion can of course be proven wrong with new discoveries.) They lived alongside small tyrannosaurs, which were by far the more common type thriving in the Jurassic and early Cretaceous times.