On a sweltering summer day in 2010, a construction worker in the southeastern Chinese city of Ganzhou was digging the foundation for a building when his backhoe smashed into something hard. Climbing down to see what it was, he probably expected the worst—impenetrable bedrock, an old water main or some other nuisance that would inevitably delay completion of the sprawling industrial park his crew was racing to finish. But when the dirt and smoke cleared, a very different culprit came into focus: bones—lots of them, some enormous.
Construction did indeed stop that day because the bones turned out to be a major discovery. The worker had stumbled on a nearly complete skeleton of a bizarre new dinosaur species related to Tyrannosaurus rex. A few years later my Chinese colleagues invited me to help study the specimen, and in May 2014 we unveiled it as the latest addition to the tyrannosaur family tree: Qianzhousaurus sinensis. The formal name is something of a tongue twister, so we gave it a cheeky nickname, “Pinocchio rex,” in reference to its peculiar long snout.
Qianzhousaurus is part of a surge of new tyrannosaur discoveries over the past decade that is transforming understanding of this group. Ever since T. rex's discovery more than a century ago, the 13-meter-long, five-ton behemoth has dominated the limelight. Yet its evolutionary history has eluded investigators. During the 20th century scientists discovered a few close relatives of T. rex that were likewise impressively large and realized that T. rex was not a mere oddity: these big predators formed their own branch of dinosaur genealogy. But they struggled to understand when the tyrannosaurs originated, what they evolved from, and how they were able to grow so large and reach the top of the food chain. These questions have remained unanswered until now.
Over the past 15 years researchers have recovered nearly 20 new tyrannosaur species at locations the world over, including the deserts of Mongolia and the frigid wastelands of the Arctic Circle. These finds have made it possible to piece together the tyrannosaur family tree, and the results are surprising: it turns out that tyrannosaurs were mainly marginal, human-sized carnivores for most of their history, achieving huge size and ecological dominance only during the final 20 million years of the Age of Dinosaurs, which began around 250 million years ago and spanned the Triassic, Jurassic and Cretaceous periods. The king of the dinosaurs, far from belonging to a dynasty of giant predators, actually had rather humble roots and was merely the last survivor of a startling variety of tyrannosaurs that lived across the globe right up until the asteroid impact 66 million years ago that brought the dinosaur era to a close and ushered in the Age of Mammals.
A star is born
The tale of how the tyrannosaur family history was uncovered begins with the discovery of T. rex, enabled by a man named Henry Fairfield Osborn. During the early 20th century Osborn was one of the most visible scientists in the U.S. He was president of the American Museum of Natural History in New York City and the American Academy of Arts and Sciences, as well as a Time magazine cover star. He used his considerable platform to push pet ideas on eugenics and racial superiority, so today he is often dismissed as just another bigot from years gone by. But Osborn was a clever paleontologist and an even better scientific administrator, and one of the best calls he ever made was to send a fossil collector named Barnum Brown out to the American West in search of dinosaurs.
Brown himself was an eccentric character, a man who hunted fossils in the dead of summer in a full-length fur coat and made extra cash spying for governments and oil companies. The guy had good instincts, however, and in 1902 he made one of the most famous discoveries in the history of paleontology: a giant, meat-eating dinosaur from the badlands of Montana.
When this dinosaur was described a few years later, Osborn gave it a name whose brand value has stood the test of time: Tyrannosaurus rex, the “tyrant lizard king.” It was an instant sensation, making headlines across the country. Osborn and Brown had unveiled the biggest, baddest land-living predator ever.
T. rex became the quintessential celebrity dinosaur, the star of movies and museum exhibits around the globe. But this fame masked a puzzle: for nearly the entire 20th century scientists had little idea of how T. rex fit into the broader picture of dinosaur evolution. It was an oddball, a creature so much larger and so dramatically different from other known predatory dinosaurs that it was difficult to place in the dinosaur family album.
Over the next few decades, though, other paleontologists discovered a handful of close T. rex relatives that lived at about the same time—in the late Cretaceous—at sites in North America and Asia dated to between 84 million and 66 million years ago. These tyrannosaurs—Albertosaurus, Gorgosaurus, Tarbosaurus—are quite similar to T. rex: humongous apex predators that thrived during the last gasp of dinosaur history. Although the fossils were impressive, they did little to illuminate the origins of the group.
Modest beginnings
A number of the recent discoveries that are helping to fill the considerable gaps in our knowledge of tyrannosaurs have come from unexpected locales. The stereotypical dinosaur find involves intrepid paleontologists trekking to some far-flung corner of the desert in western North America, Argentina, the Gobi or the Sahara and braving the heat, dust and snakes to hack fossils out of their rocky tombs. But dinosaurs, including tyrannosaurs, are turning up across the world now, even in the far northern reaches of Russia, where paleontologists instead need to cope with bone-chilling cold in winter and humid, mosquito-infested summers.
Alexander Averianov, my colleague from the Zoological Institute of the Russian Academy of Sciences in St. Petersburg, is one of these paleontologists. In 2010 his team announced a provocative discovery from the vast Krasnoyarsk region of central Siberia: a jumble of bones from a small, meat-eating dinosaur that lived well before T. rex—about 170 million years ago in the Middle Jurassic period—and would have been about the size of a human. They named it Kileskus, based on the word “lizard” in a local language. It turned out to be a vital clue to the rise of tyrannosaurs.
At first glance, Kileskus does not appear very impressive. It certainly does not look anything like T. rex. If T. rex had been living in Russia during the Middle Jurassic, it could have swatted Kileskus away like a fly, even with its pathetic little arms. But Kileskus exhibits unmistakable similarities to another small carnivore, Guanlong, that lived about 10 million years later in China and was described in 2006. For instance, both animals have a gaudy mohawklike crest of bone running along the top of the skull. And Guanlong, which is known from much more complete specimens than Kileskus, possesses features that are seen only in tyrannosaurs, such as fused nasal bones in the snout. These shared traits are signs of common ancestry: the modest, almost forgettable Kileskus and Guanlong are the ancestral stock from which the great T. rex arose.
These two finds painted a startling picture of the dawning of tyrannosaurs. They revealed that the group did not begin as massive superpredators, as many researchers had thought when the first tyrannosaurs were found, but rather as second- or third-tier carnivores living in the shadow of giant predators from distantly related groups such as allosaurs and ceratosaurs. In addition, tyrannosaur roots run far deeper than anyone expected. They lived at a time when the supercontinent Pangea had not yet fully broken apart, so that animals could disperse relatively easily across landmasses. This geography explains why early tyrannosaurs have since turned up in Russia and China, and slightly later species have emerged from the U.S., U.K. and perhaps even Australia (the taxonomic affiliation of some Australian predatory dinosaurs is a matter of debate). Together the specimens also show that it took an amazingly long time for tyrannosaurs to rise to power: there was a greater separation in time between the ancestral tyrannosaurs and T. rex (at least 100 million years) than between T. rex and humans (66 million years).
Warm and fuzzy
Although the tyrannosaurs were slow to reach truly gigantic proportions, that delay does not mean their evolution stagnated in the interim. Mounting evidence indicates that the group underwent considerable diversification long before the likes of Tarbosaurus and T. rex appeared. Striking examples of this proliferation have come from Liaoning Province in northeastern China.
Liaoning is not the most beautiful place in the world. Even though I grew up in the boring plains of the American Midwest, I find it difficult to keep my eyes open during the three-and-a-half-hour train ride from Beijing through kilometer after kilometer of hilly countryside draped with haze and dotted with farms and billowing smokestacks. But this is a holy land for fossil hunters.
Over the past two decades farmers all around this region have collected thousands of dinosaur skeletons. Repeated volcanic eruptions around 120 million to 130 million years ago quickly buried the ill-fated creatures in ash and mud, preserving their remains in exquisite detail. Among the many beasts found in this Cretaceous Pompeii are two intriguing species of tyrannosaurs. My colleague Xu Xing of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing announced the first of these species, Dilong, in 2004. Roughly the size of a golden retriever, it had long arms for catching prey and a lithe, leggy skeleton built for speed. The second species, which Xu described in 2012, is a very different type of animal. At eight to nine meters long and about a ton in mass, this animal—named Yutyrannus—probably sat at or near the top of the food chain. Both tyrannosaurs are found in the same rock formation and may have lived side by side approximately 125 million years ago. Together these species, which exhibit the fused nasal bones and other hallmarks of tyrannosaurs, demonstrate that by the Early Cretaceous tyrannosaurs had branched off into a range of new species filling different ecological roles and that some were experimenting with larger body size.
Dilong and Yutyrannus are also important for another reason. Paleontologists once viewed dinosaurs as scaly, plodding, overgrown reptiles. In recent years, however, researchers have found evidence that a number of dinosaur species sported downy coats rather than scales and were generally more dynamic and smarter than they had previously been thought. Which is to say, they were more like birds than reptiles. Dilong and Yutyrannus establish beyond a doubt that tyrannosaurs fit this description. The bones of these species are covered with a thick, feathery fluff—not the quill-pen feathers that make up the wings of living birds but simpler, filamentlike feathers that look like hair. Unlike birds, tyrannosaurs certainly were not flying. Instead they probably used their feathers for display or to keep warm. The presence of feathers in tyrannosaurs and many other kinds of dinosaurs makes it very likely that the great T. rex was feathered, too. If the tyrant king was not fearsome enough already, just imagine it as an energetic, intelligent, Big Bird from Hell.
Rise of the kings
The new discoveries from Russia, China and elsewhere show that tyrannosaurs were doing quite well from the Middle Jurassic into the Early Cretaceous. They may not have been top dogs, but they had found their niche as a steady, if unspectacular, group of stealthy, fleet-footed predators. But then something changed. Sometime between 85 million and 110 million years ago, during the middle part of the Cretaceous, dinosaur ecosystems underwent a radical restructuring. The allosaurs and ceratosaurs that had long occupied the apex of the food pyramid largely disappeared, and tyrannosaurs assumed the top predator role on the northern continents. Exactly why this happened is unclear because dinosaur fossils from the Middle Cretaceous are extremely rare. A mass extinction about 94 million years ago—when temperatures increased and sea levels fluctuated—may be to blame.
However they got there, tyrannosaurs flourished once they reached the top of the food chain. During the final 20 million years of the Cretaceous, tyrannosaurs reigned supreme in North America and Asia as multiton, tiny-armed, huge-skulled superpredators. They bit so hard that they crunched through the bones of their prey. They grew so fast that they put on a few kilograms of mass every day during their teenage years. And they lived so hard that paleontologists have yet to find an individual that was more than 30 years old when it died.
Yet as successful as the colossal tyrannosaurs were in North America and Asia, they do not seem to have ever gained a foothold in Europe or the southern continents, where other groups of large predators prospered instead. Reconstruction of the earth's climate and continental configuration during the Late Cretaceous hints at why. The world at that time was very different from the setting in which smaller tyrannosaurs first evolved. The continents had drifted much farther apart, reaching positions similar to the ones they occupy today. Furthermore, dramatically higher sea levels bisected North America and reduced Europe to a smattering of small islands. T. rex's earth was a highly fragmented planet. As a result, champions in one region might not be able to conquer another for one simple reason: they could not get there.
Persistent variety
In the regions where mega tyrannosaurs such as T. rex did rise to power, one might expect these brawny forms to have edged out other, daintier tyrannosaurs. The latest fossil discoveries suggest otherwise. Amazing new finds have revealed an unappreciated diversity of tyrannosaurs up and down the food chain, even during those last few million years of the Cretaceous, when T. rex and kin ruled unchallenged.
The Pinocchio-nosed Qianzhousaurus, from the Chinese construction site, is a prime example. When my colleague Junchang Lü of the Chinese Academy of Geological Sciences' Institute of Geology first showed me photographs of the specimen at a conference in 2013, I was gobsmacked. Here was a tyrannosaur from the latest part of the Cretaceous that was strikingly different from the tyrant lizard king. It was noticeably smaller—just about eight to nine meters long and probably around a ton in mass. Still not any animal you would want to run into in a dark Cretaceous alley but a waif compared with T. rex. Even weirder, its skull was long, narrow and delicately constructed, unlike the deep, muscular, bone-crunching skull of its famous cousins.
Lü invited me to help him describe the new Chinese fossil because I had studied two other peculiar long-snouted tyrannosaurs, discovered decades earlier, that had long confused scientists. The first of these creatures was known from part of a skeleton found in Mongolia by a Russian team in the 1970s. They called it Alioramus remotus and suggested that it was an aberrant long-skulled tyrannosaur. But very few paleontologists were able to study the specimen during the cold war, so it remained debatable whether it was a weird new species or just a young representative of the existing tyrannosaur species Tarbosaurus. A few decades later, in the early 2000s, a joint Mongolian-American team led by my Ph.D. adviser, Mark Norell of the American Museum of Natural History, discovered a much more complete and better preserved specimen of Alioramus. On the first day of my Ph.D. studies Norell took me into the museum's prep laboratory, showed me the skeleton and told me to get to work studying it. In 2009 we announced it as a new species, Alioramus altai. This skeleton appeared to be distinct from Tarbosaurus, but because it came from a juvenile individual (as revealed by its internal bone structure), we could not rule out the possibility that its seemingly unique traits were instead the product of incomplete growth.
Sometimes debates like this one rage for decades, as paleontologists await new fossils to break a stalemate. In our case, it took only a few years, along with the dumb luck of the backhoe operator. The Qianzhousaurus skeleton discovered in Ganzhou had the same long snout, delicate build and small body size of Alioramus, but it clearly belonged to a much older and more mature individual. It was the clincher: long-snouted tyrannosaurs were distinct species that lived across Asia during the very end of the Cretaceous, probably filling a second-tier predator role on the food chain below Tarbosaurus.
Qianzhousaurus was not the only petite tyrannosaur to share the planet with the heavyweights. Just about two months before we published our description of Qianzhousaurus, my American colleagues Anthony Fiorillo and Ronald Tykoski, both at the Perot Museum of Nature and Science in Dallas, Tex., revealed an even stranger latest Cretaceous tyrannosaur from the frosty Arctic Circle of Alaska, called Nanuqsaurus. It is known from just a few bones that look a lot like the corresponding bones in T. rex but with one incredible difference: they are about half the size. The obvious explanation is that the bones belong to a baby tyrannosaur, but shockingly they have thickened sutures—the “seams” between adjacent bones that are only seen in adults. Fiorillo and Tykoski came up with an idea that may sound farfetched but one I think is very plausible: Arctic tyrannosaurs evolved small bodies because their resource-poor habitat could not support larger species. Many modern island-dwelling animals have undergone dwarfing for that same reason. So while T. rex ruled to the south, a Mini Me tyrannosaur patrolled the northern wilderness.
Beyond their control
These latest additions to the tyrannosaur family tree have illuminated the evolutionary history of this charismatic group, but key questions remain. Where did tyrannosaurs originate, and did they get their start even earlier than the Middle Jurassic, perhaps during the Early Jurassic, a time when we have few fossils from around the world?
Did tyrannosaurs also live across the southern continents during the Middle Jurassic/Middle Cretaceous? Most of their fossils are from the northern continents, except for one enigmatic bone from Australia. But we know that many dinosaur groups were globally widespread during the Middle Jurassic/Middle Cretaceous, so maybe tyrannosaurs were, too. Other unknowns concern their biology. What kind of feathers did the largest tyrannosaurs like T. rex have, and what purposes did they serve? And how did Qianzhousaurus and Alioramus use their distinctive long snouts?
Incomplete as the tyrannosaur story is, it still reveals a deeper truth about evolution—namely that is not predictable. When tyrannosaurs originated more than 170 million years ago, no one would have guessed that these puny stalkers would come to rule entire continents. They were not preordained for success. Rather they had to navigate more than 80 million years of living in the shadows, biding their time until environmental changes gave them the opportunity to become apex predators. And then one day, when the tyrannosaurs were at the peak of their game, an asteroid fell from the sky and they disappeared. Their strength and size could not save them as wildfires raged, ecosystems collapsed and mammals began their march to the top.