by Brian Switek
When Evans, Ridgely, and Witmer looked at the inner ears of the hadrosaurs, they found that juvenile dinosaurs were better attuned to picking up a wider range of sound, while adults specialized in detecting lower frequencies of the sort their crests were capable of making. More than that, the brains of these dinosaurs had large cerebral hemispheres—the part of the brain associated with complex behaviors. While not absolutely definitive proof that creating and hearing sounds drove the evolution of hadrosaur headgear, the conclusion was consistent with the idea that dinosaurs like Parasaurolophus were vocal, social creatures who communicated with each other from the time they hatched to the time they died.
The same principle can help us outline what other dinosaurs might have sounded like. Most dinosaurs didn’t have musical crests, but we can gain some insight into their vocal range by estimating what particular species could hear.
Should you ever find yourself in the company of big predatory dinosaurs, remember this—they probably won’t be able to hear you. (Cartoon by Mike Keesey)
In 2005, Otto Gleich and colleagues used the relationship between inner-ear anatomy and the ability to detect sound in birds and crocodylians to estimate what the huge dinosaurs Allosaurus and Giraffatitan could hear. In their estimation, dinosaurs were attuned to very low frequencies—lower than 3 kiloherz. As journalists were quick to point out when Gleich’s study was published, 3 kHz is about the pitch of a human scream, so should you ever find yourself in the Jurassic, you can shriek to your heart’s content without having to worry about the biggest dinosaurs homing in on you. But, barring such time slips, the important lesson is that dinosaur ears may be the best way for us to understand what sorts of sounds dinosaurs made.
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Brain scans can do far more than give us a rough estimate of what dinosaurs heard. Reconstructions and casts of dinosaur brains also contain information about their other senses, such as eyesight and smell. Lawrence Witmer has been at the forefront of these investigations. While Witmer spends much of his time considering the sensory lives of prehistoric animals, he has tackled these questions through an approach melding old-school comparative anatomy with high-tech innovations. By understanding how certain structures function in modern animals, Witmer’s lab has begun to reconstruct aspects of dinosaur lives that were once thought to be beyond the realm of scientific inquiry.
In late 2011, Witmer, Darla Zelenitsky, and others used reconstructed brain images to track how dinosaurs’ sense of smell changed through time. The key was the olfactory lobes. These are the parts of the brain that process incoming information about smells, and the general rule goes, the larger the olfactory lobe, the better the animal can sniff out odors. The idea has to do with what’s called the principle of proper mass: the bigger a portion of the brain is relative to other regions, the more important that particular function is to the animal’s life. In an animal that primarily relies on sight, you would expect the parts of the brain that process visual information to be relatively large. The same is true with smell.
Some dinosaurs turned out to have very fine-tuned senses of smell. Bambiraptor—a switchblade-clawed predator not as cuddly as the name suggests—had a brain about as dedicated to smell as a turkey vulture or a black-footed albatross. Both of these are carnivorous birds that pick up on scents to find food, and it’s not a stretch to think that feathery little Bambiraptor did the same. Looking at the big picture of bird evolution, the avian descendants of the deinonychosaurs—the kin of Bambiraptor—retained their good scenting abilities during their early evolution. It was only later, as birds continued to diversify, that some lineages became more visually oriented while other groups retained (or re-evolved) a powerful sense of smell.
Raptors weren’t the only dinosaurs with especially sensitive noses. Tyrannosaurus rex is notorious for having very large olfactory bulbs. This feature, along with several of the dinosaur’s other peculiarities, put it at the center of a strange debate that has had more to do with Cretaceous PR than with science.
For as long as I can remember, the predatory power of Tyrannosaurus was self-evident. The answer to the question “What did a T. rex eat?” was “Anything it damn well pleased.” But early on, some paleontologists saw tyrannosaurs as immense, clumsy scavengers. In 1917, nine years after Tyrannosaurus itself was named, the Canadian paleontologist Lawrence Lambe proposed that the large, slender tyrannosaur Gorgosaurus was a “recumbent” carnivore and “played its useful part in nature” as a living garbage disposal—cleaning up the carcasses of ceratopsids and hadrosaurs from the Cretaceous landscape.
Decades later, Jack Horner proposed a scavenging life for Tyrannosaurus rex, too. At 1994’s Dino Fest conference, Horner pointed out that T. rex had “beady eyes,” tiny arms, and a deep skull better suited to crushing than slicing. The tyrant seemed less a predator than a scavenger, and this controversial interpretation instantly became a news hook. Nearly every Tyrannosaurus discovery—about how fast the dinosaur could run or how powerfully it could bite, based on tooth-marked remains of other dinosaurs it had consumed—was brought into the context of whether the dinosaur was a consummate predator or a filthy scavenger.
Most paleontologists didn’t see the point of the controversy. Tyrannosaurus was certainly capable of both hunting and scavenging. The tyrant did have a fine-tuned sense of smell, but the carnivore also had forward-oriented eyes—T. rex was one of the few dinosaurs that could have zeroed in on you with binocular vision. And even though the dinosaur’s arms have frequently been ridiculed, T. rex traded grasping arms for a heavy skull capable of delivering devastating bites. There’s nothing about the dinosaur that would have precluded the tyrant from hunting Edmontosaurus and Triceratops, or from wolfing down carrion, when the opportunities presented themselves. T. rex was an all-purpose carnivore, capable of dismantling rotting carcasses as well as taking down prey on the hoof.
The intricate details of the tyrant’s skull can give us an idea of how the dinosaur perceived the outside world. Thanks to new techniques and a renewed interest in dinosaurs as animals, paleontologists are slowly putting together a more comprehensive understanding of how dinosaurs interacted with the world around them. Oddly enough, some of the most detailed information about dinosaur biology doesn’t come from imagining how they walked, bit, and fought, but from signs of disease and injury preserved in their skeletons. The ailments that chipped away at a dinosaur’s health can tell stories about how the animal actually lived.
Nine
In the Bones
Tyrannosaurus rex is immortal. In fiction and in scientific restoration, no creature has ever been as vicious or fearsome. The dinosaur’s name isn’t just a figurehead title, but a reminder that flesh-tearing, bone-crushing power reached its apex in Tyrannosaurus. In fiction, especially, Tyrannosaurus is less an animal than a force of nature. The dinosaur’s steps shook the earth, its hellish howl had the power of a hurricane, and Tyrannosaurus was beautifully adapted through millions of years of evolution to efficiently separate flesh from bone. Tyrannosaurus is the epitome of all that is truly wild. Maybe that’s why we refuse to believe that this dinosaur had more than a few weak spots. Indeed, until I started riffling through the literature on paleopathology, I had no idea that dinosaurs frequently suffered broken bones, scratched at parasites, fought off infection, and otherwise faced the slings and arrows of life.
Sue suffered plenty. She is the most complete Tyrannosaurus ever found, as well as one of the largest, and is a fossilized lesson in Cretaceous hardship. Her battered skeleton is on display at Chicago’s Field Museum. The last time I came close to her digs, I was moving cross-country to my new home in Utah with three pissed-off cats in the backseat of my tiny car. A brief stop at the Field was out of the question, but fortunately for me, I knew that a Sue facsimile was going to be reassembled in my neighborhood soon. I’d at least get the chance to visit a Sue clone. The Museum of Idaho, part of the urban-suburban sprawl of Idaho Falls a few hours north from Salt Lake City, was se
t to host a traveling exhibit about the world’s most famous Tyrannosaurus rex.
I did a little homework before I drove up to Idaho. There were specific parts of Sue I wanted to check out. I had heard that she’d endured several broken bones, and that an unknown trauma had injured her jaws. I wanted to be able to focus on these damaged bones. It’s easy to look at a dinosaur skeleton without really seeing it, the same way you might look at a house, office, or church and overlook the small details. There are subtle clues hidden in the awesome architecture of dinosaur skeletons—bumps, knobs, muscle scars, and flanges that together represent the creature’s essential nature and tell us about its life. These are the signposts declaring that we’re not just looking at static lumps of rock, but at the remains of a once-living animal more fantastic than anything we could have imagined on our own.
I studied the University of Iowa paleontologist Chris Brochu’s exhaustive monograph on Sue’s skeleton. Brochu found multiple signs that the Tyrannosaurus had seen some combat in her life. Several of Sue’s ribs were broken, as well as the dinosaur’s right humerus and shoulder. The damage on her right side was probably caused by a single traumatic event, he reasoned, and fractured ribs on the dinosaur’s left side, too, hinted that broken bones were just part of life for a successful tyrannosaur. And internal infections had altered the bones in other parts of Sue’s skeleton, such as the left fibula and two of the vertebrae. Sue survived: the tyrant’s bones show signs of healing, and she didn’t ultimately collapse from an infection from the breaks. But Brochu noticed something critical when he began to explore the architecture of Sue’s jaws. He saw that they showed signs of extensive damage that may have made it impossible for her to eat toward the end of her life. This conundrum especially intrigued me—what could have weakened the massive tyrant?
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When I arrived at the Idaho museum, I walked right past the small gallery of hands-on exhibits and headed straight for Sue. What I wanted to know wasn’t in the traveling exhibit’s signage, but in the dinosaur’s bones. Fortunately for me, Sue was big enough that—even behind the exhibit barrier—the tyrannosaur’s trauma is easy to spot. Gnarly lumps of bone gave away healed breaks and places where the dinosaur’s body fought off infection, but most impressive of all were those jaw wounds. Sue’s lower jaws look as if someone unloaded a bevy of heavy-gauge shotgun shells into the dinosaur’s mouth. Both sides of her jaws are perforated with large, smooth holes.
Shortly after Sue was discovered, Peter Larson—part of the crew that unearthed the tyrannosaur—said that the holes were bite wounds. This hypothesis fit a pattern of injury other paleontologists had found among other tyrannosaurs. In a survey of theropod head wounds, Darren Tanke and Philip Currie discovered that large predatory dinosaurs often fought by biting each other on the face. Particular specimens of the tyrannosaurs Albertosaurus, Gorgosaurus, and Daspletosaurus all had bite wounds on their skulls that could have been inflicted only by their own kind. It wasn’t a stretch to think that Tyrannosaurus did the same, and the fact that the holes in Sue’s lower jaw didn’t show any sign of healing meant that this mighty dinosaur was attacked and killed by a rival.
But something was amiss. As Brochu and other paleontologists examined the jaw injuries, they realized that the distribution and shape of the holes didn’t match a tyrannosaur’s teeth. The pathology wasn’t what scientists would expect for a crushing bite from another huge Tyrannosaurus. The question of who killed Sue was reopened, and paleontologists now suspect that organisms much, much smaller than any dinosaur were to blame.
The veterinarian Ewan Wolff, the paleontologist Steven Salisbury, and their team solved the mystery of Sue’s demise in 2009. They announced that Sue and her fellow tyrannosaurs were afflicted by a microorganism that commonly infests the mouths and throats of modern hawks. In living birds, the microscopic pest is called Trichomonas gallinae, and the mode of transmission is simple: pigeons (for example) drink water that harbors the microorganism, the parasite takes up residence in the birds and infects them, and raptors that prey on pigeons get infested too. There are various strains, and some of them show no symptoms, but others eat away at the lower jaw of the bird, creating lesions in the bone and ulcers in the soft tissue. In the most severe cases, the damage is so severe the birds can no longer eat or drink properly.
While the exact same species of parasite wasn’t around when Tyrannosaurus lived, Sue and other tyrannosaurs were undoubtedly afflicted by a hitchhiker of the same ilk. (Indeed, if you look closely at many Tyrannosaurus skeletons on display at museums around the world, you can easily spot these large, smooth-sided holes.) Sue surely suffered. The bone lesions, dead tissue, and ulcers must have made her normally enjoyable feasts (or so I’d like to imagine) so painful and difficult that Sue simply starved to death. The most fearsome carnivore of all time was brought to its knees by an even more vicious predator so small that it can’t even be seen with the naked eye.
At the end of her life, Tyrannosaurus Sue suffered from an oral infestation of parasites that ate away her jawbones to the point where she may not have been able to eat. (Illustration by Chris Glen, The University of Queensland, from doi:10.1371/journal.pone.0007288.g004)
No one knows how Sue wound up with a mouthful of harmful microorganisms. There’s more than one possibility. Thanks to the specimens Currie and Tanke described, as well as punctures on the snout of a young T. rex nicknamed “Jane,” we know that tyrannosaurs bit each other on the face when they tussled. And, if the attacking tyrannosaur had a mouth brimming with parasites, this would be a great way to transmit the disease. Coarsely serrated teeth were comfortable homes for bacteria and protozoa feeding on tidbits of leftover meat from dinosaur dinners, and such dirty teeth would have driven deadly microorganisms deep into the heads of other tyrannosaurs. Yet Sue doesn’t have any bite marks. If an infested tyrannosaur didn’t attack her, then the parasite must have found its way into her system some other way.
Cannibalism is another possibility. Of the many T. rex specimens collected to date, at least four are gouged by bite marks made by another large, carnivorous dinosaur. The details of these fossils show that the tooth marks were made after death and record feeding, rather than fighting. In the Hell Creek Formation in Montana where we’ve found Tyrannosaurus remains, there has been only one predator of the size and power to inflict such damage—and that’s T. rex itself. Tyrannosaurus ate their own kind when they had the chance. And if a Tyrannosaurus just so happened to feed upon one of its kin that was infested with parasites, maybe the little buggers could have jumped ship. Cannibalism is a great way to spread diseases around.
Sue’s perforated jaws reveal that other organisms harried, invaded, and infested even the mightiest dinosaurs. Their damage can’t always be seen on the bones; Sue’s injured jaws were an exceptional case. For the most part, parasites thrived in and on the soft tissues of dinosaurs. When their host’s body decayed, the parasites disappeared, too. But a few unusual fossils show that dinosaurs provided homes for whole ecosystems of parasites.
Terrible prehistoric parasites bring to mind a modified version of the tagline for John Carpenter’s body-snatching gore fest The Thing: for some parasites, dinosaurs were the warmest places to hide. And since dinosaurs were so fantastic, we might expect their parasites to be even more horrifying. In the realm of fiction, we’ve had some fun conjuring up how awful dinosaur parasites must have been. When he created a lush jungle filled with prehistoric survivors for his King Kong remake, director Peter Jackson conjured the imaginary carrion-eating monster “Carnictis”—a kind of flesh-eating worm that lived in the guts of tyrannosaurs, but took on a gruesome existence of its own when a dead dinosaur’s viscera spilled out into a rocky pool of just the right conditions. Long before that, in his short story “Poor Little Warrior!,” science fiction author Brian Aldiss imagined the horrible little arthropods that swarmed over the bodies of the biggest dinosaurs. When Claude Ford, a time-traveling hunter bent on proving his masculin
ity by gunning down a monstrous trophy, fells a wallowing sauropod, the creature’s battalions of parasites look for the closest warm body they can find. That happens to be Claude. “You struggle and scream as lobster claws tear at your neck and throat. You try to pick up your rifle but cannot, so in agony you roll over, and next second the crab-thing is greedying it on your chest,” Aldiss imagined. The thought had never occurred to a “little shrimp” like Claude that a dinosaur’s parasite “would be a good deal more dangerous than their host.” (And in the realm of science fact, in Parasite Rex, the science writer Carl Zimmer wondered if dinosaurs hosted tapeworms, and if so, how terrifying those parasites must have been. Big dinosaurs could have sustained gargantuan parasites. Thus far, no direct evidence of tyrannosaur tapeworms had ever been found, but it’s not out of the question.)
Sadly for B-movie fans, most dinosaur parasites were not actually as gigantic or as terrifying as we have feared. They weren’t so different from modern parasites. We know this, at least in part, thanks to dinosaur shit. Much like fossil tracks, dinosaur coprolites—or fossilized feces—don’t get the attention they deserve. You’re never going to see a blockbuster museum exhibit centered around the great piles deposited onto Mesozoic soil. But preserved dinosaur scat contains plenty of information worth sharing about dinosaur biology. Tyrannosaur coprolites have shown that these dinosaurs swallowed huge quantities of flesh and bone, and their digestive systems were so fast that they didn’t even completely digest the parts of other dinosaurs they ate. And the pats left behind by sauropod dinosaurs have helped paleontologists track the makeup of prehistoric ecologies, as well as the evolution of grasses and other plants. Furthermore, some organisms considered dinosaur crap to be both food and shelter—some well-preserved dinosaur turds contain snails that ate and lived inside the excrement. If you know what you’re looking for, you can find tiny, tiny parasites in coprolites, too.