by Brian Switek
Dinosaurs may have heard the prehistoric relatives of alligators and crocodiles make noises like this in prehistoric swamps—just imagine the sounds a forty-foot alligatoroid like Deinosuchus would have been capable of—but whether they made similar sounds is speculative at best. Tempting as it is, we can’t really take a handful of birds and crocodylians and extrapolate the sounds made by those animals to a vast range of different dinosaurs. That alligators and crocodiles vocalize is a clue that dinosaurs could have called to each other, but we can only try to reconstruct dinosaur sounds from the little evidence we have.
Obviously, we would need to know a lot more about dinosaur soft tissues—the anatomy of their throats, especially—to reconstruct the sounds they made. And, fortuitously, there are some delicate skeletal clues that can help us determine how dinosaurs made themselves heard. At least one group of dinosaurs exhibit clues to their sonic abilities in their elegant headgear. Parasaurolophus was one such Mesozoic musician. From neck to tail, this hadrosaur wasn’t that different from its close relatives. The body of this dinosaur is just about as average as you can get—Parasaurolophus had the short arms, long legs, and deep tail that were typical of hadrosaurs. What immediately sets Parasaurolophus apart is its gorgeous skull. Jutting out from the back of the dinosaur’s cranium is a slightly curved, tubular crest. The development of the crest differs from one species to the next. Parasaurolophus cyrtocristatus from Late Cretaceous New Mexico and Utah had a shorter crest than its slightly later relatives P. tubicen from New Mexico and P. walkeri from Alberta, Canada. Nevertheless, all three share similar, instantly recognizable adornments. The beauty of the Parasaurolophus ornamentation isn’t just the adornment’s outward appearance. Having a big tube sticking out of your skull is one way to get attention, but it’s the internal anatomy of the dinosaur’s crest that held the real communication secret.
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I was fortunate enough to see the feature firsthand when I visited one of Utah’s vast southern badlands on a sunny May morning in 2010. Grand Staircase–Escalante National Monument is a gorgeous, isolated swath of southern Utah desert. During my trip to the park’s Cretaceous exposures, the paved road rapidly gave way to a well-groomed dirt track that changed colors with the rocky, sage-filled landscape on the route to the pastel yellow monument dubbed Grosvenor Arch. I wished I had the chance to enjoy the rest of the park’s scenery beyond that point. The flattened trail quickly turned to a long one-lane runnel studded with rocks and pocked by potholes as the track—a road in name only—oscillated up and down on the way up to the Kaiparowits Plateau.
I tried to calm myself as my car rattled and jolted over “The Cockscomb” and down “The Gut.” I had spent my entire life in the well-paved eastern states, and the roughest roads I had typically encountered were gravel parking lots. I kept a death grip on the steering wheel, stared straight ahead, and tried not to imagine the conversations with my insurance agent about what had happened to the rental car I was driving. (“Now, what were you doing at the time of the incident?”) The last stretch before I hit the University of Utah camp on Horse Mountain was the worst—the last jog of dirt road looked like something out of an Indiana Jones movie. A few small chunks of road had fallen away along the curve, a definite reminder to aim very carefully. Imagine my delight when, the morning after my arrival at camp, the experienced fossil finder Scott Richardson had me take the same route in the other direction to get down to a very special fossil out in the monument.
The hollow crests of some hadrosaurs—such as this Parasaurolophus—may have allowed these dinosaurs to bellow a range of low notes over long distances. (Photograph by the author at the Natural History Museum of Utah)
The exposures in the BLM-managed park aren’t quite what you’d expect for the West. Parts of the Kaiparowits Formation—a roughly 75-million-year-old stack of rocks formed when southern Utah was a coastal swamp near a shallow seaway that cleft North America in two—are covered by shrubs and short trees sunk into sandy soil. The dinosaurs in these shrub-covered sections of the formation don’t so much tumble out of naked rock as they seem to roll out of the dirt. After Scott and I parked our cars and started out to where the Parasaurolophus was, Richardson pointed out a solitary dinosaur toe bone resting in the soil. The chocolate-colored bone had simply fallen out of the encasing sediment, though Richardson couldn’t touch it. There are strict and specific BLM rules about collecting fossils in this area, and the proper permits had to be arranged before plucking up so much as a fragment of bone. The dinosaur toe would have to wait.
When we finally arrived where the Parasaurolophus rested, I didn’t immediately understand what I was looking at. All I could see was a strange dark patch on a huge block of gray rock. The exposed part of the fossil looked a little bit like the cartoon illustrations of chromosomes I used to cut out for oversimplified class projects in Biology 101: inside a rind of tan bone were two pairs of oblong blobs, the bottom blobs longer than the top ones. I suddenly realized I had seen this dinosaur shape before, and Richardson confirmed my guess. I was looking at a cross section through the crest of Parasaurolophus, looking into the hollow tubes that ran through the skull ornament.
As I found out when I later looked up the history of this hadrosaur, the Canadian paleontologist William Parks saw something similar when he described Parasaurolophus walkeri in 1922. While the dinosaur’s elongated crest looked solid enough on the outside, a fortuitous break showed that the structure was actually hollow. Thin partitions of bone separated a pair of tubes that ran from the nose to the back of the skull and down into the mouth—a super-long nasal passage unlike any seen before.
Why a dinosaur would need such an elaborate set of skull plumbing was a mystery. Even more perplexing was the fact that closely related hadrosaurs had similar arrangements, but contained within different crest shapes. In the greater hadrosaur group, there are three main branches: the hadrosaurines, the saurolophines, and the lambeosaurines. The first group, which includes dinosaurs such as Hadrosaurus from my former home state of New Jersey, were unornamented forms; and the saurolophines—such as the seemingly ubiquitous Edmontosaurus and the “caring mother” dinosaur Maiasaura—either lacked flashy ornaments or had relatively simply bumps and crests on their heads. The lambeosaurines had more charismatic headgear. While similar in body to their comparatively plain relatives, these dinosaurs typically had elaborate crests of various shapes. In addition to the tube-headed Parasaurolophus, other classic members of this group include Corythosaurus (a dinosaur with a domed crest) and Lambeosaurus (a form with a large L-shaped crest). In each genus and species, the nasal passage invades the crest to create a circuitous pathway from nose to mouth.
When I was a kid, the outdated books in my school library and a cheap dinosaur album my parents bought me at a local Hallmark store were unanimous about the crest’s function. The ornate headgear was obviously used as a dinosaurian aqualung that allowed Parasaurolophus and other lambeosaurines to spend most of their time underwater. In his children’s book Dinosaurs and Other Prehistoric Reptiles, artist Rudolph Zallinger painted a Parasaurolophus and Corythosaurus synchronized-swimming under the surface of a Cretaceous lake, and a sticker book created by the Italian company Panini depicted a whole herd of Parasaurolophus grazing underwater. This made perfect sense within the completely wrong supposition that these dinosaurs were amphibious herbivores. After all, the hadrosaurs had been named “duck-billed” dinosaurs—a name that has stuck despite its inaccuracy. Well-preserved hadrosaur specimens have shown that these dinosaurs had expanded beaks that looked like a pair of interlocking vertical shovels. And certain hadrosaur specimens with intact skin impressions were misinterpreted to cast the dinosaurs as dedicated swamp dwellers. In 1912, the paleontologist Henry Fairfield Osborn described an exceptional Edmontosaurus specimen that had been discovered by Charles H. Sternberg, with preserved skin impressions over most of its body. It appeared as though the dinosaur’s hand was enclosed within a fleshy mitt. Osborn b
elieved that this was a kind of webbing that would have helped the dinosaur scull around Cretaceous lakes and rivers. Actually, the condition was a common hadrosaur feature that helped keep the “fingers” together in a single unit for support while walking on all fours.
Before we knew the truth about hadrosaurs, an aquatic mode of life seemed like the perfect explanation for the weird crests of some species. While Parks was uncertain about the function of the Parasaurolophus crest—maybe the feature acted as some kind of visual signal?—in 1933 Alfred Sherwood Romer suggested two possible hypotheses. Maybe the crests acted as air tanks, or perhaps they allowed the dinosaurs to snorkel. The fact that there were no external crest openings for the dinosaurs to breathe through sank the snorkel idea, but, as I would learn so many years later, the air tank idea had popular appeal.
Not that all paleontologists agreed with Romer’s notion, but other experts still envisioned the crests as having something to do with an aquatic setting. Charles Mortram Sternberg—one of the sons of the famous fossil collector Charles H.—suspected that the U-shaped bend in what he called “hooded hadrosaurs” prevented water from entering the respiratory system of dinosaurs like Lamebosaurus when they dunked their heads underwater to feed. The German paleontologist Martin Wilfarth had a more fanciful idea. The expanded areas of bone were anchors for trunks, he proposed, which would have allowed Corythosaurus and kin to reach up to the surface for a breath of air when needed.
Even John Ostrom, who criticized and tossed out the other ideas, still viewed the expanded nasal passages within a watery milieu. “It is quite probable that hadrosaurs lived a rather passive, perhaps even retiring existence,” he wrote, and pointed out that “[t]hey possessed no horns, no claws, no sharp teeth, they carried no clubbed or spiked tail, and they had no bony armor. They certainly were not constructed for rapid flight and they cannot be considered giants for their time.” What was a defenseless hadrosaur to do?
Ostrom thought their nasal extensions might be the solution. The passageways could have increased their ability to smell predators from a long way off and escape into the water when necessary. This idea didn’t catch on either. There was no indication that the part of the hadrosaur nose that detected smells stretched much beyond the area where the nostrils met the outside world; furthermore, the need to smell predators would predict that hadrosaur crests would have a consistent shape that would optimize the dinosaurs’ sniffing ability. The fact that hadrosaur crests were so diverse hinted at a different explanation.
As it turned out, Parks had been on the right track. When James Hopson reviewed the various theories about hadrosaur crests in 1975, he concluded that the structures were visual signals. The main purpose of a crest was simply to be seen. But Hopson cautioned that this scenario did not preclude other explanations. The long, circuitous passageways could have acted as resonating chambers that would have allowed dinosaurs like Parasaurolophus to bellow to each other over long distances. Years before, when the Swedish paleontologist Carl Wiman described the species Parasaurolophus tubicen, he remarked that the dinosaur’s crest resembled a musical instrument called a crumhorn. I truly hope Parasaurolophus sounded more impressive than the woodwind instrument, which reminds me of a fancy kazoo. If Parasaurolophus, a dinosaur more than thirty feet long that weighed nearly three tons, sounded so wimpy, it must have been one of the most unintentionally hilarious creatures of all time.
Fortunately, thanks to researchers who have investigated the acoustic abilities of Parasaurolophus, we know that the dinosaurs didn’t sound so ridiculous. The Johns Hopkins University paleontologist David Weishampel used an improvised model of a Parasaurolophus crest to explore the dinosaur’s range. I remember seeing Weishampel toot his dinosaurian horn in more than one documentary when I was a kid. Painted with green and orange stripes, the instrument was simply a U-shaped horn made out of PVC pipe roughly the length of a Parasaurolophus crest. The sound was like a foghorn—a deep, booming call that would have carried far over the Cretaceous bayous where this dinosaur lived. Watching the clips over again now, the demonstration reminds me of one of Ray Bradbury’s short stories. In “The Foghorn,” Bradbury imagined a lonely prehistoric monster rising from the sea upon hearing a foghorn—a sound it had mistaken for the voice of one of its own. Alas, the sound of Weishampel’s hadrosaur tuba has never drawn out any hiding dinosaurs.
Of course, dinosaurs weren’t creatures of paint and PVC. Hadrosaur crests were complex structures of bone and soft tissues connected to the biological rhythms of a living animal. The models can be only vague approximations of how dinosaurs sounded. But there are subtle clues that hadrosaurs made such calls, and that their repertoire even changed as they grew. The evidence is in their ears.
Weishampel got some documentary airtime thanks to his own instrument, but he also wrote several papers on the acoustic properties of hadrosaur crests. From a musical perspective, he determined, the well-ornamented Parausaurolophus walkeri had a range from G two octaves below middle C to B below middle C, while the blunter-crested Parausarolophus cyrtocristatus was capable of sounds from D one octave below middle C to F# above middle C. And, as Weishamphel outlined in a 1981 Paleobiology paper, the delicate ear anatomy of hadrosaurs such as Corythosaurus seemed to be consistent with the idea that these dinosaurs were sensitive to a wide range of low-frequency sounds of the sort their crests could have made.
This applied only to the adult animals. Juvenile hadrosaurs, Weishampel hypothesized, lacked the well-developed crests of the adults and may have chirped like baby alligators do. There was a good reason for this. The calls of the little dinosaurs were high-pitched so that their squeaks didn’t travel too far and garner unwanted attention from predators. As the dinosaurs grew, however, communication with other members of the same species—particularly potential mates—became especially important, and talking long distance is best accomplished through low-frequency sounds. Modern African elephants are a rough parallel here. The rumbles of these mammals allow the modern behemoths to chat from afar.
Weishampel based his hypothesis on previous studies of hadrosaur ears and the anatomy of modern birds and crocodiles. For the most part, paleontologists didn’t have much to go on when it came to dinosaur ears. Well-preserved hadrosaur skulls were rare and valuable, so destroying part of a skull to look inside the ear was not acceptable; and while paleontologists had begun employing CT scanners that allowed a look inside at dinosaur skulls, they were expensive to use. It has been only recently, as we develop more accessible and higher-resolution scanning technologies, that we’ve been able to tell what hadrosaurs were actually capable of hearing.
David Evans is one of the paleontologists who picked up Weishampel’s work on hadrosaur sound. In a 2006 Paleobiology study he concluded that display, sound, and possible physiological benefits shaped different hadrosaur crests, and in 2009 Evans teamed up with colleagues Ryan Ridgely and Lawrence Witmer to investigate how the development of the dinosaurs’ crests, nasal passages, and brains related to each other. After creating three-dimensional scans of skulls representing different age stages of Lambeosaurus, Corythosaurus, and another dome-crested form called Hypacrosaurus, the paleontologists tracked the nasal passages and investigated the anatomy of the brain and inner ear.
The skulls of these dinosaurs held some surprises. In a specimen of Hypacrosaurus altispinus, the researchers found that the dinosaur’s nasal passage looped around in a very elaborate path. As air entered the dinosaur’s nose, it traveled up to about the level of the eyes before dropping back down, twisting, rising back up, peaking high in the crest, and looping back downward into the throat. In a juvenile Lambeosaurus, by contrast, the nasal passage followed a simpler path in which it swerved briefly at a portion called the S-loop before rising up into the young dinosaur’s underdeveloped crest and coming back down into the throat. In Hypacrosaurus, the nasal passage was far more circuitous than was apparent from the dinosaur’s rather plain crest. Looking at the outside of the skull, you woul
dn’t guess that it contained such complicated plumbing. Whatever function the nasal passages had, they must have evolved for different reasons than the crest. The outside form of the crest was probably modified as a biological signpost, and the internal nasal passages were adapted according to different pressures relating to sound and physiology.
We can deduce why a dinosaur might have such complicated nasal plumbing by looking inside its braincase. Paleontologists can create detailed virtual models of dinosaur brains by scanning the vacant brain cavities inside fossil skulls. The various lumps and bumps outline how important certain senses were to a dinosaur. Likewise, high-definition scans of dinosaurs’ inner ears can help us understand what these dinosaurs could actually hear.