My Beloved Brontosaurus

by Brian Switek

  The minimum number of animals—estimated from collected bones—is fifty-seven, but there were undoubtedly many, many more. Based on the sheer number of bones in the quarry, there may have been hundreds of individuals. And while most of the bones that paleontologists have discovered are attributable to adult animals, sub-adult and juvenile Centrosaurus were among the casualties, too. The sheer number of fragments and skeletal scraps outlines what must have been one of the most breathtaking sights in prehistory: scores of Centrosaurus walking together. There are simply too many animals in one place to deny that the dinosaurs moved as a herd. The site is not a secret Centrosaurus graveyard, where animals had slunk off to die over years and years. It’s a snapshot of a local disaster.

  The bonebed is only where the dinosaurs were buried, and the evidence hints that they died a short distance away. The most popular hypothesis imagines the herd of Centrosaurus trying to cross a river, much as caribou, wildebeest, and other herbivorous mammals do today. But something went wrong. Maybe the waters were too high or the animals began to panic, but whatever the immediate cause, much of the herd drowned. The Centrosaurus weren’t buried right away. Their rotting bodies must have bobbed along and been tossed up onto the riverbank, and there was too much meat for scavengers to dispose of. The local tyrannosaurs, Gorgosaurus, ate their fill of the carcasses and left shed teeth behind as evidence, and small tooth marks on some Centrosaurus bones put small mammals at the bonanza, too. Eventually, floodwaters rose and washed the bones of the dinosaurs together, rolling and tumbling them along the river bottom until they arrived at their final resting place. This bonebed didn’t come together over many seasons, and can’t be construed as a group of dinosaurs squabbling over the same food source. The site records a quick death for a massive number of animals that were moving together—a fleeting vision of how dinosaurs lived and died together.

  Bonebeds are one line of evidence that paleontologists draw on as they examine dinosaur sociality. Some of the best clues about dinosaur lives come from a different set of clues: footprints. Admittedly, they’re not quite as sexy as dinosaur skeletons. I remember how disappointed I was as a child when I saw tracks left by dinosaurs that once roamed New Jersey. My parents had escorted me to the “Dinosaur Den” of the nearby Morris Museum. The small, dark hallway featured a big Stegosaurus—an armored giant with no connection to my home state whatsoever—and a few casts, but the only authentic local fossils were three-toed tracks left in reddish stone slabs. They just looked like big bird prints—not altogether surprising, since, before the discovery of dinosaurs, such footprints in the Connecticut Valley were colloquially referred to as “turkey tracks”; the Amherst geologist Edward Hitchcock thought they were traces of moa-like birds. The footprints didn’t speak to me. I wanted real dinosaur skeletons—to see the animals themselves and imagine their fearful powers.

  I didn’t understand what dinosaur footprints really were: fossilized behavior. Think of a horned dinosaur—let’s say the highly ornamented Styracosaurus—walking across the muddy shore of a prehistoric lake. As the spiky herbivore trundles along, each foot leaves an impression in the firm mud, recording—in detail—how the dinosaur actually moves. Of course, paleontologists don’t always know which dinosaur made a specific track. The quirks of fossil preservation tend to preserve footprints in some places and skeletons in others, and unless you find a dinosaur literally dead in its tracks, you can offer only a range of possible candidates. The problem is called the Cinderella Syndrome—matching the right foot with the right fossil “slipper” is really tough to do. Still, dinosaur tracks are distinctive enough that paleontologists can narrow down the range of candidates. Scores of sites all over the world have yielded dinosaur highways that reveal patterns demonstrating that they walked, ran, and sometimes traveled together. Some of the tracks found in the Connecticut Valley go in the same direction in parallel—showing that several dinosaurs walked next to each other.

  The Connecticut Valley tracks were created by small and medium-sized dinosaurs, most of them theropods. Rare trackways elsewhere show that even huge dinosaurs sometimes ambled along together. The paleontologist Roland T. Bird brought the site to the attention of his colleagues, even if he didn’t discover it himself. People who lived in and around Glen Rose, Texas, already knew about the dinosaur tracks when Bird rolled into town in 1938. In fact, rumors of huge dinosaur tracks were what had drawn Bird—who was looking for a set of tracks to lay behind the American Museum of Natural History’s “Brontosaurus” skeleton—to the vicinity of the Paluxy River in the first place, and there was already a cottage industry based on excavating dinosaur tracks for garden ornaments, of all things. And of course there were forgers, too, some of whom got a kick out of making human-like footprints in the same rock to convince the gullible that people and dinosaurs once walked together.

  There was no shortage of footprints around the Paluxy. Some seemed to record the pathways of lone animals, while a special few sites were pockmarked by the movements of multiple dinosaurs. On his maiden voyage to the site, Bird headed in to investigate a tip about some carnivorous-dinosaur tracks on the Davenport family ranch. He found that there were indeed tracks left by a large predator—possibly the ridge-backed relative of Allosaurus named Acrocanthosaurus—but there was also a field of fossil potholes created by sauropods. Bird considered excavating these, but as he wrote in his journal at the time, “Mrs. Davenport proved to be as big a problem as any in the field.” She didn’t like the idea of an East Coast scientist swooping in to take tracks off her land. Bird wasn’t the first person to ask about them—local track collectors had been pestering the Davenports—and Mrs. Davenport was adamant that the tracks would stay put. “No one had ever removed any tracks from Davenport property to her knowledge,” Bird wrote, “and not even the prestigious American Museum of Natural History of New York was about to do so now or at any future date. Any charm exuded by the said museum’s field representatives was as so much sweetness and fevered breath wasted on the desert air.” Ultimately, Bird appealed to Davenport’s sense of curiosity about what sort of animal actually made the tracks to get permission to dig on her land, but that was all: he didn’t get the OK to bring any treasures back to New York City.

  Bird and his assistants were eager to get to work nonetheless, and they peeled back layer after layer of limestone over an area where a set of sauropod tracks disappeared under the surface. And to his astonishment, there was a wide swath of dinosaur tracks that had been hidden under the top layers. “[T]he more we found,” Bird wrote, “the more there was left to be found … [T]here seemed to be no end to the number nor to the length of the sauropod parade.” When the site was fully uncovered, Bird couldn’t believe his eyes:

  Here was not a single sauropod trail as I had found on the Paluxy; here a herd of giants had stampeded, or moved, as a single entity. I tried for an accurate count, but it couldn’t be done. There were to begin with prints of seven individuals, in a twenty-foot space, but a few feet beyond this well-trodden area the tracks broke down into a hodge-podge. One fact stood out above all; they moved in the same direction, and presumably at the same time.

  Not all the footprints were the same size, either. The big and little prints told Bird that teens and adults traveled together. And, since these huge dinosaurs were thought to be swamp-dwelling sluggards at the time, Bird took the general absence of tail impressions to mean that these dinosaurs had been sloshing through shallow water. One set of tracks, in Bird’s estimation, had a long drag behind it, and he wondered if this individual was extra large, tired, or “supposed to sign off with his tail.” (Bird wrote, “Perhaps scientists of future ages may be able to settle” the question of why dinosaur trackways were not associated with tail drags, as they should have been if anatomical restorations of the time were correct. Bird was merely working within the milieu of his time, and the mystery isn’t a mystery at all—dinosaurs did not drag their tails. Bird’s observation that “most of my dinosaurs run high-t
ailed” was a reflection of a reality not recognized at the time he uncovered the tracks.)

  The dense accumulation of footprints at the Davenport ranch continued to fascinate paleontologists long after Bird. All those sauropod tracks moving in the same direction were solid evidence that some of these animals traveled in herds. One idea, suggested by the paleontologist Robert Bakker, is that the little ones plodded along in the middle of the group so they would be protected from predators.

  But sauropods may not have been such attentive and aware parents. The problem with trackways is that multiple animals, even moving in a single group, often walk over each other’s tracks and obscure the trail. In the case of the Davenport herd, composed of at least twenty-three dinosaurs, the little tracks overlap with the big ones, and, according to the track expert Martin Lockley, show that there was no special arrangement to protect the vulnerable young dinosaurs. The smaller dinosaurs simply followed the larger ones, unprotected. The dinosaurs passed through some kind of bottleneck, and so the herd passed through the narrow space in a line rather than as a spread-out group.

  Whether sauropods in general formed herds is impossible to say. While these dinosaurs all shared the same body plan, the group was diverse, disparate, and long-lived. It would be silly to assume that such a varied group of animals all behaved in the same way; some were probably solitary, while others plodded en masse. Trackways, like the ones in Texas, indicate that some sauropods formed groups at least some of the time, but the precise details of their social structure are almost entirely unknown to us. We know some sauropods were gregarious, but that’s almost all the rock record has told us so far.

  Trackways are the most direct evidence we have of dinosaur social lives. Even though the idea that Deinonychus and other raptors were pack hunters was originally based on flimsy evidence, for example, unique trackways show that deinonychosaurs occasionally traveled together. The peculiar feet of these dinosaurs are the key to their identification. Most theropod dinosaurs supported themselves on three-toed feet, but deinonychosaurs stood on only two, like modern ostriches. Their second toe—the one closest to the midline of the foot—supported their big, retractable claw and was lifted off the ground, creating a very distinctive type of footprint. One trackway made by these dinosaurs records the movement of several individuals in the same direction, spaced about a body width apart—good evidence that this was a social group. And another trackway found in Niger shows where one raptor moved and altered the path of another dinosaur—a subtle moment of early Cretaceous time that gives us another clue that these dinosaurs interacted with each other. Visions of highly organized, socially stratified raptor packs are the products of sensational speculation rather than scientific fact, but at least some forms of these sharp-clawed predators strutted together.

  Evidence from trackways and bonebeds undermines the classic image of dinosaurs that I absorbed when I was a child. The dinosaurs I initially encountered were always grouchy-looking, solitary animals—Stegosaurus and “Brontosaurus” browsed for soft plants on their own, confident that their respective armor plates and giant size would save them, while carnivores like Allosaurus and Tyrannosaurus were rogues, always lurking behind the next tree. In the books, movies, and museum displays I grew up with, only Triceratops and Deinonychus were regularly shown in groups, for defense and to better subdue meals, respectively.

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  Dinosaur Provincial Park’s Centrosaurus bonebed, the Paluxy tracks, and other sites show that many dinosaurs were gregarious. But not all dinosaur social interactions were friendly. We love to bring dinosaurs back to life to see them tear each other to pieces. From the very beginning, paleontologists have been unable to describe dinosaurs without pondering their impressive powers of attack and defense. In his 1838 treatise on the manifestation of God’s power in nature, naturalist William Buckland cast the formidable Megalosaurus—which he viewed as an enormous carnivorous lizard—as an intelligently designed killing machine. In fact, Buckland proclaimed, Megalosaurus was so well suited to killing that the efficiency of the predator actually “tend[ed] materially to diminish the aggregate amount of animal suffering. The provision of teeth and jaws, adapted to effect the work of death most speedily, is highly subsidiary to the accomplishment of this desirable end.”

  Even herbivores were restored as vicious brutes. In one of the earliest dinosaur paintings—composed by the celebrated painter of biblical catastrophes John Martin—a serpent-like Iguanodon opens a mouth full of jagged teeth to bite an attacking Megalosaurus in protest. The art looks less like a scientific restoration than like a medieval vision of wyrms or wyverns that have ensnared each other, millions of years before there was a St. George to slay them.

  While not quite as apocalyptic as what Martin envisioned, a single bone at the Utah State University Eastern Prehistoric Museum records a traumatic battle between Allosaurus and Stegosaurus. The small museum squats along the main drag in Price, not far from the Cleveland-Lloyd quarry. A high-kicking, featherless Utahraptor in bronze greets visitors entering the parking lot, and a skeletal reconstruction of the same dinosaur mimics the ninjutsu pose inside the museum’s foyer. The last time I visited, after my failed first attempt to visit the nearby dinosaur boneyard, many of the other skeletons were off display, scheduled to have their tails lifted and spines readjusted to fit the twenty-first-century dinosaur bauplan. But there was still plenty to see in the gallery, including an Allosaurus vertebra with a Stegosaurus spike driven through it.

  The fossil wasn’t actually found like this. The original Allosaurus tailbone—uncovered at the Cleveland-Lloyd bonebed—had a weird, C-shaped puncture along one side. According to the museum director Kenneth Carpenter and his colleagues, a Stegosaurus spike fits snugly in the wound. (Inspired by one of Gary Larson’s The Far Side cartoons, Carpenter has also proposed that a stegosaur’s array of tail spikes should be called a “thagomizer.”) The pathology is a vestige of a Jurassic fight, and the Stegosaurus even left part of itself behind. The Allosaurus bone didn’t heal properly—a clue that the tip of the Stegosaurus spike broke off and was left embedded inside the carnivore’s body. This might have been a frequent hazard for both the attackers of Stegosaurus and the spiky dinosaur itself. About 10 percent of Stegosaurus tail spikes show healed breaks at their tips. Most often, the researchers suspected, Stegosaurus spikes would have slashed open long wounds on the sides of would-be predators, but, if the carnivore approached from a particular angle, the spikes would have been more likely to break off and lodge inside the assailant’s body. Even though the actual event is lost to prehistory, the fact that Allosaurus and Stegosaurus faced off is preserved in bone.

  Members of the same dinosaur species fought each other, too. Healed skull wounds, like those on the young Tyrannosaurus “Jane” at the Burpee Museum of Natural History in Illinois, show that tyrannosaurs fought by biting each other right on the face. Nor was intraspecies bickering the sole province of carnivores. Triceratops bore signs of their battles on their skulls.

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  While passing through Claremont, California, in the fall of 2011, I stopped by the Raymond M. Alf Museum of Paleontology to visit my friend, the paleontologist Andy Farke. The main reason for my visit was to see the museum’s new paleontology hall, and I also wanted to check out one of Farke’s Triceratops models. A few years before, Farke had used a pair of dinosaur sculptures to visualize how the famous ceratopsians locked horns. As he led me to his office, I asked Farke if I could see the polyresin sources of his inspiration. He gladly handed the model to me from its spot on the shelf: a detailed sculpture of a Triceratops skull roughly 15 percent of actual size. Matching it with an identical model, Farke staged a mock battle to show how Triceratops might have fought.

  As explained in countless dinosaur tomes, Triceratops had two long horns over the eyes, a short horn on the nose, and a broad, solid frill (or, if the “Toroceratops” hypothesis is upheld, at least solid for most of its life). To generations of paleontolog
ists, as well as to kids in sandboxes in New Jersey, these adornments looked like weapons honed for combat. When the Yale University paleontologist Richard Swann Lull described a pathological specimen of this dinosaur—which might be called Nedoceratops if it turns out to be a distinct genus—he wrote, “the supraorbital horns are the sole aggressive weapons while the widely expanded frill served admirably to withstand the shock of the adversary’s horns. We have here a precise analogy with the knight of old tilting with his spear and shield.” Whether goring a Tyrannosaurus or fending off a rival, Triceratops surely used the weapons to jab, block, and parry.

  No one actually investigated the defensive abilities of Triceratops in detail. Its defensive prowess seemed self-evident. But Farke wondered how the dinosaurs would have jousted. When he manipulated the two models to see how Triceratops fought, Farke found only a few possible horn-lock positions. Triceratops could have angled their heads so that only one brow horn of each individual hooked around the other’s; they could have tilted their heads even further down so that both brow horns locked around each other’s; or they could have offset their heads to the side so that their brow horns locked and their nasal horn jutted up into the frill.

  Farke wasn’t just fooling around with toys. By determining the range of horn-locking positions, he could look to actual Triceratops skulls for signs of combat. If the dinosaurs were fighting in the ways Farke predicted, then the parts of their skulls the horns scraped against should have been damaged. Farke followed up on his proposal with colleagues Ewan Wolff and Darren Tanke in a 2009 paper, “Evidence of Combat in Triceratops.” The paleontologists looked at the different parts of the skull to see if there was any clear sign of regular combat. As anticipated by Farke’s models, the lower bones on the outside of the Triceratops skull—the squamosal and the jugal, or cheekbone—had the highest incidence of injury.