My Beloved Brontosaurus

by Brian Switek

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  A few years before the Triceratops debate broke out, Goodwin and Horner proposed that two spiky, dome-headed dinosaurs—Dracorex and Stygimoloch—were really just younger versions of Pachycephalosaurus. All three lived in the same place at the same time, and evidence indicates that the cranial spikes of the younger ones were resorbed as the animals matured and a thick dome formed over the skull. Voilà!—three different dinosaurs filled out the life history of just one form. Similarly, the shovel-beaked hadrosaur Edmontosaurus is extremely common in the Late Cretaceous strata of Montana and the surrounding region, and we thought that this dinosaur lived alongside an even larger hadrosaur appropriately named Anatotitan. But according to Nicolás Campione and David Evans, Anatotitan was actually the bigger, more mature form of Edmontosaurus.

  Even the tyrant king himself figures into the swelling argument over how dinosaurs grew. In 1946, Charles Gilmore described a strange tyrannosaur head found in Montana. The skull was small, with a long and low profile, and Gilmore thought that he had found a new species of the dinosaur genus Gorgosaurus—a tyrannosaur that was a bit slimmer and more agile than T. rex. He named the animal Gorgosaurus lancensis. Four decades later, however, paleontologists proposed that the skull actually represented a “pygmy tyrant” that sprinted through the same haunts as its more famous relative. On the basis of the skull Gilmore collected, they rechristened the animal Nanotyrannus, and that choice has opened up a debate about how drastically tyrannosaurs changed as they grew.

  As with Triceratops, paleontologists regularly mistook young tyrannosaurs—leggy, gangly dinosaurs with shallow skulls that lacked the crushing capabilities of the adults—for distinct genera or species. In addition to Nanotyrannus, paleontologists named at least four different tiny tyrannosaurs, including the recently announced Raptorex, on the basis of small, often incomplete, specimens. All of these have been discarded. As Thomas Carr and other experts have discovered, the younger growth stages of some of the last tyrannosaur species often had long, low skulls that vaguely resembled earlier forms of tyrannosaur. As the animals grew, however, their skulls deepened, and the number of teeth in their mouths changed as their dental equipment transformed from slashing blades into crushing railroad spikes. “Some people become lobotomized around tyrannosaurs” and confuse characteristics of young tyrants for signs of new species, Carr told me, and our unfettered love for them has led us to overlook differences due to growth or variation so that we can crown a new tyrant species. As it stands now, there is only one tyrant dinosaur known from the latest Cretaceous of North America, and that is still our old faithful Tyrannosaurus rex.

  Young dinosaurs didn’t look exactly like their parents. This reconstruction of a juvenile Tyrannosaurus rex, on display at the Natural History Museum of Los Angeles, shows how much the little tyrants changed as they aged. (Photograph by the author)

  I’ve always found it intriguing that the most heavily debated cases of metamorphosis all concern Hell Creek Formation dinosaurs. Triceratops, Edmontosaurus, Pachycephalosaurus, and Tyrannosaurus were all among the last non-avian dinosaurs. This affects the debate over just how catastrophic the end-Cretaceous mass extinction really was. If Triceratops, Torosaurus, Dracorex, Stygimoloch, Pachycephalosaurus, Edmontosaurus, Anatotitan, Nanotyrannus, and Tyrannosaurus all coexisted, then dinosaurs maintained their diversity right up until the very end. But if all the proposed growth series are correct, this diversity is instantly cut in half, and the Late Cretaceous looks emptier.

  North America’s Late Cretaceous dinosaurs have been the most intensely scrutinized so far, but the argument affects all dinosaurs. Now that we have a better idea of how much dinosaurs changed, we are better equipped to spot infant dinosaurs in the field and in museum collections, too. Our newfound interest in nesting and growth isn’t just about mapping the trajectory of dinosaur lives. The narrative of how dinosaurs started life and how they changed after hatching incorporates various threads of dinosaur biology—from their social behavior to their physiology. In fact, the way dinosaurs grew up also has important implications for one of the most puzzling aspects of paleobiology: How did dinosaurs like Apatosaurus get so big?

  Five

  Jurassic Thunder

  From the time I scampered through the American Museum of Natural History’s fossil halls as a child, I wanted to find a way to visit the institution’s dinosaur warehouse. The mounted skeletons were great—some of the finest dinosaurs ever uncovered—but I desired more. I wanted to see the scores of dinosaurs that had never made it to the exhibit galleries. And the bigger the bones, the better.

  Public displays are where dinosaurs live. Reconstructed in bone, fiberglass, and plaster on painstakingly crafted metal armatures, the Mesozoic celebrities pose for their adoring public. But no museum has space to display everything it has obtained, or even the most fascinating fossils in its care. If museums put their entire fossil collections on exhibit, visitors would have to slog past rows upon rows of mammal teeth, turtle shells, and petrified bone fragments before reaching the relatively few dinosaurs complete enough to even consider mounting. Not to mention the fact that it’s hard to study the intricate details of a dinosaur’s anatomy when the skeleton is locked into a mounted pose. If we’re going to extract the secrets of dinosaur lives, paleontologists need easy access to the ever-increasing wealth of bones in safely kept museum back rooms. Display halls present the products of paleontological research, but the long rows of metal shelves heaped with fossils and drawers cradling delicate specimens provide the basis for much of what we have learned about prehistoric life.

  My dream finally came true on a crisp March evening in 2011. The AMNH was preparing to debut its World’s Largest Dinosaurs exhibition, and invited bloggers and Twitter addicts like myself to preview the show. The C train from Penn Station to the museum’s uptown stop couldn’t go fast enough. In addition to a sneak preview of the displays, the program promised a tour of the AMNH’s private fossil storerooms. After two decades of waiting, I’d finally get my chance to stroll, slack-jawed, through the institution’s cherished dinosaur storehouse.

  When I reached the meet-up in the fourth-floor Hall of Saurischian Dinosaurs, I wasted no time signing up for the earliest collections tour on the schedule. I was sure the Gorgosaurus and Apatosaurus on display would understand—they were old friends from long weekend afternoons in the museum’s galleries, and this was my chance to see their disassembled counterparts. Senior scientific assistant Carl Mehling met my tour group after a quick elevator ride down to the museum’s extensive basement. Carl was an amiable host, even when I made him a little nervous by leaning over a beautiful articulated skeleton of an oviraptorid, nesting chicken-style over a brood of eggs, to get a few close-ups of the dinosaur’s wishbone. The skeleton, part of a pair, was a gorgeous specimen from the Cretaceous of Mongolia. The dinosaur’s cream-colored bones popped out against the rusty orange of the surrounding rock.

  But most of the storeroom was home to sauropod bones. Individually cataloged and nestled on beds of foam, long strings of vertebrae were arranged along the shelves, as if the museum had been playing a game to see how many sauropods they could fit into a single room. This was paleontological Tetris. Each and every bone was a beautiful monument to evolution. On one shelf sat a single neck vertebra of Barosaurus—a particularly slender giant that reached over eighty feet in length. Compared to the short stack of bones that make up my own neck, the single dinosaur bone is a natural masterwork, beautifully sculpted into an elongated central shaft indented by air pockets and ornamented with short wings of bone.

  I even bumped into an old friend as I poked around the lower shelves. Tucked away among miscellaneous bones and casts was a copy of the museum’s outdated “Brontosaurus” head—a double of the tiny, grinning skull that seemed to be anchored so impossibly high above my head so many years ago. A worn, dusty card kept with the model confirmed that this was the skull of the “great amphibious dinosaur” modeled by the mus
eum’s preparation expert Adam Hermann a century before. Even though she was kicked off exhibit, I was glad to see the old girl again.

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  But there was another piece of dinosaur history I desperately wanted to see. Paleontologists have discovered many sauropods, but the AMNH was supposed to be the home of the biggest of the big, the ultimate in superlative dinosaur size. If I thought he could have helped, I would have asked Mehling if he could locate the fossil, but the bone I was looking for went missing right around the time that Hermann created that “Brontosaurus” skull. I was after Amphicoelias fragillimus—what may have been the largest creature ever to walk the Earth.

  If Amphicoelias was hiding among the crowded and dusty shelves, it wouldn’t have looked like much. Just a strange lump of bone, albeit one four and a half feet high. The long-lost fossil was a yet another Bone Wars dinosaur—much like Apatosaurus and Triceratops—but this creature was one of Edward Drinker Cope’s prizes. Like many dinosaurs first discovered in the nineteenth century, though, Amphicoelias has a complicated history.

  In 1878, when his competition with O. C. Marsh was at full intensity, Cope announced the discovery of what he believed to be an absolutely enormous dinosaur. He detailed the find in a four-page paper, “On Amphicoelias, a Genus of Saurian from the Dakota Epoch of Colorado,” and that brief missive is a perfect example of how quickly the dueling paleontologists created new dinosaurian identities on paltry remains. On the basis of just a few bones—including a femur, several tail vertebrae, and part of a hip—Cope thought he could distinguish two different species of Amphicoelias, and in a move sure to frustrate dyslexics, named them A. altus and A. latus. Both species were similar to previously discovered sauropods such as Apatosaurus, but they were considerably larger. The femur Cope assigned to A. altus was six feet, four inches long. This dinosaur was surely a giant among giants.

  Another Amphicoelias specimen was bigger still. In a dispatch published in August that same year, Cope wrote, “I have recently received from my indefatigable friend, Mr. O. W. Lucas, the almost entire neural arch of the vertebra of the largest saurian I have yet seen.” This was just a scrap—the uppermost portion of a single vertebra, once connected to the rounded centrum—but it was a big scrap. Cope believed this bone belonged in the middle part of the spinal column of a new species of Amphicoelias—A. fragillimus. When complete, Cope ventured, the bone might have been at least six feet tall, and a back-of-the-envelope calculation based upon the skeletons of other dinosaurs led Cope to believe that the animal’s femur would have been over twelve feet high. No dinosaur was bigger, and, in a bit of extra speculation, Cope cited the rough resemblance of the partial vertebra to the backbones of deep-sea fish as evidence that “these beasts may have walked in deep water and browsed on precipitous shores.” Though he based it on frustratingly fragmentary evidence, Cope could now claim the largest dinosaur in his battle with Marsh. Even the giants Diplodocus and “Brontosaurus”—rapidly described by Marsh within the following year—did not come close to the dimensions of Amphicoelias fragillimus.

  If Cope was correct, in fact, Amphicoelias fragillimus was the largest of all dinosaurs. (The fossils from A. altus and A. latus turned out to be Diplodocus bones, and so paleontologists recognize only A. fragillimus today.) Based upon the proportions of dinosaurs such as Apatosaurus and Barosaurus, the paleontologist Kenneth Carpenter estimated that A. fragillimus would have stretched an impressive 190 feet long—nearly twice as long as the next contender for prehistory’s longest dinosaur. If you stood at the tip of the dinosaur’s nose and set out at a leisurely walking pace, it would take over half a minute to reach the tip of its whiplash tail.

  The only evidence of the dinosaur’s existence disappeared decades ago. After Cope’s death in 1897, many of his fossils were acquired by his friend and pupil Henry Fairfield Osborn at the American Museum of Natural History. When Osborn and colleague Charles Mook sifted through Cope’s collection in 1921 in an effort to reanalyze some of the older finds, though, Amphicoelias was gone. Maybe the bone is lost among the shelves somewhere, perhaps someone destroyed the fossil, or the bone may have simply crumbled into scores of irreparable pieces in the days when preservation techniques were still crude. No one knows. And in 130 years of prospecting among the West’s Jurassic badlands, no fossil hunter has ever found another specimen. Even the original quarry is tapped out. When Carpenter tried to relocate the Amphicoelias site in 1994, there was no sign of the dinosaur anywhere. Most of the skeleton may have even eroded away by the time Lucas discovered the bonebed in 1877—the neural arch was all that was left of a true titan. How could such a gigantic dinosaur have so thoroughly disappeared?

  The same question could be asked of other giants. Our frustratingly incomplete inventory of the sauropods makes the question of who was the largest dinosaur of all time exceedingly difficult to resolve. No surprise, then, that a series of giant dinosaurs have tried to lay claim to the title of the biggest of the big.

  Before I knew about Amphicoelias—the dinosaur that got away—I was told that the eighty-five-foot-long, twenty-ton Brachiosaurus was as big as dinosaurs got. The dinosaur was so huge, a Time Life Young Readers Nature Library book told me, that Brachiosaurus must have been bound to deep Jurassic lakes. Only later would I learn that Brachiosaurus and kin were bound to the land. The air sacs that pervaded their skeletons made them surprisingly buoyant for their size, and if they had somehow managed to stand still up to their noses in water, the pressure would have fatally constricted their chests. Improbable though it seems, sauropods moved their bulk about on dry land.

  Not long after I learned about Brachiosaurus from outdated books, I heard about three recently discovered sauropods that have robbed the “arm lizard” of the Largest Dinosaur title. Two of them stand at the Museum of Ancient Life in Lehi, Utah. A twenty-minute drive south of Salt Lake City, the museum boasts an enormous collection of casts placed in eerie reconstructed environments where skeletal crocodiles tear at a fallen Stegosaurus and a pair of Tyrannosaurus snarl at each other over a kill. The skeletons don’t simply pose—they seem ready to step off their pedestals and start devouring guests as if they were in a direct-to-DVD horror flick. Not all the exhibits feature Mesozoic death, though. Other dinosaurs simply posture for their admiring public, and the largest of all are a pair of sauropods in the first dinosaur hall. Looming over an archway leading to the next exhibit hall is the dinosaur formerly known as Ultrasaurus. The Brigham Young University paleontologist Jim Jensen dug up a few parts of this dinosaur from Dry Mesa, Colorado, in the late 1970s. The sauropod was heralded as the largest ever before it was even properly described. When I tuned in to the Walter Cronkite–hosted documentary Dinosaur! in third grade, the show used some low-grade special effects to manifest the dinosaur at BYU’s football stadium, with fleeing cheerleaders for scale. Looking at the dinosaur’s fossils in the museum, there’s no doubt that it was enormous. Below the facsimile mount is the reconstructed shoulder blade that inspired Jensen’s hype about the dinosaur; the lone bone, riddled with cracks and breaks, was just as tall as Jensen was. And up against the far wall is another of Jensen’s discoveries from Dry Mesa: a 100-foot-long skeleton of Supersaurus. The huge dinosaur, similar to an overgrown Diplodocus, runs almost the entire length of the exhibit space.

  A reconstruction of Diplodocus carnegii. This 80-foot dinosaur was among the largest animals of all time, although there were a handful of giants that got even bigger. (Illustration by Scott Hartman)

  The paleontologist David Gillette added another contender in 1991—a slender sauropod said to extend 170 feet from nose to tail tip. This was Seismosaurus. I loved saying the name of this dinosaur and its gargantuan competitors. Ultrasaurus, Supersaurus, and Seismosaurus—the names alone deeply resonate when you say them, a portent of a giant’s arrival. But only Supersaurus remains today. These three dinosaurs—named from scrappy portions of skeletons—have a complicated and intertwined taxonomic history that is best left to
technical journals. Ultimately, three dinosaurs entered, and only one left. That big “Ultrasaurus” scapula came from a Brachiosaurus that was not as gigantic as Jensen had supposed, and Gillette’s Seismosaurus turned out to be a big Diplodocus that topped out at about 110 feet, given a substantial boost by the very long tail typical of the genus. Only Supersaurus has survived the revisions and nomenclatural shifts.

  The problem is that we don’t know exactly how big the largest dinosaurs grew to be. Supersaurus and Gillette’s big Diplodocus are top contenders, but as yet no one knows how large these dinosaurs really were. Even the dinosaur that paleontologists most often cite as the biggest of all—Argentinosaurus, thought to have been about 100 to 120 feet long—is incompletely known. Paleontologists can only estimate the size of the most gargantuan dinosaurs based upon what we know about their better-known relatives. The largest dinosaurs have been named on some of the most fragmentary material. This dearth of complete skeletons makes perfect sense. Small, delicate dinosaurs can quickly and easily be covered in sediment. A little bit of saturated mud or wet sand is all that’s needed. But a 100-foot-long, 60-plus-ton behemoth is another matter. A local flood or other natural disaster capable of moving massive quantities of sediment would be needed to entomb such an animal, and that’s to say nothing of the scavengers which undoubtedly attended the burial of the biggest dinosaurs. (The death of one enormous dinosaur enriched the lives of many carnivores.) There were many more giant sauropods than ever found their way into the fossil record, and even among those left behind as fossils, few died in circumstances amenable to high-quality preservation. If a nearly complete skeleton of Supersaurus, Argentinosaurus, or even Amphicoelias exists, no one has found it yet.