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A reconstructed Acrocanthosaurus at the North Carolina Museum of Natural Sciences. Photo by the author.

Compared to mounted dinosaur skeletons, fossil footprints might seem like mundane objects. They only record one small part of a fantastic creature, and it is harder to envision a whole dinosaur from the ground up than the wrap flesh around a skeletal frame. But we should not forget that dinosaur footprints are fossilized behavior—stone snapshots of an animal’s life. And sometimes, trackways record dramatic moments in dinosaur lives.

In 1938, American Museum of Natural History paleontologist Roland T. Bird traveled to Glen Rose, Texas to investigate rumors of huge dinosaur tracks found in the vicinity of the Paluxy River. Bird found them in abundance, but one site was especially intriguing. Set in 113-million-year-old rock were the footprints of a huge sauropod dinosaur—and it seemed that the long-necked giant was followed. The large, three-toed footprints of a predatory dinosaur, probably the ridge-backed Acrocanthosaurus or a similar theropod, paralleled and eventually converged on the footsteps of the sauropod. And at the point of overlap, the predator seemed to skip a step—a little hop that Bird took to mean that the carnivore had sunk its teeth into the herbivore and was lifted out of its tracks a short distance.

Bird excavated the trackway in 1940. About half of the long trail went to the AMNH and can now be seen stretching out behind the museum’s Apatosaurus mount, despite the fact that Apatosaurus lived millions of years before the tracks were made. The other portion is housed at the Texas Memorial Museum in Austin. Bird’s hypothesis about how the tracks were made has inspired exhibits at other museums, such as the Maryland Science Center and the North Carolina Museum of Natural Sciences. Yet not everyone agrees about what the tracks represent. Do they record an Acrocanthosaurus attack as it happened? Or could the trackway simply be a fortuitous association of tracks from dinosaurs that walked the same ground at different times?

Artist David Thomas and paleontologist James Farlow went back to Bird’s notes and the trackway evidence to reconstruct what might have transpired. The association between the sauropod and theropod tracks seemed too tight to just be coincidence. The predatory dinosaur very closely followed the pathway of the larger herbivore, both moving along a broad left curve. Near the end of the excavated area, both the theropod and sauropod turned abruptly to the right. If the two dinosaurs had passed at different times, then we’d expect that the sauropod or theropod would have continued on in the same trajectory and crossed another set of tracks preserved nearby. Based on the fully reconstructed image, the sauropod and theropod were interacting with each other.

And there’s something else. Just before the enigmatic double-right-footprints made by the theropod, there is a drag mark made by the sauropod’s right hind foot. This might be where the titan was attacked and faltered, or maybe the sauropod threw its weight to avoid being bitten. Frustratingly, we can’t know for sure. And the missing left theropod footprint isn’t a clear sign of an attack, either—all we know is that there’s a missing track right where the animals were in close proximity.

Whether or not the Paluxy River Trackway records a successful Acrocanthosaurus assault is uncertain. But the tight connection between the theropod and sauropod tracks suggests that the carnivore at least stalked the herbivore, and perhaps even took a swipe at it. Specimens like this test our ability to draw brief moments in time from stone. The task is made all the more complicated by the gradual loss of information contained within the rock. While they look sturdy, trackways are actually fragile fossils, and the half of the trackway at the Texas Memorial Museum has significantly deteriorated since it was put on display. The museum is trying to raise a million dollars to properly conserve and house this historically and scientifically significant fossil. If you wish to learn more about their campaign, you can find more information here.

Camarasaurus, as envisioned by Erwin Christman. From Osborn and Mook, 1921.

Sauropods were swamp monsters. At least, that’s what books, movies, and illustrations taught me when I first encountered the huge dinosaurs. If Diplodocus and Brachiosaurus didn’t actually spend most of their time in the water, then the dinosaurs always stayed close to watery refuges where they could escape from Allosaurus and other predators.

But starting in the 1960s, a renewed scientific interest in dinosaurs overturned this cherished imagery. Sauropods were wholly terrestrial creatures. These giants did not possess any features related to an aquatic or amphibious lifestyle—Apatosaurus and kin were often plunked down into bogs and lakes in reconstructions because that environment seemingly answered nagging questions about the biology of these animals. But early 20th century paleontologists didn’t think that all sauropods were equally adept at life in the water. Rather than take the line that all sauropods were skilled swimmers, paleontologists identified at least one Jurassic sauropod that probably spent more time on land.

In 1920, a trio of American Museum of Natural History scientists published a pair of short papers on the sauropod Camarasaurus. This dinosaur, with a blunt head and spoon-shaped teeth, was one of the better-known members of the classic Morrison Formation fauna, and the AMNH paleontologists had just completed a major reexamination of the dinosaur’s remains. In the first note, Henry Fairfield Osborn and Charles Mook briefly summarized the results of their study, and in a second, accompanying missive, William Gregory outlined the dinosaur’s life habits.

Camarasaurus didn’t seem suited to a life wallowing in a Jurassic lake. While Gregory mentioned that the dinosaur “might well have been an efficient wader,” the dinosaur was also “positively devoid of special adaptations for swimming.” The dinosaurs limbs, shoulders and hips were clearly suited to supporting the animal’s bulk, and Gregory considered the “relatively small and feeble” tail of Camarasaurus to be of no help in swimming. While Gregory did waffle on the habitat the dinosaur preferred, the overall picture was of a relatively straight-limbed dinosaur that carried its body high off the ground. Sauropods did not drag their bellies through the Jurassic mud, as other paleontologists had suggested under the supposition that sauropods were like lizards or crocodiles, writ large.

The following year, when Osborn and Mook published their massive revision of sauropods collected by Edward Drinker Cope, they similarly cast Camarasaurus as a dinosaur that was “terrestrial in gait but adapted to an amphibious life.” And the plates of that paper present some of the restorations and reconstructions previously mentioned in the PNAS papers. A model of Camarasaurus, created by artist Erwin Christman under Gregory’s direction, showed the dinosaur walking on land with slightly bent forelimbs, similar to how the museum mounted its great “Brontosaurus” skeleton years before. Christman and Gregory also collaborated on a pair of skeletal reconstructions—one with the head of Camarasaurus held high, and the other in a droopy pose, with neck and tail slung low.

Osborn, Mook and Gregory’s insistence that Camarasaurus was an amphibious dinosaur, or at least frequently waded, is puzzling. The paleontologists didn’t justify this part of their argument. Sauropods were simply considered synonymous with warm, luxuriant swamps. Contrary to this belief, the experts explicitly pointed out evidence that Camarasaurus walked tall and had a skeleton well-suited to holding up the animal’s weight while walking on land. Even before the “Dinosaur Renaissance” forever changed dinosaurian imagery, early 20th century paleontologists were already cataloging the same evidence. They just saw that evidence differently, in the context of a lazy Mesozoic world filled with shuffling, basking sauropods.

References:

Gregory, W.K. 1920. Restoration of Camarasaurus and life model. PNAS. 6, 16-17

Osborn, H.F., Mook, C.C. 1920. Reconstruction of the skeleton of the sauropod dinosaur Camarasaurus Cope (Morosaurus Marsh). PNAS. 6, 15

Osborn, H.F., Mook, C.C. 1921. Camarasaurus, Amphicoelias, and other sauropods of Cope. Memoirs of the American Museum of Natural History, new series, 3, 247-387 (plates LX-LXXXV).

Taylor, Michael P. 2010. Sauropod dinosaur research: a historical review. pp. 361-386 in: Richard T. J. Moody, Eric Buffetaut, Darren Naish and David M. Martill (eds.), Dinosaurs and Other Extinct Saurians: a Historical Perspective. Geological Society of London, Special Publication 343.

The original AMNH mount of Brontosaurus, reconstructed in 1905. Image from Wikipedia.

“Brontosaurus” will always be special to me. The shuffling, swamp-dwelling dinosaur never really existed, yet, for my younger self, the Jurassic behemoth was an icon of everything dinosaurs were supposed to be. The skeleton mounted at the American Museum of Natural History is what really hooked me on the sauropod. When I first visited the skeleton in the late 1980s—before the museum’s dinosaur halls were renovated in the late 1990s—I was astonished. I had seen illustrations of Brontosaurus before, but seeing the animal’s actual bones was a transcendent experience for me. I already liked dinosaurs, but after standing in the shadow of those column-like limbs and intricate vertebral column, I loved dinosaurs.

Today we know that the specimens once assigned to Brontosaurus excelsus really belonged within the genus Apatosaurus. That issue was settled decades before I was even born, although museums and paleontologists themselves were slow to adopt the change. (It wasn’t until the proper head of Apatosaurus was rediscovered—the specimen was excavated at Dinosaur National Monument in 1909 but confused for a Diplodocus skull for decades—that the move to publicly shun Brontosaurus started in earnest.) Indeed, in 1903 paleontologist Elmer Riggs recognized that Brontosaurus excelsus was extraordinarily similar to the skeleton of another sauropod, named Apatosaurus ajax. Both had been named by Yale paleontologist O.C. Marsh at the height of the Bone Wars era, when many dinosaur specimens, no matter how subtle their differences, were given a new genus or species designation. In this particular case, the fact that the Apatosaurus ajax specimen came from a relatively young animal and the Brontosaurus excelsus specimen was an older animal led Marsh astray. Both forms, Riggs concluded, belonged to the same genus, and Apatosaurus had priority since it was named first.

The American Museum of Natural History mount went up in 1905. The dinosaur was promoted as Brontosaurus, not Apatosaurus. Even though Riggs’ case would eventually win out, AMNH paleontologists Henry Fairfield Osborn and William Diller Matthew didn’t agree with the name change. Exactly why Brontosaurus was allowed to live on—much to Riggs’ frustration—is unclear. But all these little quirks of nomenclature and procedure had a major influence on the popularity of Brontosaurus over Apatosaurus. The AMNH mount was the first reconstruction of this dinosaur ever attempted, and in 1905, it was one of a kind. (The original material Marsh used to describe Brontosaurus was held at Yale, but Marsh never made an effort to publicly display the partial skeleton his crew found at Como Bluff, Wyoming. The specimen, carrying a Brontosaurus name plate and the wrong head, was not reconstructed at Yale until 1931.) The AMNH Brontosaurus mount was the introduction of sauropods to the fascinated public.

William Diller Matthew recounted the process of mounting his museum’s Brontosaurus in an American Museum Journal article and a news item for the Independent. The skeleton was a Frankenstein. The principal part of the mount was an incomplete skeleton found near the Nine Mile Crossing of the Little Medicine Bow River in Wyoming. This one site yielded most of the vertebral column, all the ribs, elements of the shoulders and hips, and a few portions of the limbs from the single sauropod. But quite a few parts were missing, so AMNH paleontologists turned to other specimens. The AMNH Brontosaurus also included various elements from specimens found at Como Bluff and Bone Cabin Quarry, Wyoming, as well as plaster casts made from the Yale Brontosaurus material and other bones already in the AMNH collections.

And, of course, there was a question of the head. No one had ever discovered a Brontosaurus skull articulated or even associated with the rest of the skeleton. (And Earl Douglass’ discovery at Dinosaur National Monument was still four years away.) A skull had to be specially designed for the AMNH mount, and the New York museum followed Yale’s lead.

While all the bones from Marsh’s original Brontosaurus specimen came from Quarry 10 at Como Bluff, there was no skull among the lot. Rather than let the dinosaur go decapitated, however, Marsh identified two skull portions from a more diverse bonebed nearby, known as Quarry 13, as belonging to Brontosaurus. The sections of upper and lower jaws were set with spoon-shaped teeth, and these are the skull portions which make up the head of the famous 1883 reconstruction of the dinosaur Marsh commissioned.

The Como Bluff jaws outlined what the front of the dinosaur’s jaws might have looked like and, assuming that Marsh was correct, indicated that the skull of Brontosaurus was very different from that of Diplodocus. Fortuitously, the same AMNH expeditions to Bone Cabin Quarry which turned up Brontosaurus parts also brought back a complete Camarasaurus skull. Prior to this discovery, no one knew exactly what the head of Camarasaurus looked like. The fact that it seemed to share the spoon-shaped teeth assigned to Brontosaurus meant that the skull was a good model for reconstructing the rest of the missing “thunder lizard” skull. As far as I’m aware, the paleontologists did not consider that the supposed Brontosaurus skull parts, found in a different quarry than Marsh’s original specimen, really belonged to Camarasaurus.

Of course, accumulating all the right bones is just the first step in preparing a mount. Today, huge dinosaur skeletons are the stars of many museums. In 1905, though, such an effort had never been attempted before, and the AMNH paleontologists were not entirely sure how the brontosaur bones should be articulated. Matthew, along with colleague Walter Granger, dissected lizards and crocodiles to investigate how their muscles attached to their limb bones, and used these distant modern analogs to give their Brontosaurus a slightly bow-legged posture.

Mounted an a raised platform, the AMNH Brontosaurus looked like an impressive terrestrial titan. Yet during his study of the bones, Matthew concluded that Brontosaurus was a great amphibious dinosaur. Drawing from the authority of anatomist Richard Owen and paleontologist E.D. Cope, Matthew pointed out that the anatomy of Brontosaurus was so well-suited to life in water that you could tell the approximate depth at which the animal submerged. While the dense, heavy limbs of the dinosaurs acted like the heavy boots of deep-sea divers, Matthew pointed out, the sauropod’s light vertebral column would have been more buoyant. The dinosaur’s back therefore represented a sort of high water line which indicated the depth at which Brontosaurus wallowed in swamps, arcing its long neck to slurp up soft water plants.

Brontosaurus, in Matthew’s estimation, spent life slogging through a warm Jurassic bath. That seemed just as well—the dinosaur’s brain was comically small for its size. This sauropod was not an intelligent, behaviorally complex creature, Matthew argued, but a dim-witted leviathan devoted to a lazy lifestyle. “Hence we can best regard the Brontosaurus as a great, slow-moving animal automaton,” Matthew wrote, “a vast storehouse of organized matter directed chiefly or solely by instinct and to a very limited degree, if at all, by conscious intelligence.”

I am glad that dinosaurs have changed dramatically since Matthew characterized them as idiotic, clumsy piles of flesh. Apatosaurus and the whole rest of the dinosaurian ensemble are far more fascinating now than they were when bound to short and savage lives in steaming jungles and marshes. The true identity of “Brontosaurus” was eventually made clear, sauropods were ushered out of the swamps, butt-brains have been refuted, and paleontologists are able to extract more information about dinosaur lives from old bones than ever thought possible before.

And yet, I still feel some affection for Brontosaurus. This isn’t because I would prefer to see dumb, blunt-headed dinosaurs sloshing through algae-filled ponds, but because the old thunder lizard represented the epitome of true dinosaur-ness when I was a child. The mountain of muscle and bone was a wonderful icon which, in memory, reminds me just how much dinosaurs have changed during the twenty four years since I first saw the sauropod’s bones. I am thrilled that paleontologists sunk Brontosaurus, and the story of the icon’s demise reflects how paleontology has matured from a contest to see who could collect the biggest skeletons to a discipline that is carefully teasing out the secrets of prehistoric lives.

References:

Matthew, W.D. 1905. The mounted skeleton of Brontosaurus. American Museum Journal.V (2), 63-70

Osborn, H.F. 1906. The skeleton of Brontosaurus and the skull of Morosaurus. Nature. 1890 (73), 282-284

Parsons, K. 2001. Drawing Out Leviathan: Dinosaurs and the Science Wars. Bloomington: Indiana University Press. pp.1-21

In the American Museum of Natural History’s Hall of Saurischian Dinosaurs, there is a great fossil mismatch. You can find the deceptive pairing in the Apatosaurus exhibit. Set in the floor behind the enormous dinosaur is a set of trackways—the Apatosaurus is posed as if the skeletal sauropod has just left the tracks behind. But there is no way that Apatosaurus left those tracks. The footprints and the long-necked dinosaur on display were separated by tens of millions of years.

Apatosaurus is an iconic Morrison Formation dinosaur. The hefty sauropod trod across prehistoric floodplains of America’s Jurassic West around 150 million years ago. But the footprints on display at the AMNH comes from a different time. The slab is part of a roughly 113-million-year-old trackway found along the Paluxy River near Glen Rose, Texas. Apatosaurus was long gone by the time the Texas tracks were created, and the shape of the footprints indicate that a very different kind of sauropod, probably belonging to the subgroup called titanosaurs, actually created the tracks.

Regardless of the inappropriate juxtaposition, though, getting those tracks out of the ground and set up at the AMNH was a massive paleontological undertaking. A YouTube video—posted above—shows actual footage of the 1938 excavation.

Although dinosaur tracks were known to local people since at least the beginning of the 20th century, it wasn’t until the late 1930s that the footprints garnered broad attention from paleontologists. Roland T. Bird, a fossil collector working with the AMNH, was roving the Southwest in 1937 when he got word of dinosaur tracks in the vicinity of the Paluxy River. When he got there, he found that the tracks supported a small local industry—everyone seemed to know about them, and many people had quarried tracks to sell for rock gardens. Fortunately for Bird, there were still plenty of tracks in the ground, including impressive trackways of multiple dinosaurs moving together.

The slab at the AMNH is one section of a large trackway that Bird had divided into three pieces. (The other two parts are at the University of Texas and the Smithsonian Institution.) Getting the tracks out was arduous, destructive work, made all the more complicated by the fact that at least some of the trackway went under the river. Bird and members of the local Works Progress Administration crew diverted the river to access and remove the tracks.

Bird’s tracks didn’t immediately go up on display. The broken pieces of excavated trackway just sat in the museum’s yard, and Bird’s health rapidly declined due to unknown causes and he was forced into an early retirement. When the AMNH decided to renovate its dinosaur halls in the 1940s, however, paleontologist Edwin Colbert asked Bird to come back to oversee the reconstruction of the trackway behind the museum’s “Brontosaurus” mount. Without Bird, the project would have been impossible—the broken trackway pieces were becoming exposed to the elements in the museum’s storage yard, and many of the fossil pieces were not labeled. The project was scheduled to take six weeks. Bird took six months, but, nonetheless, Bird and his collaborators were able to restore the steps of a Cretaceous giant.

The backside of Diplodocus, photographed at the Utah Field House of Natural History. Photo by the author.

In 1991, the American Museum of Natural History unveiled one of the most fantastic fossil displays ever created. Placed at the center of the renovated Theodore Roosevelt Rotunda, an adult Barosaurus rears back to protect its offspring from an oncoming Allosaurus. The defending sauropod’s head is 50 feet up in the air, although whether or not such an immense, long-necked dinosaur could have pulled off such a feat has been a continuing point of contention. Even in a typical posture, Barosaurus must have had a powerful heart to pump blood along its 25-foot neck, and who knows how hard the dinosaur’s heart would have to work to continue bloodflow to the animal’s head if it reared up? Some paleontologists consider such a feat physically impossible, but as paleontologist William Gallagher pointed out while teaching my Paleontology 101 class at Rutgers University, male Barosaurus had a good reason to rear up. How else would the huge dinosaurs have positioned themselves to mate?

Exactly how dinosaurs got it on has inspired no small amount of speculation. The largest dinosaurs of all, the sauropods, have been especially perplexing. We often say that these dinosaurs “shook the earth” with their footsteps, but did they also make the bed rock with their lovemaking? (I apologize for that joke, and will keep the geology puns to a minimum. Promise.) Paleontologist Beverly Halstead famously wondered about dinosaur sex in public lectures and articles, and he suggested that standard “dinosaur style” was for a male to come alongside the female and throw its leg over the female’s back as she lifted her rump into the air to move her tail out of the way. In the case of sauropods such as Diplodocus, Halstead even imagined that the amorous dinosaurs might intertwine their tails. While other paleontologists have considered the tail-twisting aspect unlikely—sauropod tails were balancing organs and were too stiff to intimately coil around each other—the basic dinosaur position Halstead promoted has remained a prominent possibility for the dinosaur kama sutra.

But not everyone agreed that giants such as Apatosaurus and Brachiosaurus were capable of such nupital acrobatics. These animals were so immense—Apatosaurus, not even the largest sauropod, is estimated to have weighed more than 23 tons—that some researchers thought the kind of positions Halstead was promoting would give the dinosaurs fractured legs and broken spines. At a symposium of vertebrate morphologists held at the University of Chicago in 1994, biologist Stuart Landry, Jr. gave a short presentation entitled “Love’s Labors Lost: Mating in Large Dinosaurs.” He did not see how sauropods could have mated on land. A large, rearing sauropod, he told his audience, “would have to support 10 to 20 tons in a precarious position two or three meters off the ground.” A male Apatosaurus would be liable to tip over and possible take the female with him. Instead, Landry suggested that the largest dinosaurs looked for muholes or bodies of water to buoy themselves up. When a conference attendee asked if he was proposing that all dinosaurs mated in water, Landry responded, “I would say the very large ones must have.” Of course, this hypothesis required a large number of Jurassic and Cretaceous hot tubs of just the right depth for sauropods to reproduce, and scientific models of sauropods have suggested that these dinosaurs were actually quite buoyant and unstable in water. Sauropods were diverse, disparate and widespread animals that roamed in terrestrial habitats all over the world—there’s no reason to presume that the largest dinosaurs had to seek out the nearest deep lake when they got the itch.

Biomechanics expert R. McNeill Alexander also considered the weight problem in his 1989 book Dynamics of Dinosaurs & Other Extinct Giants, but came to a different conclusion. Even though a male sauropod would have rested a great deal of weight on the back of the female during mating, Alexander pointed out that the stresses and strains would not have been any more severe than those caused while the female dinosaur was walking. (After all, walking requires shifts in weight as the dinosaur balances and goes through each step cycle, and so a dinosaur’s skeleton had to be strong enough to cope with these shifts.) “If dinosaurs were strong enough to walk they were strong enough to copulate,” Alexander wrote. “They were presumably strong enough to do both.”

Without living specimens to observe, we will never know all the intimate details of sauropod sex. Still, there are only a limited number of positions that could have worked for the dinosaurs. For reasons that I’ll write about later this week, the consensus among paleontologists is that male dinosaurs probably had relatively small penises. (Shhh! Don’t tell Tyrannosaurus. He’s already upset about all those “useless forelimbs” jokes.)  An amorous male would have to position his cloaca—the orifice used for both expelling waste and mating in crocodylians, birds and probably dinosaurs—right up to the cloaca of a female, and the female’s tail would have undoubtedly presented an obstacle. Rather than simply leaning straight against the top of a female like an elephant or rhinoceros does, a male sauropod would probably have to rear up at a relatively oblique angle, and the female would have to assist by moving her tail (which is also a way in which female dinosaurs could have exerted mate choice and confounded any hot-under-the-collar males they would rather not mate with). Perhaps some museum will look into the problem and try to mount a pair of coupling sauropod skeletons—much like the mating Tyrannosaurus at Spain’s Jurassic Museum of Asturias—but even then we are limited by what we can imagine. Whether we want to imagine a pair of Brachiosaurus in flagrante delicto is another matter altogether.

This post is the first in a short series of articles on dinosaur reproduction that will run through Valentine’s Day. Because nothing spells romance like dinosaur sex.

References:

Alexander, R. M. 1989. Dynamics of Dinosaurs & Other Extinct Giants. New York: Columbia University Press. pp. 57-58

Anderson, J. The Perplexing Puzzle of Maladroit Mating. Chicago Tribune. August 30, 1994.