DINOSAURS WITHOUT BONES
DINOSAUR LIVES REVEALED BY THEIR TRACE FOSSILS
ANTHONY J. MARTIN
To my wife, Ruth, whose traces of love suffuse and nourish me every day.
Contents
CHAPTER 1 Sleuthing Dinosaurs
CHAPTER 2 These Feet Were Made for Walking, Running, Sitting, Swimming, Herding, and Hunting
CHAPTER 3 The Mystery of Lark Quarry
CHAPTER 4 Dinosaur Nests and Bringing Up Babies
CHAPTER 5 Dinosaurs Down Underground
CHAPTER 6 Broken Bones, Toothmarks, and Marks on Teeth
CHAPTER 7 Why Would a Dinosaur Eat a Rock?
CHAPTER 8 The Remains of the Day: Dinosaur Vomit, Stomach Contents, Feces, and Other Gut Feelings
CHAPTER 9 The Great Cretaceous Walk
CHAPTER 10 Tracking the Dinosaurs Among Us
CHAPTER 11 Dinosaurian Landscapes and Evolutionary Traces
Notes
Acknowledgments
Index
Image Gallery
CHAPTER 1
Sleuthing Dinosaurs
Vestiges of a Cretaceous Hour
The Triceratops was vexed. As the largest adult male in this part of the river valley, the challenge coming from a younger male strolling through his long-established territory was intolerable. Appropriately, then, it provoked a prompt and assertive response. From a stationary start, feet planted firmly on the damp muddy sand of the floodplain, he stared at his rival, started walking, and picked up speed. As he moved, he lowered his huge, broad head while pointing his three-horned face directly at the other Triceratops, communicating his intentions unambiguously. While accelerating, his rear feet registered directly on top of where his front feet had just pressed, and then exceeded them, leaving a varied pattern of tracks behind him.
From a distance, the other male seemed to stand his ground. Yet his feet shuffled, blurring their outlines in the underlying sand, as he tried to decide whether to stand his ground or turn and flee. Regardless, he was in big trouble.
In between the two Triceratops, a group of small feathered theropod dinosaurs with stubby forearms—similar to the Asian alvarezsaur Mononykus—and a nearby bunch of slightly larger ornithopod dinosaurs (Thescelosaurus) looked on warily. Each of these groups of dinosaurs had been striding unhurriedly across the floodplain, tolerating one another’s presence, spurred on by intriguing scents wafting down the sunlit valley. Nevertheless, a charging Triceratops provided a good reason to temporarily abandon their long-term goals and deal with this more immediate problem.
In unison, they all looked up at the advancing Triceratops, its profile and rapidly increasing pace causing it to appear ever larger as it neared. Next to them, a mixed flock of toothed birds and pterosaurs all turned and aligned themselves with the wind at their backs. They began hopping while flapping their wings, and then were aloft, chattering loudly. This was all the motivation one of the more skittish theropods needed to start running, and the rest of his group followed suit. The ornithopods only hesitated a second or two before doing the same. First, though, more than a few of both species lightened the load before taking off, involuntarily voiding their bowels and leaving variably colored and sized scat, peppered with seeds, on top of their distinctive footprints. In her haste, one Thescelosaurus slipped on a muddy patch and fell on her side. She quickly righted herself and bolted to catch up with the others, leaving a long, smeared body impression on the sand among the tracks.
A band of Mononykus-like theropods became too crowded near the edge of the river and, in a moment of desperation, jumped into the water and began swimming. In the river shallows, their feet touched the bottom and claws on each toe gouged the sand, making parallel grooves with each stroke. Bodies buoyed by the water and swimming with the current, their tracks were nearly twice as far apart from one another as if they had been on land.
These three separate but related shockwaves—the aggressive movement of the big male Triceratops, the raucous flocks of birds and pterosaurs, and the panicked stampede of the theropods and ornithopods—triggered overt and subtle changes in the behaviors of nearly every dinosaur nearby. In this respect, it was like many other days in the Cretaceous Period, where long periods of quiet stability were occasionally interrupted by near-chaotic commotion.
On higher ground above the floodplain, a male–female pair of predatory theropods (Dromaeosaurus) paused from digging up small mammals from out of their burrows with their rear feet. They raised their heads, looking for whatever had provoked alarm calls from the birds and pterosaurs now passing overhead. Their footprints and excavations had disturbed a considerable amount of soil in the area. But they had much more to do before leaving. Once convinced the alarming behavior below had nothing to do with them, they went back to uncovering their furry morsels, sniffing the ground and scraping with their rear feet.
Near the Dromaeosaurus couple and next to the river levee, a few other theropods (Troodon) shifted anxiously on their sediment-rimmed ground nests. Several weeks earlier, Troodon mothers had laid eggs in the nests two at a time, a function of their dual oviducts. Some had deposited as many as two dozen eggs, a dinosaurian form of labor that took about a week and a half to complete. After laying, each mother Troodon vertically oriented the eggs in the center of the doughnut-shaped nest. Now it was the job of the male to sit above and otherwise guard the precious egg clutch for nearly fifty days. Almost nothing could motivate them to leave their nests, so they continued to squat above them, albeit nervously.
Two smaller feathered theropods, potential egg predators and only a few meters away, gave the Troodon fathers further incentive to stay put. These dinosaurs gnawed on a recently dead pterosaur, scraping their teeth across its limb bones to strip whatever flesh was left. One of them, though, unsettled by the ripple effects of the dinosaur duel below, succumbed to caution and scrambled up a tree, her hands and feet imparting sets of scratch marks on the smooth bark.
The spreading disturbance initiated by the Triceratops provoked several small ornithopod dinosaurs, distantly related to Thescelosaurus, to retreat into their burrows. The burrows, which they had dug previously into the banks of the levee, had entrances only slightly wider than the ornithopods’ bodies, making for a tight fit as they scrambled inside. These burrows twisted to the right and then left as they descended, making S- or Z-shapes, before expanding into a main living chamber at their ends. This zigzag design effectively deterred predators while maintaining livable temperatures and humidity in each burrow, essential features for these ornithopods to safely raise their young.
Several hundred meters downstream from the two Triceratops—which were only seconds away from ramming each other—a group of ostrich-like theropods (Struthiomimus) walked along a gravel bar at the edge of the river. They stopped every now and then to swallow pea- and marble-sized quartz-rich rocks. These “stomach stones,” also known as gastroliths, would lodge in a muscular gizzard just above their intestines, where they served as digestive aids to grind varied omnivorous foodstuffs like internal mortars and pestles. These theropods were aware of but mostly unperturbed by what was taking place upstream, as the “Mononykus” and Thescelosaurus groups were still far away from them. Their tracks, accented by sharp claw marks, left vague outlines on the gravelly parts of the bar, but were crisply defined on the sandier patches. The female tracks were only slightly larger than those of the males, but otherwise identical in form.
Meanwhile, located between the Dromaeosaurus and the Struthiomimus, a male–female pair of Ankylosaurus, joined in coitus, ignored just about everything else happening around them. These heavily armored dinosaurs, each weighing about five tons, typically would have left large, deep tracks
regardless of what they were doing. But the addition of the male’s weight to the rear of the female meant her hind feet sank much deeper than his into the moist sand below. Together they made a six-footed impression, the front four coming from her and two rear ones from his hind feet. The earth moved, however briefly, for both partners.
Where the “Mononykus” and Thescelosaurus once stood on the lower part of the floodplain, at least a few of their tracks and some of their scat had been squashed under the thumping feet of the older Triceratops as he closed the gap between himself and the younger male. The latter could no longer stay in one place and commenced walking, then trotting, toward the other. Snails, clams, and salamanders in the floodplain sediments unlucky enough to be in the same place as a Triceratops foot were summarily crushed.
The collision between the two male ceratopsian dinosaurs, which had a combined weight of about twenty tons, was surprisingly subdued and dull. They did not hit one another at full speed, as the additive impact would have killed both instantly. Instead, they first stopped, and then clashed their mighty heads together. Their wide head shields not only served as great billboards for attracting mates and recognizing their species, but also dissipated much of the energy when they struck. Still, the left upper horn from the older male punched through the right side of the younger one’s skull, leaving a round wound about the width of the older male’s horn.
Injured badly, although not fatally, the younger male turned and began limping away. His right humerus also had been fractured by the impact and started to ache. Accordingly, his trackway acquired a new asymmetry, with both legs stepping shorter on the right side and a shallower impression in the right-front footprint as he put less weight on it. The larger male Triceratops, satisfied for now, walked away but stopped periodically to turn and look, making sure that no further bad behavior would come from this young upstart.
This was the right moment for the Tyrannosaurus. She had been standing stock-still in the higher, vegetated part of the floodplain, waiting to take advantage of the chaotic situation and get an easy meal. If she could have felt disappointment, though, she would have experienced it then as she looked down from her elevated position and saw both Triceratops walking off the floodplain, each very much alive. She used a mixture of hunting and scavenging to feed her six-ton frame, and had dined often on Triceratops that eventually died from battles with one another. Whenever she found a dead one, she would take small bites of sweet meat on the face, and then grab the head shield from the back with her teeth to tear off the head, exposing its delectable neck muscles. It was tempting for her to follow the limping one to see if he would expire, but attacking too soon entailed much risk. Millions of years of natural selection had not resulted in one of the world’s largest land carnivores taking on prey that could also kill it.
Flexible in her menu choices, she turned her gaze toward a nearby group of hadrosaurs (Edmontosaurus) of varying ages that were unknowingly sharing this grassy–shrubby part of the floodplain with her. Most of these dinosaurs were on all fours, grazing on the plants around them. But they occasionally reared up on their hind legs and looked down to their left, having been distracted by the battle below and its effect on the surrounding fauna. As the hadrosaurs pulled up low-lying plants and chewed, tiny bits of silica in the leaves and stems imparted microscopic scratches on their teeth. Unnoticed by either the hadrosaurs or the tyrannosaur, the herd was attracting the attention of thousands of dung beetles. These insects burrowed industriously into the feces and laid their eggs, the dung providing both a home and high-quality food for their progeny. Snails also grazed on the droppings, gaining plenty of nutrition from the digested plant material.
The Tyrannosaurus turned her massive head slowly to the left to get a better fix on a potentially easy target. There: a half-grown Edmontosaurus had become curious about the sounds of ceratopsian conflict and separated himself from the rest of the herd. She locked her vision onto him and shifted her weight to that side. Mud oozed between her toes and a prominent ridge formed on the outside of her left foot. Starting with her feet together, she began stalking, taking a series of steps punctuated by pauses. Right, left, stop; right, stop; left, stop; right, left, stop. The breeze flowed down the river valley in the opposite direction of her movement, masking her sounds and scent. Within minutes, she was close enough to pounce, and did.
Her tiny arms were useless for grabbing the Edmontosaurus, so she lunged forward with her best asset: a mouth full of stout, banana-sized serrated teeth, backed up by the most powerful bite of any land animal that had ever evolved. Unfortunately for her, but fortunately for the hadrosaur, a crunching branch on her next-to-last step revealed her close presence in an otherwise stealthy approach. The Edmontosaurus lurched to his left just when the tyrannosaur’s jaws clamped down on the uppermost part of his tail. This action removed a chunk of the hadrosaur but left the rest of the animal running away on his back two legs, hooting loudly in pain and fright. The other hadrosaurs in the herd likewise switched to faster bipedal postures and quickly moved away, making much noise and stomping on mud, plants, dung, dung beetles, and snails with their stout feet.
The tyrannosaur trailed the retreating hadrosaurs for a few minutes, just in case the one she had bitten would falter from his wound, or some other straggler in the herd would make itself available to her. Her tracks first paralleled and then turned in harmony with the herd’s trackways as she followed them. As the distance separating them became greater, though, she stopped. Going any farther would expend more resources than it was worth, so she would just have to wait for another opportunity.
She went back to the fresh, bite-sized amuse-bouche of hadrosaur remaining on the ground at the site of her attack, sniffed it, picked it up with her mouth, chewed, and swallowed. The next day, its remnants would come out the other end of her digestive tract with most of its useful nutrients absorbed, but with bits of etched vertebral bone and a few of the hadrosaur’s muscle fibers preserved in the feces. She walked down into the lower part of the floodplain and moved along the trend of the river valley. Her huge, thickly padded, three-toed and clawed footprints obliterated some of those left only fifteen minutes before by the terrified “Mononykus” and Thescelosaurus.
Unknown to this Tyrannosaurus and every other dinosaur in the area that day, the river valley itself was also a remnant of a former dinosaur presence. Herds of immense long-necked sauropod dinosaurs had moved through this place several tens of million years before, but were now mostly gone from this part of the world. Trails made by these sauropods, caused by habitual movements over hundreds of generations, had breached levees and cut across river channels. These alterations provided new avenues for flooding water that eventually changed local drainage patterns, which in turn impacted regional flow.
The sauropods had grazed and trampled the local vegetation sufficiently that plant roots no longer bound sediments near the river channels. As a result, every flood rapidly eroded banks, broadened channels, and spread sand and mud farther out onto floodplains than before. A combination of flooding and windblown sand from this widened river built up levees, which provided banks suitable for hosting the burrowing ornithopods. Over time, this synergism between sauropods—and later, large ornithopods and ceratopsians—with sediments, and moving water irrevocably changed the neighboring habitats and became the “new normal” for all dinosaurs that lived and evolved there afterwards.
The sauropods had also made extensive nesting grounds, maintained on the upper parts of the floodplains, that covered hundreds of acres and stacked on top of one another over thousands of years. The annual visits of these huge dinosaurs and their nurseries not only inhibited plant growth, but also imbued this area with a bumpy surface, like clothes left under a blanket. In a recursive way, then, these coalescing and superimposed nests, which now formed hardened layers several meters below the Troodon nests, made the area amenable for laying eggs and raising young for later dinosaurs. Wasps, beetles, and other insects likew
ise were attracted to the well-drained soils in between the Troodon nests. These insects burrowed into the sand and made brooding cells, which were later occupied by their larvae and cocoons. The former nesting horizons of the sauropods also restricted the burrowing of small mammals, which brought them closer to the surface and made things a little easier for the Dromaeosaurus and other predators to find them.
Yet another trace left by the sauropods was an evolutionary one, evident in the plant communities. The hadrosaurs had been grazing on vegetation that was the result of intense selection pressures placed on past plants by sauropods through both stomping and eating. These changes in plant communities consequently shifted evolution in the animal communities that used them, reinventing entire ecosystems.
Thus the stage for this Cretaceous drama was constructed and its actors were unwittingly directed by the lasting marks of these vanished sauropods and other dinosaurs. In this sense, traces begat traces, and the dinosaur vestiges of the past influenced those of the present, an expansive canvas gently suggesting where the next brushstrokes should go.
Tracing Dinosaur Lives
Most of the scientists I know try hard not to write fiction. After all, our primary goal each day is to understand just a bit more about whatever we study and clearly communicate our newly realized comprehensions to others. Even so, as a practicing paleontologist, I thought that a fictional scenario would best encapsulate much of what this book is about, while also introducing its main topic in a way that engages and encourages our imaginations.
Perhaps more than any other part of paleontology, the research specialty of ichnology—the study of trace fossils (tracks, trails, burrows, feces, and other traces of behavior, including fossil examples)—is about that exciting intersection between science and flights of fancy. When applied to dinosaurs, ichnology becomes even more stimulating. In fact, I like to argue that for us to truly grasp how dinosaurs behaved, to really know how they lived as animals and interacted with one another and their environments, we absolutely must study their trace fossils, and not just their bones, in order to paint the most vivid picture imaginable of their world.
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