Dinosaurs Without Bones (2 page)

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 likewise 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.

In the story above, I placed together dinosaurs that may not have been in the same time and place, although most are from near the end of the Cretaceous Period (about 70 million years ago) and in an area defined approximately by Montana and Alberta, Canada. Furthermore, even those dinosaurs overlapping in both respects still may not have encountered or affected one another. However, in this deliberate mash-up of dinosaurs and their behaviors, real dinosaur trace fossils inspired nearly every element of this story.

Even better, many of these trace fossils have been discovered or studied just recently. Because of these finds, paleontologists are reconsidering some of what we thought we knew for sure about dinosaurs, either confirming long-suspected behaviors or revealing astonishing new insights into their lives. In other words, dinosaur trace fossils very often fulfill or exceed our expectations of these most celebrated of fossil animals.

Let’s start with the
Triceratops
fight as an example. It turns out a good number of
Triceratops
head shields, which are composed of paired parietal and squamosal bones, bear deformities in the squamosals. These look like former healed wounds and are consistent with injuries caused by
Triceratops
horns. Ceratopsians, a group of dinosaurs that includes
Triceratops
and related horned dinosaurs, also made tracks, which are preserved in Cretaceous rocks from about 70 million years ago in the western U.S. and Canada. Ceratopsian tracks can be identified from their size, numbers of digits—five on the front foot and four on the rear—and are preserved in rocks the same age as those with ceratopsian bones. These same tracks also show that ceratopsians walked with an upright posture. This implies that these dinosaurs could move more efficiently than previously supposed from skeletal evidence: more like a rhinoceros, and less like a lizard.

Did large ceratopsians like
Triceratops
trot or gallop? We don’t know for sure yet, but their tracks would provide one of the best ways to test whether they moved faster than a walk. So even though I only imagined two ceratopsians trotting toward one another and knocking heads, it’s feasible that someone could find tracks showing that such fights did indeed happen. Moreover, this possible future discovery is given hope because other trace fossils—the healed wounds—suggest that ceratopsians occasionally became cross with one another, whether over territory, mates, food, or all of the above.

Was there ever a dinosaur stampede like the one described, composed of a mix of diminutive dinosaurs and different species? Maybe, although this is now being disputed. In the Cretaceous
Period, about 95 million years ago and on a lakeshore in what is now Queensland, Australia, nearly a hundred small, two-legged dinosaurs ran in the same direction and at high speed. Paleontologists who originally studied this tracksite think that species of theropods and ornithopods were together in the same limited space. Evidently, they were then panicked by the arrival of a much larger dinosaur on the scene. These tracks also say something about different species of dinosaurs tolerating one another in the same environments, as well as reacting to the same stimuli. And just what was the identity of the large dinosaur that caused such distress? And was it really a stampede, or can this unusual tracksite be explained by other means? That’s a story in itself, which, along with the science behind it, I’ll gladly discuss later.

How about the scene with the swimming dinosaurs? Once again, trace fossils confirm a concept that has gone back and forth among paleontologists, but is now certain: we know that at least a few dinosaurs left land and got into the water. One grouping of swim tracks, made by seven separate theropods, is preserved in Early Cretaceous rocks (110 million years ago) of Spain. Many more swim tracks are in Early Jurassic rocks from about 190 million years ago in southwestern Utah. The latter site has shattered any doubts about dinosaurs swimming, with more than a thousand such tracks, linked to theropods and ornithopods, showing how they paddled against, with, and across currents. Until lately, swimming was thought of as an extremely rare behavior in dinosaurs. Hence, these tracks have impelled paleontologists to reexamine their presumptions, and they are now looking for more evidence that some dinosaurs were comfortable in water, or even that they may have occasionally gone fishing and taken advantage of the plethora of food waiting for them beneath the water’s surface.

Dinosaur digging, whether used for making burrows in which they lived, to acquire underground prey, or to make ground nests, is yet another newly diagnosed behavior in dinosaurs, and one based mostly on their trace fossils. In 2007, I helped two other paleontologists document the first known burrowing dinosaur (
Oryctodromeus
cubicularis
, a small ornithopod) from Cretaceous rocks (95 million years old) in Montana. Incredibly, this dinosaur was found in its burrow with two partly grown juveniles of the same species. Two years later, I interpreted similar burrows in older Cretaceous rocks (105 million years old) of Victoria, Australia. Why would small dinosaurs burrow? Some of the reasons were proposed in the story, such as protection of young and maintaining a controlled underground environment, although these are still subject to debate.

A different type of digging by other dinosaurs also has been inspired by unusual trace fossils found in Late Cretaceous rocks (about 75 million years ago) of Utah in 2010. These are interpreted as claw marks made by predatory theropods. The close association of these marks with underlying fossil burrows, inferred as those of mammals, adds another previously unconsidered dimension to dinosaur behavior, which was their preying on small subterranean mammals. Sediment-rimmed nests made by theropods like
Troodon
and some sauropods also imply that these dinosaurs dug up and mounded soil to make these protective structures.

Related to this, a renaissance in our understanding of dinosaur eggs, babies, and the rearing of young has revolved around their trace fossils, too.
Troodon
, a Cretaceous dinosaur from 70 to 75 million years ago and found in parts of western North America, was the first known North American example of a theropod that made rimmed ground nests. These nests also contained clutches of paired eggs, which were arranged vertically in the nests by one or both of the parents after egg-laying. All three trace fossils of
Troodon
behavior—the making of rimmed ground nests, pairing of the eggs, and their post-laying arrangement—provide insights we never would have figured out from their skeletons.

Similarly, a spectacular find of Late Cretaceous nests in Argentina from 70 to 80 million years ago and attributed to gigantic sauropods called titanosaurs shows that dinosaurs other than
Troodon
made ring-like enclosures for their eggs. The sauropod nest structures, however, only superficially resemble those of
Troodon
and are bigger, more abundant, and stacked on top of one another, representing
many episodes of sauropod breeding in the same general area. In this sense, then, did these enormous dinosaurs act like modern migratory birds, returning to the same nesting grounds for hundreds of thousands of years? Once again, this and other questions are ones that trace fossils can help to answer.