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.
The seemingly odd depiction of the gangly theropod Struthiomimus consuming rocks along a riverbank and using these as gastroliths is not too far off from the truth, either. Paleontologists have long suspected that some herbivorous dinosaurs, similar to modern birds or crocodilians, swallowed rocks and used them in their digestive tracts to grind food. This especially made sense for dinosaurs with teeth poorly adapted for chewing yet somehow needing to eat difficult-to-digest plants. What has surprised paleontologists in recent years, though, is the realization that a few theropods, a group of dinosaurs once assumed to have been exclusively carnivorous, also have these “stomach stones.” Paleontologists just assumed that strong stomach acids were sufficient for digesting anything consumed by a theropod. Although no one has yet found gastroliths directly associated with Struthiomimus, some of its relatives, collectively called ornithomimids (“ostrich mimics”), do have them. This fact has prompted paleontologists to start thinking about what these theropods might have eaten other than meat: insects, plants, or a blend of both? Or did these gastroliths have some other uses we still don’t quite understand? And why would some herbivorous dinosaurs with teeth unsuited for chew
ing, such as most sauropods or stegosaurs, not have gastroliths?
Speaking of food, yet another dimension of dinosaur behavior that is much better comprehended through their trace fossils regards what they ate. Traces woven into the opening narrative, such as healed bite marks, toothmarks on bones, wear on teeth caused by plants, and coprolites (fossil feces), tell us much more about dinosaur dietary choices than any other means of fossil evidence. For instance, we can now surmise that Edmontosaurus and Triceratops must have been quite tasty for some tyrannosaurs. This is backed by healed toothmarks caused by a large predatory theropod preserved in a few bones of Edmontosaurus, including at least one with a smoking gun (or tooth, as it were) linking it to Tyrannosaurus or its close relatives.
Triceratops bones also bear toothmarks that could only have been made by tyrannosaurs, including those that mark the front of the face and others showing where they grabbed a Triceratops head shield to separate its head from the rest of its body. Amazingly, not one but two colossal coprolites attributed to tyrannosaurs have been documented, each with finely ground bone and one containing fossilized muscle tissue. From coprolites, we also suspect that at least some Late Cretaceous hadrosaurs ate rotten wood. (Why? Sorry, can’t reveal everything just yet.) We even figured out from dinosaur coprolites that at least a few animals—namely, dung beetles—depended on dinosaur feces as “manna from heaven” to ensure their survival. Hence, these trace fossils bring us much closer to reconstructing ancient ecosystems, piecing together food webs in dinosaur-dominated ecosystems from more than 65 million years ago.
What else can dinosaur trace fossils tell us? Considering extreme ranges in dinosaur sizes, diversity, numbers, geographic dispersal—including the North and South Poles—and evolution throughout their 165-million-year history, dinosaurs clearly played key roles in the functioning of land environments. Again, this is where dinosaur trace fossils have been and will be used to augment or surpass other fossil or geological information. For example, did sauropods and other dinosaurs actually change the courses of rivers or otherwise alter landscapes through their tracks, trails, and other traces? All signs point to yes. Did polar dinosaurs live year-round in those icy environments, or did they migrate seasonally like modern caribou? Trace fossils, such as dinosaur tracks and burrows in sedimentary rocks from formerly polar environments, tell us that they likely stayed put during the winters. How about dinosaur evolution and extinction: What do trace fossils tell us about the timing and causes of these large-scale biological facts of life for dinosaurs? In one recent study, the earliest ancestors of dinosaurs were proposed on the basis of not-quite-dinosaur tracks in 245-million-year-old rocks in Poland from the earliest part of the Triassic Period.
Another important evolutionary step in dinosaurs we’ve documented quite well is that birds evolved from a lineage of small theropods. This relatedness has been certified through many lines of evidence, including fossilized feathers directly associated with the skeletons of more than thirty species of theropods. But we still have questions about this evolutionary transition that are hard to answer from just bones and feathers. For example, when did these small dinosaurs start to climb trees, or fly from the ground up, or land after flight?
The hotly debated question of whether any dinosaurs survived a post-apocalyptic landscape caused by a meteorite impact 65 million years ago is also potentially answerable by trace fossils. A single dinosaur bone in 64-million-year-old rocks is nearly always regarded suspiciously, the fossil equivalent of an online-dating ad in which a person misleadingly underreports his or her age. These bones are very likely recycled, having been eroded out of older rocks, re-deposited, and buried a million years or more after the dinosaur originally died. On the other hand, a single dinosaur track in 64-million-year-old rocks would be hard-to-refute evidence that at least one dinosaur was walking around after they supposedly all died.
So although dinosaur trace fossils certainly can answer questions that range from what an individual dinosaur was doing at a given moment during the Mesozoic to the big picture of how dinosaurs originated, evolved, and went extinct, paleontologists still hope for more. For instance, some of the trace fossils on their “wish lists” are those that flesh out some of the more dramatic encounters dinosaurs very likely had. One that comes to mind would be more trace fossil evidence supporting the oft-depicted scene of large predatory theropods stalking other dinosaurs, or pack-hunting behavior in theropods of all sizes. The first scenario is portrayed in our story toward its end as the tyrannosaur, after her botched attack on the hadrosaur, follows it and the rest of the hadrosaur herd. A similar behavior can be inferred from tracks in Early Cretaceous rocks of east Texas, in which the footprints of a large theropod paralleled and then crossed those of a sauropod, apparently shadowing it. Other compelling theropod trackways include some from the Cretaceous of China that tell of six theropods, equally spaced and all moving in the same direction, which very much looks like evidence of pack hunting. More evidence of pack-hunting theropods is suggested by parallel trackways in Early Jurassic rocks of Utah. Oh, and I should also mention that the two-toed tracks left by the Chinese theropods show they were deinonychosaurs, sickle-clawed theropods related to Dromaeosaurus and Velociraptor. You can bet these theropods weren’t digging for their food that day.
Lamentably, trace fossils clearly illustrating dinosaur mating, like those conjectured for an amorous pair of Ankylosaurus, have not yet been recognized. Because we don’t know for sure what “dinosaur whoopie” trace fossils might look like, these will require an active (perhaps overactive) imagination to detect them, as I have tried to do above. Nonetheless, I expect such trace fossils, including those that precede mating (“wooing” traces, so to speak), will be eventually found and identified, adding to our understanding of what were very likely complex dinosaur sex lives.
In short, the story at the start of this chapter and the myriad dinosaur trace fossils that contributed to its creation demonstrate the huge advantages afforded by these sometimes-underappreciated records of dinosaurs’ daily lives. The main point of the rest of this book, then, is to justify a shift in perspective and start thinking of traces. In other words, after this book, you will no longer just visualize mounted dinosaur skeletons in a museum. Instead, you will think of those skeletons covered by muscles, tendons, and skin, then moving, breathing, mating, eating, fighting, swimming, taking care of their young, and other behaviors, and of traces left by these behaviors. You will also think about the number and variety of traces dinosaurs would have left behind during their normal lifespans, from infancy to old age, and realize how these marks would far outnumber any of their bones. Once you’ve done all of that imagining, you’re ready to look deeper at dinosaur trace fossils, a different way of thinking that’s guaranteed to change what you thought you knew about these long-extinct but ever-popular animals.
CHAPTER 2
These Feet Were Made for Walking, Running, Sitting, Swimming, Herding, and Hunting
Why Dinosaur Tracks Matter
If through some miraculous disaster every dinosaur bone in the world disappeared tomorrow (or the next day, for that matter), the fossil record for dinosaurs would still be represented quite well by their tracks alone. The main reason for this is very simple: each dinosaur only had, on average, about two hundred bones per individual. Yet you could bet that those dinosaurs that made it from mere hatchling to rambunctious juvenile to surly angst-filled teenager to a full-fledged responsible adult probably made many more than 200 tracks during their lifetimes. This supposition alone implies—although we’ll never know for sure—that dinosaur tracks probably far outnumber their bones in Mesozoic rocks worldwide.
The number and variety of dinosaur tracks out there is astonishing. Thus far, dinosaur tracks have been found in eighteen states of the U.S. and on every continent except for Antarctica, with thousands of newly discovered ones each year. Dinosaur tracks range in latitude from the North Slope of Alaska to southern Argentina, and are in roc
ks dating from the beginning of dinosaurs, about 230 to 235 million years ago (mya), to their very end, 65 mya. Although it’s tempting to think of all dinosaur tracks as potholes that would easily swallow a tricycle and its dinosaur-admiring rider, tracks also varied in size from less than the width of a thumbnail to depressions that could be used to park a Smart Car.
Other than their sheer abundance, another comforting thought about dinosaur tracks is that they very often are in places where dinosaur bones are rare or absent. Moreover, they also convey snapshots in time, reflecting a vast variety of dinosaur behaviors in the moment, telling us about a former dinosaur presence in a given place and what they were doing in whatever environment they traversed. Conversely, very few dinosaur bones were buried where a dinosaur lived; that is, most bones were likely moved some distance from their original habitats. As a result, I like to argue that dinosaur tracks constitute the “real” fossil record of dinosaurs rather than their bones, which are nice but, well, just a little too dead. Tracks breathe life back into dinosaurs.
Dinosaur Feet and Footprints through Time
Before jumping into a more detailed discussion of dinosaur tracks, it’s probably a good idea to learn about the main groups of dinosaurs and their feet, which helps to identify dinosaur trackmakers. Paleontologists classify dinosaurs through anatomical traits, and these traits are nearly always related to dinosaurs’ evolutionary history, or their shared ancestry. Ideally, then, each recognizable dinosaur bone can be correlated with about six broad groups of dinosaurs: theropods, pro-sauropods, sauropods, ornithopods, thyreophorans (stegosaurs, ankylosaurs, and nodosaurs), and marginocephalians (pachycephalosaurs and ceratopsians). These groupings of dinosaurs that share a common ancestor—called clades—are best expressed graphically through a branching diagram called a cladogram.
Dinosaurs Without Bones Page 2