Dinosaurs Without Bones (3 page)

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 chewing, 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.

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 rocks 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
.

In the simplest cladogram for dinosaurs, theropods are more closely related to prosauropods and sauropods than they are to ornithopods, whereas stegosaurs, ankylosaurs, and nodosaurs are more closely related to one another than they are to marginocephalians. Also, because birds descended from theropod ancestors and thus qualify as dinosaurs, these are included on any self-respecting dinosaur cladogram. But if you want to be even more of a “lumper” with classifying dinosaurs, you could go back to their initial split into
saurischians
(“lizard-hipped” dinosaurs), which includes all theropods (birds too), prosauropods, sauropods, and
ornithischians
(“bird-hipped” dinosaurs), which are all ornithopods, thyreophorans, and marginocephalians.

Dinosaur feet can be roughly correlated with the evolutionary history of dinosaurs, based on the appearance and disappearance of these clades in the fossil record. For example, the hypothetical “first dinosaur,” which would have evolved about 235
mya
and was the common ancestor to both saurischians and ornithischians, probably walked on its rear two legs (bipedal) and its feet would have had three prominent toes (digits) pointing forward, one toe off to the side, and all toes tipped with claws. Its front feet, had it also used these for walking (making it quadrupedal), would have had five fingers (also digits), again all pointing forward and with claws. All subsequent dinosaur feet were modified from this basic body plan, whether certain dinosaur lineages stayed bipedal, went to quadrupedal, or used some mixture of the two. As a result, dinosaur tracks made by feet that had more than four digits in the rear are rare—happening only with sauropod tracks—and more than five digits in the front would be really weird. If anything, most dinosaur feet reduced or lost digits throughout their evolutionary histories. Unneeded toes or fingers, which can be evolutionarily expensive, were weeded out by survival and propagation of species, in which it was advantageous to get rid of these over time.

Just as modern trackers might classify mammal tracks by the number of toes, the same can be done with dinosaurs. For the major evolutionary groups of dinosaurs, the following modes of movement
and digit numbers, with only a few exceptions, can be applied to help with identifying their tracks:

• Theropods—rear feet only (bipedal), three digits pointing forward, and sometimes another small one off to one side.
  
• Prosauropods—rear and front feet (bipedal or quadrupedal), four digits on the rear, four on the front.
  
• Sauropods—rear and front feet (quadrupedal), five digits on the rear and five on the front (although digits are almost never visible in sauropod front-foot tracks).
  
• Ornithopods—rear feet only or all four feet (bipedal or quadrupedal); on the rear feet, three digits pointing forward, sometimes another digit off to the side, whereas front feet had five or less.
  
• Stegosaurs—rear and front feet (quadrupedal), rear feet with three digits and front with five.
  
• Ankylosaurids and nodosaurids—rear and front feet (quadrupedal), four digits in the front and four or three in the rear; most ankylosaurids (perhaps all) had three.
  
• Ceratopsians—rear and front feet (quadrupedal), four digits in the rear and five in the front.