Dinosaurs Without Bones (24 page)

However, this experiment also served as a reminder that for scientists to stay authentic, we should never rest on our laurels, however hard-won those might be. For example, the effort expended to make three trips and back from Atlanta, Georgia to Knowledge Creek in Victoria, Australia, and discovering and documenting possible dinosaur burrows in Cretaceous rocks there, does not give me the inviolable right to omit the word “possible” when discussing them. After all, someday someone with more knowledge, experience, technology, and luck than me may reinvestigate those strange structures and decide that, no, they are something else entirely. On the other hand, someday someone somewhere else might find far better examples of dinosaur burrows, and that dinosaurs we never expected to have burrowed made them. (Still, I am not holding
my breath about sauropods or tyrannosaurs as probable burrow dwellers.)

All of this would be perfectly fine, a happy circumstance of how paleontology, like any science, progresses through the slaying of old ideas and the inclusion of newer, better-tested ones. Burrowing dinosaurs are no different in this respect, but thanks to ichnology, a previously obscure idea about dinosaurs is now there for us to consider, poking its head out of the ground for a look around.

Dinosaurs and Their Embodied Trace Fossils

This is where I cheat. Take a look at the title of this book again, and you will see an absence—“without bones”—as a central theme. So in defiance of that dictum, I will now turn to dinosaur bones and other body parts, such as teeth, for whatever wisdom these can provide.

This concession to body fossils is necessary because many dinosaur trace fossils are in their bones and teeth, or in bones and caused by teeth. A few of these trace fossils announce themselves as broken or otherwise injured bones, some of which could only have been inflicted by other dinosaurs. Sometimes this evidence of dinosaur-on-dinosaur violence suggests that it originated from within a species and perhaps was inspired by competition, whether over territory, a mate, or food. In other instances, injuries were caused by another species of dinosaur that attempted to kill and eat the assaulted dinosaur, although most trace fossils in bones
are marks made after their owners perished. In short, the bodies of dinosaurs or other vertebrates—not sediments—were the places where these marks of dinosaur behavior were recorded.

How do you imagine such trace fossils? You already have. Recall that this book began with a piece of fiction set about 70 million years ago, opening with two rival male
Triceratops
that squared off in combat, and with one losing face. A few other dinosaurs in this tableau—small anonymous feathered theropods—were chewing on pterosaur bones while this ceratopsian drama played out nearby. A tyrannosaur, initially interested in the outcome of the ceratopsian battle but ultimately disappointed with its results, switched her ravenous attention to a herd of
Edmontosaurus
. With these dinosaurs, she bungled her ambush of a young male
Edmontosaurus
but managed to get a snack by biting a chunk of flesh and bone out of his tail. The hadrosaur escaped with only a wound, and he lived happily ever after until dying of some other cause and getting fossilized. For the rest of his life, though, he ate plants near the ground, which were more likely to include grit that scratched his teeth. Although this scenario was imagined, the trace fossil evidence for all such behaviors is real.

This realization that bodies were substrates for dinosaur behaviors enables us to discern much more than just looking at the rocks surrounding and entombing dinosaur bodies. For example, how can one tell whether or not dinosaur toothmarks on a dinosaur bone were punched into or scraped against it while the prey was still alive, dying, or dead? How does one go about identifying dinosaurs that left toothmarks, especially if they were not kind enough to leave a calling card in the form of a dislodged tooth?

Dinosaur toothmarks are not just recorded in the bones of other dinosaurs but also in bones of vertebrates that lived at the same times and places as dinosaurs. Even dinosaur teeth, which in some places might be the only body fossils reflecting a dinosaur presence in Mesozoic rocks, can host trace fossils, too. Some of these traces are extremely subtle, such as the microscopic marks imparted when dinosaurs chewed plants with grit on their leaves or with
mineralized parts. Other traces are more overt, such as broken teeth of carnivorous theropods which must have bitten off more than they could chew.

By looking at these body parts and their trace fossils, many questions about dinosaurs become more answerable. For instance, how did they relate to one another, especially if they became upset with another of their own species? Were predatory dinosaurs picky eaters, only going after one species of prey, or were they more opportunistic, eating whatever appealed (and was available) at the time? Did they normally eat big adults, or did they primarily seek out juveniles, or the old and weak? How did they eat: daintily, with a furtive nibble here and there, or with the gluttonous bad manners of a Renaissance-festival banquet? Did some dinosaurs prey at all, or did they live their lives mostly as oversized vultures, relying on the kills of other dinosaurs or other already-dead, ready-to-eat meals? Did any dinosaurs ever succumb to the evolutionary taboo of cannibalism by eating—or at least biting into—their own kind?

All of these are behaviors about which we might otherwise speculate if not for trace fossils becoming more pieces in the puzzle and completing pictures of how dinosaurs lived their daily lives.

It’s Only a Flesh Wound

Through the power of trace fossils, paleontologists can tell by looking at a dinosaur’s body that another dinosaur attacked it. Yet, as is typical in science, such dramatic interpretations can be equivocal. Such a quandary is exemplified by a 2013 discovery of fossilized dinosaur skin from South Dakota. Although this “skin” was actually just a natural cast of the original skin, it was closely associated with the bones of
Edmontosaurus annectens
, a Late Cretaceous hadrosaur from the western U.S. and Alberta, Canada. Yet the skin itself is not the trace fossil. Instead, a probable trace fossil is
in
the skin. Whether this trace fossil was made by the hadrosaur or inflicted by another dinosaur, though, is subject to vigorous debate, as explained here.

Owing to their rarity, dinosaur skin impressions give paleontologists good reason to celebrate such finds. These unusual body fossils were normally formed first as impressions against soft sediment, and then naturally cast in sandstone, similar to how many dinosaur footprints were preserved. (Along those lines, the best-preserved dinosaur tracks have scale impressions, but these are the marks of living skin, not the skin itself.) This particular patch of skin had an irregular feature which paleontologists interpreted as an apparent healed injury. This interpretation provoked a spirited discussion about what constitutes good trace fossil evidence for a dinosaur attacking another dinosaur.

The skin impression and the irregularity on the skin are surprisingly small. The patch of skin measures only about 12
X
14 cm (4.7
X
5.5 in), about the size of a notebook that can fit in a shirt pocket. The oddity on that patch stands out from the main scaly pattern, looking like a partly closed human eye and coincidentally about the same size as one. It has a raised exterior and indented interior, as if something sharp had punched through the skin, then the skin annealed around the wound.

So here are two scenarios for the given evidence, in which everyone accepts the basic premise that this is indeed a healed injury:

Scenario A: Hadrosaur was walking peacefully through a forest, minding its own business, while unknowingly stomping on and otherwise terrorizing insects, amphibians, and small mammals. Suddenly, a ferocious tyrannosaur bursts out of hiding, ambushes the hadrosaur, and gets in a good bite. At least one tooth punctures the hadrosaur’s skin and damages the bone underneath. Luckily for the hadrosaur, but sadly for the tyrannosaur, the injured dinosaur escapes and lives long enough for its skin to heal. The end.

Scenario B: Hadrosaur was walking peacefully, but awkwardly, through the forest; perhaps it was a teenager.
Suddenly, it stumbles into a tree with prominent, sharp spines. At least one spine punctures the hadrosaur’s skin and damages the bone underneath. Fortunately for the hadrosaur, it extracts itself from the tree and lives long enough for its skin to heal. No one knows, or cares, what happened to the tree. The end.

So which hypothesis do you want to be true, especially if you absolutely adore the notion of huge and horrific theropods chomping on hapless ornithopods? Which do you think would make for a sexier story in a news release written about the study? Which one is most likely to be recreated through the latest CGI technology and show up in a cable-TV special on dinosaurs in the next few years, narrated by semi-comatose celebrities or overexcited paleontologists? Why, yes, A, A, and A are all correct answers. Yet Scenario B has not been disproved, nor have all sorts of variations on the interpretation that this fossil malady was not caused by a ravenous predator. How about a sharp rock? Another hadrosaur in a fight over a mate or meal? Or, most ignoble of all, a festering sore caused by a bacterial or fungal infection?

Just like the hadrosaur, you get the point. Trace fossils of injuries preserved in dinosaur bodies become difficult to interpret when we can’t distinguish whether these wounds were caused by another dinosaur, a non-dinosaur animal, or self-inflicted (albeit accidentally). Also, some of these marks may have been made through other means, such as infections, which are not trace fossils at all. This is where the study of dinosaur health problems enters a realm of forensics that gets contentious, just like evidence for a court case going to trial.

Acting as the prosecuting attorney, however, I purposefully withheld some crucial evidence until the last part of the trial so that it would have a maximum effect on the jury. You see, the skin impression from this hadrosaur was directly next to its skull, and the skull had toothmarks. Furthermore, the toothmarks were widely spaced, and most were from an animal with pointy teeth and large enough
to attack an adult
Edmontosaurus
. Even better, these toothmarks on its skull also show signs of healing. So, ladies and gentlemen of the jury: Is it not true that these toothmarks came from a carnivorous dinosaur? Is it not true that they match the lineup of teeth in the mouth of a tyrannosaur? Is it not true that tyrannosaurs liked to dine on
Edmontosaurus
? Is it not true, then, that the puncture mark on the skin came from the same attack as the skull?

Well, maybe. After all, even with healed toothmarks on its skull and a nearby patch of skin with a healed puncture wound, this specimen of
Edmontosaurus
may have had two mishaps separated in time, with only one inflicted by a large theropod. However, if I were a betting ichnologist, I would wager that the toothmarks in the skull comprise the real evidence of a tyrannosaur attack. In contrast, the healed wound in the skin impression is more circumstantial and might be from a separate injury that the hadrosaur did to itself.

So why did these trace fossils get reversed in importance by the paleontologists who studied them? Probably because dinosaur skin impressions are rare, evidence of wounded skin is rarer, and evidence of healed skin is exceptional. Hence it was the skin that justifiably became the focus of the study, whereas the multiple toothmarks in the bone below it became additional information, shuffled to the background because they seemed so ordinary—which they most assuredly are not.

Old Ailments, Traces, and False Traces

The discernment of dinosaur disorders falls under the category of
paleopathology
, which is the study of ancient ailments. Although not all of paleopathology concerns itself with dinosaurs—much of it centers on evidence for diseases in pre-historic human remains—this science is being applied enthusiastically to dinosaurs. In the practice of paleopathology, physicians and veterinarians sometimes collaborate with paleontologists in a neat mix of old and new.

Still, we must always return to basic principles of ichnology before allowing ourselves to be dazzled by other sciences. For any paleopathologic evidence in fossil bones, skin, or teeth to qualify
as a dinosaur trace fossil, it must have behavior behind it. This criterion is necessary regardless of whether a trace fossil was made by the dinosaur with the injury, from another dinosaur, or from a non-dinosaur animal.

For example, one specimen of
Psittacosaurus
from Early Cretaceous rocks of China also, like the
Edmontosaurus
, has a skin impression associated with its body, and the skin impression has two apparent puncture marks. But these marks show no sign of healing. Accordingly, paleontologists would simply say these traces are from predation or scavenging by another animal that intended to eat this already-dead
Psittacosaurus
. Possibly the tracemaker was a theropod, or maybe it was another animal that also had pointed teeth, such as a lizard or crocodilian. So rather than showing how this dinosaur escaped an attack, with its punctured skin later closing up and leaving a noticeable scar, this was evidence that its inert body was included on some animal’s meal plan.

Thus basic ichnology is combined with basic paleopathology whenever a paleontologist looks at a dinosaur skin impression or bone and notices an abnormality: holes, dents, breaks, enlargements, or other traits that stand out as something extra, something that was not part of its original anatomy. Even chipped teeth fall into this category, in which a theropod lost part of a tooth. When this happens, the most basic of questions a paleontologist can ask is “Dead or alive?” As in, was the dinosaur out of commission for good, or was it still moving under its own power when the feature was added? If living, the trace fossil might be attributed to the dinosaur itself, although in some instances another dinosaur might have caused it while the two were tangling with each other. If dead, though, another tracemaker was entirely responsible for whatever trace fossil is preserved in a dinosaur’s bones or teeth, which in some instances might have been a dinosaur, too. However, a composite trace fossil also could have been made from the behavior of the dinosaur with the preserved injury combining with the behavior of the dinosaur that dealt the injury.