Dinosaurs Without Bones

by Anthony J. Martin

  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.

  Paleontologists and paleopathologists alike also must remember to be good scientists by asking a follow-up question, “How could I be wrong?” This is when they contemplate the possibility that their supposed paleopathologic evidence is anything but that. After all, holes, dents, breaks, and other marks in Mesozoic bones (dinosaurian or otherwise) could be trace fossils from other vertebrates, trace fossils from invertebrates, or—most upsetting of all—not trace fossils at all.

  Among vertebrate trace fossils on dinosaur bones, these could be toothmarks made by animals that lived in environments completely separate from dinosaurs. We know that because, for example, some dinosaur bones ended up in shallow-marine sediments. Yet because they were land-dwelling animals, their decaying gas-filled bodies must have washed out from land and floated along until they had a burial at sea. (This explanation for how dinosaur parts got into marine sediments is nicknamed the “bloat-and-float” hypothesis.) Amazingly, a few of these bones have toothmarks and embedded teeth from scavenging sharks, which evidently could not pass up an exotic (albeit rotting) carcass passing through their neighborhood.

  Meanwhile on land, lowly mammals imparted their distinctive incisor incisions on a variety of Late Cretaceous dinosaur bones from Alberta, Canada. These toothmarks were not made by mammals feeding on dinosaurs, exacting revenge for all of their consumed relatives. Instead, they were trace fossils like the tooth-marks left by modern rodents that chew bones to wear down their constantly growing teeth and get more calcium into their diets.

  Insect borings are other trace fossils in dinosaur bones that, at first glance, might be mistaken for dinosaur toothmarks. Carrion beetles, which dine on dead bodies, or termites, which make small pits or tunnels in bones, could have made these. Carrion beetles would have chipped bones with their strong mandibles while stripping flesh for food, just like they do today. Not all termites restrict themselves to wood or soil, and some modern species drill into bone, making themselves at home. As many people might know from watching lurid crime shows on TV, specific insects and their lifecycles are directly associated with colonizing dead bodies. Hence these insect trace fossils tell dinosaur researchers much about what happened after the dinosaurs died, such as whether their bodies were exposed for long or buried quickly.

  So between sharks, small mammals, and insects, recently dead dinosaur bodies were valuable sources of nutrition and potential homes, with the bones lending themselves to lots of tracemaking. What about marks left on bones that were not traces of animal behavior? This is where paleontologists and overzealous ichnologists (guilty as charged) must exercise caution. For instance, plant roots can invade any available spaces in bones and push them apart, making trace fossils that could be mistaken for insect borings. Bones also can gain nicks, scrapes, and dinks from being dragged along a stream bottom. There’s also something about having several kilometers of sedimentary rock overhead that tends to crush bones. All of these non-living phenomena could have wrought false traces, the bane of ichnologists everywhere.

  Bang Your Head, Wake the Dead

  Once in a while, two dinosaur partners of the same species contributed to making one type of trace fossil on bones. Among my favorite examples of these are big holes in the head shields of Triceratops. When paleontologists first noticed these holes, they were a bit mystified. For one, these were not normal parts of head-shield anatomy, which admittedly can become quite holey in some ceratopsians such as Chasmosaurus or Torosaurus. Although most had roundish outlines, these holes also varied in size and location on the head shield. Most important, though, the holes showed signs of injury and healing of the bone, meaning they were lesions. This implied they were wounds acquired during the lifetimes of the dinosaurs, and thus not post-death artifacts caused by erosion, dissolution, or fracturing of the bones and surrounding rock over tens of millions of years.

  So paleontologists Andrew Farke, Ewan Wolffe, and Darren Tanke, in an attempt to make sense of these holes, took a closer look at the sizes, shapes, and placements of them on Triceratops and another ceratopsian, Centrosaurus. In Triceratops, most of these injuries were toward the rear and bottom part of the head shield, having been registered on the squamosal and jugal bones; almost none were in other bones. This clustering suggested that some type of behavior was behind them. After all, if these lesions had been from bone infections, they more likely would have been evenly distributed throughout the head. So were they toothmarks, delivered
by their theropod contemporaries and presumed archenemies Tyrannosaurus rex? No, because Centrosaurus head shields showed almost no holes. Yet because Centrosaurus lived at the same time as Triceratops and Tyrannosaurus, it also should have been on a T. rex menu. (Paleontologists currently have no evidence of T. rex being a fussy eater.) Granted, we also know through trace fossils that Triceratops was eaten by T. rex, and with much gusto. But that ichnological point will have to wait until later.

  Left without disease and predation as explanations, the paleontologists who studied these injuries identified Triceratops itself as the culprit. What anatomical traits did Triceratops possess that could impart such grievous traumas? Were these not mere herbivores, possessing no real means of defense other than through herding together as big happy families? Did their means of meanness ever show on their faces?

  As many dinosaur fans can relate—especially those under ten years old—the genus name Triceratops, assigned in 1889, translates as “three-horned face.” For generations, paleontologists and laypeople alike have looked at those three horns, imagined them pointing forward on a charging Triceratops, and thought, “I would hate to be skewered by those.” Two of these horns, the longest, are positioned one above each eye and thus are nicknamed “brow horns.” These horns give it a bull-like appearance, but remember they were on an animal that weighed about ten times as much as a modern bull. The third horn is more centrally located above the beak (prefrontal) that defined the top of a Triceratops mouth. Paleontologists are also now sure that Triceratops horns changed in size and shape throughout their lives, becoming more formidable with age.

  These horns took considerable energy to grow and were very much a part of these dinosaurs’ lives. Hence it makes sense, evolutionarily speaking, that these horns had some useful purpose, whether to prolong life to reproductive age (“Back off, predatory theropod!”), ensure reproduction (“Hey, baby, check out my horns!”), correctly identify others of your species (“We can do this, right?”), or fend off rivals within your species (“I found her first!”).

  At first, people weaponized these horns, imagining them as deadly counterpoints to ravaging tyrannosaurs. Paleontological artist Charles Knight (1874–1953) most famously depicted such a scenario in a 1927 mural, in which a lone Triceratops stands defiantly in front of a Tyrannosaurus, its paired brow horns pointing suggestively at the predator’s soft underbelly. Since then, children and adults alike have imagined Triceratops fatally goring its attacker with those horns, or Tyrannosaurus somehow slipping past this armature to get in a triumphant killing bite.

  Sadly, ichnology has little to tell us about whether or not Triceratops ever practiced such self-defense arts against Tyrannosaurus or any other predator. For instance, if Triceratops horns ever did pierce tyrannosaur skin, we have no record of it. Tyrannosaurus skin impressions are thus far unknown, and even if such spectacular finds are uncovered, these may or may not have recorded any wounds, let alone ones attributable to Triceratops. One would also think that the force generated by a jousting Triceratops would have left distinctive marks on tyrannosaur leg bones or other skeletal parts, but these are likewise unidentified.

  Only one tantalizing Triceratops skull tells of a living battle with a Tyrannosaurus, in which one of its brow horns and a squamosal bone were chomped and later healed. But this evidence is just enough to provoke far too many questions. Who started the fight, the tyrannosaur or ceratopsian? Was this an argument over food—as in, the Triceratops was potentially food for the tyrannosaur, but objected—or was it over territory, in which one of two powerful animals needed more space? And of course, who ended the fight? Did the tyrannosaur walk away with no injuries, some injuries, or worse? All we know is that the Triceratops survived this testy encounter. Otherwise, without the trace fossils so helpfully left by a Tyrannosaurus on a Triceratops skull, we might not have known about it at all.

  Lacking any clear evidence that horns were used as implements of self-defense against large theropods, paleontologists turned away from violence and reached for sex. That is, Triceratops horns combined with head shields are now regarded more as sexual advertisements than battle gear. In this role, their large heads would have been quite useful for recognizing the same species and perhaps gender, thus neatly avoiding two egregious mistakes sometimes made whenever the urge to mate takes over nearly all reason. Moreover, once these ceratopsians correctly recognized the same species and opposite gender in another ceratopsian, these horns and head shields then may have served a second overlapping purpose as implements for combat. These contests most likely would have been over mates, but also could have been inspired by a need for more territory, scarce food and water resources, protecting young, or an ill-tempered ceratopsian simply deciding to take out its frustrations on another. As a result, colliding heads and piercing horns left their marks as healed holes in Triceratops skulls, leaving us with intriguing scenarios of ceratopsian conflict.

  Pachycephalosaurs, like Triceratops, are also great candidates for having made trace fossils caused by their own species and having those trace fossils left in their skulls. Pachycephalosaurs, such as Pachycephalosaurus, Stegoceras, and others, are relatively rare dinosaurs and only found in Cretaceous Period rocks. They are distant relatives of ceratopsians, having shared a common ancestor before the Cretaceous. Although their limbs are poorly known, they were bipedal, and their teeth look perfectly adapted for a plant-eating lifestyle. So far, no one has interpreted pachycephalosaur tracks, which is completely forgivable as their feet are unknown. Fortunately, pachycephalosaurs are well represented by their opposite ends, which are the tops of their heads. These skullcaps are incredibly thick, and in Pachycephalosaurus can be nearly 25 cm (10 in) thick. They are composed of parietals and closely associated skull bones, which are sometimes accompanied by crowns of spikes and horns. In many instances, such bones are the only evidence of pachycephalosaurs in a given time and place.

  Paleontologists have long wondered why these dinosaurs were such boneheads. Growing bone is energetically expensive and must have meant this trait had some adaptive advantage during the evolutionary history of pachycephalosaurs. How could this have helped them, whether to survive, to have sex, or survive to have sex?

  Nearly everybody agrees that these robust skulls must have been used for butting, as in, these were the Mesozoic equivalent of crash helmets used to protect their heads as they smashed them against something. What reasonable people disagree on, though, is what kind of butting? Did pachycephalosaurs use their heads for defense against predators, such as running full-tilt into the flank of a theropod, leaving it to limp away and make small mental notes not to attack pachycephalosaurs? Did they use them to drive off other herbivores from their favorite plants? Or did they turn against one another and, like their ceratopsian cousins, hit each other, whether for mates, establishing territory, or both?

  The consensus is that pachycephalosaurs’ thick skulls were most likely used for violent confrontations with one another. But figuring out why or how they used their heads is worth lots of discussion. Fortunately, explanations for their skulls can be reduced to just two. One is that pachycephalosaurs were head-bangers, knocking into each other directly with skull-to-skull contact. The other is that pachycephalosaurs went for softer targets, such as torsos, because breaking ribs might have been less risky for an attacking pachycephalosaur than taking on a skull like its own.

  Let’s think ichnologically, then, about what trace fossil evidence would be needed to figure out which of the two scenarios actually happened. Ideally, two pachycephalosaur trackways made at the same time would do the trick. For head-to-head combat, each trackway would have long stride lengths (running at high speed) and along the same line, but directly opposed (pachycephalosaurs ran toward each other), and abruptly ending, with one or perhaps both trackways connecting with much shorter and irregular steps off to the side of the previous trackways (staggering away dizzily). For flank-butting combat, the trackways would be nearly the same exc
ept one of them might be at right angles to the other, ending where they intersect. However, as mentioned before, not one pachycephalosaur track has been identified, let alone a trackway, let alone two trackways, let alone two intersecting trackways made at the same time. So as much as this ichnologist hates to admit it, we must rely on bones to resolve this problem.

  In 2011, two paleontologists, Eric Snively and Jessica Theodor, had head-butting in mind when they looked at skulls of Stegoceras and Prenocephale, both Late Cretaceous pachycephalosaurs from North America. Using a combination of CT (computer tomography) scans and computer modeling of stresses and strains that would have been transmitted to the head and neck by this behavior, they tested whether the additive impacts of two pachycephalosaurs were feasible or not. After all, any such behavior, if performed regularly, might have resulted in permanent head or neck injury, and hence would not have lasted very long in a pachycephalosaur lineage. They then repeated their analysis on three species of modern mammals that habitually head-smash one another: white-bellied duikers (Cephalophus leucogaster), which look much like deer; giraffes (Giraffa camelopardalis) of Africa; and musk oxen (Ovibos moschatus) of North America. Other mammals they studied as control groups were: bighorn sheep (Ovis canadensis), which head-butt when younger but stop such shenanigans once older; pronghorns (Antilocapra americana) of North America, which hit each other with their antlers, but not their skulls; and three mammals that eschew head-knocking altogether, llamas (Lama glama), elk (Cervus canadensis), and peccaries (Tayassu tajacu).