Dinosaurs Without Bones

by Anthony J. Martin

  The paper was reviewed in a timely way and fortunately was not rejected outright. Still, the reviewers and editor expressed concern about its content, requiring responses to specific points. As mentioned before, this is where many scientists might give up on both revising and resubmitting a manuscript, without the guarantee of acceptance. However, I had already planned to go back to Australia and promptly put Knowledge Creek on my itinerary to answer a few questions, such as the exact nature of the sedimentary rocks surrounding the structures I claimed to be dinosaur burrows.

  Thus my third (and perhaps last) visit to Knowledge Creek took place in May 2009. This time only three of us went: my wife Ruth (her second visit); Mike Hall from Monash University of Australia, an experienced field geologist who also was an expert on sediments and sedimentary rocks; and myself. Mike had never been there, and it had been long enough for me that we became disoriented and displaced—some might call it “lost”—on the way down to the site. This was a disheartening déjà vu for Ruth, echoing her experience from three years previous with me and other geologists, and justifiably shaking her faith in our navigational abilities. Her unease grew more pronounced when Mike proposed that we circumvent the nearby sheer sea-cliffs by following a wallaby trail through the thick coastal scrub forest. Travails notwithstanding, we made it to the site okay, and once more Ruth and I stood together on the nearly flat marine platform of Cretaceous strata of Knowledge Creek. This time we had plenty of daylight ahead of us, though, and Mike’s help as a geologist to double-check my field results.

  This trip was well worth it, as we noted what might have been yet a third partially preserved dinosaur burrow. It was shorter than the other two but almost the same diameter and had a similar sandstone fill. This implied that all three had once been hollow structures and had likely been filled by the same sedimentary processes at the same time. Elsewhere on the outcrop, in strata above the supposed dinosaur burrows, we also measured and otherwise documented dozens of invertebrate trace fossils, all small-diameter burrows. With only three of us there, we even had enough time after my data collection to explore our gorgeous surroundings.

  It was a magical experience, a sense of wonder evoked by walking on the remains of polar rivers from more than 100 million years past, as waves from a present-day sea churned violently against these same rocks just behind us. All paleontologists who have done field work end up having favorite places where they feel lucky to learn and realize something new about the history of our planet. Knowledge Creek had become one of mine.

  Once back in the U.S., I finished the revisions of the manuscript, wrote a rebuttal to the reviewer remarks that nearly eclipsed the length of the manuscript itself, inserted a mention of a possible third burrow at the same site, and resubmitted it. Gratifyingly, the editor accepted the paper the following week, which had the title “Dinosaur Burrows in the Otway Group (Albian) of Victoria, Australia, and Their Relation to Cretaceous Polar Environments.” Unexpectedly for me, though, the accepted manuscript was also posted online only a week after that, which meant that science reporters began calling me, asking about these dinosaur burrows in Australia, the oldest interpreted from the geologic record.

  My university media folks and I scrambled to put together a press release, and we used edited video footage from Knowledge Creek to accompany it as a sort of modern-day newsreel. As of this writing, the video had the most views of any in the history of my university, and the press attention was flattering. So two places in the world—Montana, USA and Victoria, Australia—had been proposed as sites with dinosaur burrows, with the hope that more would be added to this list in upcoming years, as paleontologists now knew what to look for.

  “Mythbusting” a Dinosaur Burrow

  Many scientists welcome media attention or other forms of interacting with the public, whether through lectures or writing popular-outreach pieces such as magazine articles or blog entries. Yet a mantra I often preach to my students and try to put into daily practice is that these attempts at public outreach and communication do not necessarily make our science truer. As I mentioned previously, science does not prove, it disproves. Hence, paleontologists who live up to this ideal by testing their own results—treating their work to the same degree of scrutiny and skepticism as they would their rivals’ research—always impress me. It is an intellectual honesty we all need to practice, such as in interpreting the indirect evidence represented by dinosaur trace fossils.

  Along those lines, Dave Varricchio assigned an undergraduate student of his, Cary Woodruff, to do just that. Woodruff’s job was to test whether the Oryctodromeus bones could have been buried while in the den (that was the original hypothesis) or whether they were carried in from outside (that’s the alternative hypothesis). If they were moved from outside of the den, this would be a strike against these dinosaurs having lived and died there and would have cast doubt on their having been the original burrow inhabitants. Instead, their dead remains may have been tossed into a big hole, burrow or not, that just happened to be in the area when a river overflowed back in the Cretaceous.

  So in an experiment straight out of the popular TV show Myth-busters, Woodruff joined PVC piping and set up scaffolding to make a half-sized version of the burrow. He then made up mixtures of sand, mud, and water, and used rabbit bones as a proxy for the dinosaur bones. Thirteen times he ran the experiment, in which he filled a plastic bucket with sediment and poured it down the hole. Sometimes rabbit bones were added to this sedimentary stew, but other times he placed the bones inside the artificial burrow chamber first, then decanted. Out of the thirteen times he did this, six resulted in the same sort of jumbled distribution of bones in the burrow chamber that Dave had observed with the original Oryctodromeus bones and their sedimentary matrix. Four of these arrangements were made with the rabbit bones already in the den, and two came from outside, with the bones in the bucket.

  Four to two: we win! Except not really, because science is not a game in which simple scores decide which hypothesis is the better one. These results still meant the bones feasibly could have been deposited into the burrow and that the Oryctodromeus bones could have come from somewhere else outside of it. Woodruff then tried to disprove this hypothesis by scrutinizing the original Oryctodromeus bones, seeing whether they held any nicks, scratches, or dents from having been bounced along the bottom of a stream. Many dinosaur bones bear such evidence, telling how those parts may have traveled far from the spot where a dinosaur died. However, in this instance the bones had no such marks. This lack of evidence implied that the bones of this probable parent and offspring did not travel far at all. They either died together just outside of the burrow, or in it.

  But wait: Could the burrow have belonged to some unknown burrowing predator that scored a super-sized meal one day by taking down an adult and two juveniles of the same species? Then maybe it dragged these bodies into its burrow to nosh on them, left its partially consumed dinner in the burrow, and was conveniently somewhere else when its prey’s bones were buried by the flooding of a nearby river? For that idea to have more support, though, more trace fossils were needed, such as toothmarks on the bones. Yet none were to be found. Thus, considering the happenstance of bones from an adult and two juveniles of the same dinosaur species being together, no signs of long-distance transport of those bones, no evidence of anything chomping on the bones, and all of the aforementioned ichnological evidence showing a match between the burrowmaker and the burrow, the hypothesis that this Oryctodromeus cubicularis was a burrowing, denning dinosaur still stands. So far, so good.

  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. Af
ter 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.

  CHAPTER 6

  Broken Bones, Toothmarks, and Marks on Teeth

  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 st
ory 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?