Dinosaurs Without Bones

by Anthony J. Martin

  Drs. Thulborn and Wade came up with this scenario once they measured the directions of the individual trackways and distances between tracks within each trackway. Using the formula devised by R. McNeil Alexander for calculating dinosaur speed based on track length, hip height, and stride, they figured these dinosaurs were moving at about 12 to 16 kph (7.5–10 mph), which is quite fast for short-legged dinosaurs. Furthermore, the tracks mostly showed toe-tips touching the ground instead of entire digits. Lastly, some tracks were greatly elongated, indicating that slipping and sliding happened during their journeys. Very simply, when small animals take big strides, get up on their toes, and lose their footing on moist ground, they are running. To find out just how many dinosaurs were involved in this collective action, they walked a line perpendicular to the direction of movement and counted a minimum of 130 animals; about 55% of these were theropods and 45% ornithopods.

  So instead of my recounting more of their evidence, I will let words from Drs. Thulborn and Wade’s first research paper about the site, published in 1979, speak for the feelings these tracks evoked. In reading this, note how they broke an unspoken rule in paleontology, in that they expressed an emotional empathy with animals from nearly a hundred million years ago:

  Persuasive circumstantial evidence leads us to conclude that they represent a stampede—that is, a wild, unreasoning and panic-stricken rush to escape the threat of danger. What could have caused such presumed panic?

  It was a great question, and in their attempt to answer it Thulborn and Wade pointed to the large three-toed tracks that entered the scene from the left. These tracks originated from the same direction taken by more than a hundred small dinosaurs, some of which ran around and onto the tracks of the big dinosaur, interpreted by the paleontologists as those of a big theropod. How big? As mentioned before, dinosaur sizes can be estimated by their tracks: just multiply the length of the footprint by 4.0, and you have the approximate hip height of the dinosaur. In this instance, the best-preserved track was 64 centimeters (25 inches) long, so its hip was about 2.5 meters (8.3 feet) off the ground. Just to put this in perspective, this is higher than the tallest basketball player in the NBA, and would have been big enough to make you and me run, too. So imagine the fright felt by a much smaller ornithopod or theropod from the approach of such an imposing predator.

  Compounding this effect would have been the contagious fear spreading instantly through a sizeable group of small dinosaurs, instigating a chain reaction of similar behaviors. Think of the arrival of a fox in a chicken yard, or even a human walking up to a group of shorebirds, and how the jittery reaction of one bird is enough to spook the others, thus causing all to share the terror.

  Bolstering this idea of a panic-inducing theropod was its trackway pattern, which was unusual. Based on their measurements, Thulborn and Wade figured it was moving slowly, at only 8 kph (5 mph), and because it crossed its right foot over its line of travel, then its left foot, it may have even paused at some points before picking up speed and turning sharply to the right. Frustratingly, this is also the point where the trackway reaches its end, forever eroded. This odd trackway pattern, expressed by only eleven footprints, led Thulborn and Wade to propose that the theropod might have been stalking, looking for prey in a target-rich environment.

  Thulborn and Wade’s map of the tracksite and other analyses of the tracksite constituted a masterpiece of meticulousness, supplying a wealth of data to support their interpretations. From their work they published two peer-reviewed papers, the first with the provocative title of “Dinosaur Stampede in the Cretaceous of Queensland,” published in 1979. The second, published in 1984, was a much longer and more detailed report modestly titled “Dinosaur Track-ways in the Winton Formation (Mid-Cretaceous) of Queensland.”

  I still point to the 1979 paper as one of the most compelling I have ever read in ichnology and often re-read it to remind myself of what constitutes a “gold standard” in the traditional study of dinosaur tracks. I also keep in mind that Thulborn and Wade completed their study of 3,300 tracks without the benefit of tools we now take for granted: no high-resolution digital cameras, digital calipers, global-positioning systems (GPS), geographic information systems (GIS), image analysis, laser scanners, 3-D modeling, Internet, or other technological shortcuts that would have made such a study far easier. Just heaps of hard physical labor, lots of surveying and other measurements, and scientific reasoning, served with a healthy dollop of intuition on top. And the end result was a tale that still astonishes.

  This story of a dinosaur stampede in Queensland was so intriguing that it is rumored to have inspired one of the more spectacular scenes in the movie Jurassic Park (the first one, that is, not its awful sequels). In this scene, a flock of the Late Cretaceous theropod dinosaur Gallimimus (“ostrich mimic”) stampedes in fear at the nearby presence of Tyrannosaurus rex, the most redemptive character of the movie, and who was also from the Late Cretaceous. (The casting of both dinosaurs, along with Triceratops, Paralophosaurus, and Velociraptor, also illustrates why Jurassic Park should have been titled Cretaceous Park instead.) Unfortunately for the prey but fortunately for the predator, one member of the flock is separated from its compatriots and singled out for sacrifice, while the others continue running for safety.

  Although perhaps more than a hundred million people have watched this scene in the movie, I’ll bet a six-pack of my favorite adult beverage that less than 1% of these people know how it mirrors the initial scientific interpretation of the Lark Quarry tracksite. Of course, locals in Winton know about this piece of fiction imitating fact, and I have heard them joke about how nice it would have been for the Lark Quarry Conservation Centre to receive a tiny royalty from the film profits as a sort of honorary tithe. Regardless, I have been pleased to contribute to the local Winton economy in a non-Spielbergian way by visiting the tracksite, bringing others there with me, staying in Winton, purchasing pub meals, singing Waltzing Matilda, and of course quenching my thirst with Queensland-produced beers, all ably demonstrating some of the touristic and economic impact of dinosaur tracks, or what I like to call “ichnotourism.”

  What Made the Three-Toed Dinosaur Track?

  In paleontology, just like any other science, a scientific hypothesis might rule for several decades or more, enter the public realm, and become part of popular lore. Nonetheless, because the science behind such stories is always changing, this means that what we took for granted as a “true story,” even one we like very much, can be upended in a way that surprises everyone, including the paleontologists revising its narrative. Such is the situation with the “giant stalking theropod dinosaur causing a small dinosaur stampede” story of the Lark Quarry dinosaur tracksite in Queensland.

  This tale, which has reigned for more than thirty years and is known worldwide by paleontologists and laypeople alike, faced a radical makeover in the light of new evidence presented in a paper published by paleontologists Anthony Romilio and Steven Salisbury in 2011. The fresh hypothesis states that the “dinosaur stampede” was not triggered by the arrival of a predator, and no stalking of other dinosaurs by a voracious predator happened either. Yet this new evidence and its accompanying explanation has not been completely accepted, and in fact is facing a fierce challenge from one of the original researchers, Dr. Thulborn. In other words, don’t grab an eraser just yet, as the controversy may not be resolved.

  The story of Lark Quarry and its dinosaur tracks illustrates very well a constant feature of science, which is that it is always evolving. This basic principle certainly has been realized in a grand way with evolutionary theory, gravitational theory, plate tectonic theory, and quantum theory. Lots of testing, scrutinizing, and arguing took place before the majority of the worldwide scientific community accepted any of these broad-sweeping, unified explanations for much of how our world works. But these theories also underwent considerable changes—an ongoing process.

  The same standard applies to more focused hypotheses of all shapes a
nd sizes, including those concerning dinosaur tracks and other trace fossils, which is that new evidence or perspectives can modify these, making for better fits. Along those lines, in science we do not prove, we disprove, meaning that old stories are sometimes revised in the face of novel finds. Furthermore, answers invariably generate more questions, meaning that although we may get ever so much closer to the truth with each investigation, we also keep in mind that we may never quite reach it absolutely, especially when we deal with events of the distant geologic past.

  The overturning of long-held paleontological hypotheses by trace fossils is not uncommon, either. As mentioned previously, fossil tracks reported from Poland dating to the Early and Middle Triassic Period (about 245 mya) indicated that the immediate ancestors to dinosaurs were extant earlier than previously thought. In my own research in 2009, I proposed the oldest known dinosaur burrows in Cretaceous rocks of Victoria, Australia, one of which was eerily similar to the first known dinosaur burrow, reported from Cretaceous rocks of Montana (USA) in 2007 (explained more in the next chapter). In the non-dinosaur realm, the oldest tracks attributed to four-legged animals from 395 mya precede their body fossil record by about ten million years. In short, trace fossils matter in paleontology, whether for supporting new hypotheses or knocking down old ones.

  So just what constitutes the evidence and scientific basis for a dramatic re-write in the story of the Lark Quarry dinosaur tracksite? Also, how could this new interpretation be wrong, exemplifying the fair warning that in science, just because something is newer doesn’t mean it’s also better? The thought of an ornithopod impersonating a large theropod—the dinosaurian equivalent of it wearing clown shoes (or, more aptly, “Bigfoot” shoes)—is not only a shocker for anyone who has grown up hearing the tale of Lark Quarry, but it borders on heresy. In science, though, we rather enjoy slaughtering sacred cows, making burgers out of them, adding a couple of slices of bacon, and putting a fried egg on top. After all, hypotheses are only accepted conditionally, and then are subject to further testing so we can find out whether or not they still hold up to scrutiny.

  Sometimes these hypotheses continue to stand (so far, so good) but more than a few get modified or knocked down completely. And if they get knocked down, it’s often because someone found data that better supports an alternative hypothesis, or “another story.” Granted, “recycling” also happens sometimes, especially with scientists who get a little too attached to a pet hypothesis, perhaps long after it’s been pronounced dead by everyone else (hey, egos happen). But for the most part, consensus is based on the evidence—not the people, their degree of self-promotion, or the volume of their message.

  So just what was the evidence at Lark Quarry supporting the previous hypothesis (which can be summarized as “big theropod maybe caused a panic because it was preparing to kill and eat an ornithopod”), and how does this contrast with the evidence supporting the new hypothesis (“big ornithopod maybe caused a panic, but for different reasons than eating another dinosaur”)? It all comes down to a common dilemma in dinosaur ichnology, and one that has been around for more than two hundred years, which is how to distinguish three-toed dinosaur tracks from one another and interpret who made the tracks.

  So let’s look at how the continuing dilemma of three-toed dinosaur tracks figures in all of this. The 2011 study of the Lark Quarry dinosaur tracksite by Romilio and Salisbury involved: a close look at the dinosaur tracks as they are preserved today at Lark Quarry; studying casts that were made of the dinosaur tracks soon after they were excavated in the 1970s; and lots of statistics, which I will do my best to explain to any non-scientific (yet admirably geeky) readers who might need it. Still, the heart of Romilio and Salisbury’s 2011 interpretation was a focused reexamination of the large dinosaur tracks. Although these large tracks dwarf the others at Lark Quarry, as mentioned before, they are relatively few in number, with only eleven such footprints recorded on a surface that contains more than 3,300 tracks. That’s right: The key plot element of the original story of Lark Quarry hinges on a sequence of only eleven tracks, and identifying what made those tracks.

  Why identifying the maker of these big tracks is so difficult is mostly attributable to the tracks only having three toes, a trait also known as tridactyl. These tracks are also more or less mirror images on either side of the middle toe, a condition called mesaxonic. Furthermore, because bipedal dinosaurs made such tracks, their trackways normally show an alternating right–left–right diagonal walking. (Incidentally, now that you know these nifty terms, be sure to incorporate them in your daily conversations, such as “Wow, your chicken leaves some of the best tridactyl mesaxonic tracks in a bipedal trackway I’ve ever seen!”)

  This little checklist helps to narrow down the possible dinosaur trackmakers, in that we know it is definitely not a stegosaur, ankylosaur, sauropod, or ceratopsian, all of which walked on four legs (quadrupedally) and had feet with more than three toes. Well, except for stegosaurs which had tridactyl rear feet, but as far as we know stegosaurs never walked bipedally, so they’re eliminated as suspects too. This means you should be thinking “ornithopod” or “theropod” for nearly all three-toed fossil tracks in rocks from dinosaur times. But also keep in mind how some ornithopods also left small front-foot impressions, a result of sometimes walking quadrupedally. Theropods, in contrast, almost always moved on just two legs, although, as you learned earlier, a few rare instances of hand impressions show up in trace fossils made where they stopped briefly to sit.

  So now let’s say you found some three-toed dinosaur tracks and you want to figure out whether these are from a theropod or an ornithopod. The most basic way to tell the difference is to measure the track length and width, then compare the two. On average, theropod tracks are longer than they are wide, whereas ornithopod tracks are wider than they are long. This means a length:width ratio for a theropod track will be >1.0, and <1.0 for an ornithopod. The tracks you’ve examined have ratios of 0.8 to 0.9, so these must be from ornithopods! Right?

  Oh, if only ichnology were so simple, where we could all be so satisfied with our results, teeming and preening with confidence. Okay, time for an exercise in humility. Let’s go through a few questions and see how you do in answering them:

  How would you describe the toes relative to the overall length of the track: thin, medium, or fat?

  Did those toes end with sharp clawmarks or blunt ones?

  What did the “heel” (posterior of the track) look like?

  Did you take into account how the substrate preserving a track, which might have been wet mud, dry sand, or moist muddy sand, might have affected the overall outline of a track?

  Did you think about how the dinosaur stopping suddenly, turning to the right, or moving its head might have distorted the track outline?

  Did the dinosaur, when extracting its foot from the mud or sand, cause sediment to collapse into the toe impressions and thus change the character of the track?

  Did you also think of how the substrate drying out might have changed a track outline before it was fossilized?

  Are these tracks undertracks, and if so, how far below the original surface were they formed?

  Do you feel clueless yet? (Welcome to my world.)

  Along these lines, three Spanish paleontologists—Josè Moratalla, Josè Sanz, and Santiago Jimenez—tried to take some of this guesswork out of distinguishing theropod and ornithopod tracks. In an article published in 1988, they used a sample of 66 Early Cretaceous tridactyl dinosaur tracks from Spain, all of which had been identified confidently as either ornithopod or theropod tracks on the basis of their qualities. With these tracks, they measured nearly every parameter they could imagine: digit lengths, digit widths, angles between digits, widths of the foot between digits, and more. Moratalla and his coauthors then compared ratios of these parameters—such as digit length:digit width—to see which ones were significantly different from one another (statistically speaking).

  From these an
alyses, they figured out “threshold” values and probabilities for some of the ratios and calculated probabilities of a ratio belonging to an ornithopod or theropod. For instance, if the length:width ratio of a tridactyl track is above 1.25, or 25% longer than it is wide, then there was an 80% probability that the track belonged to a theropod. Fortunately, they didn’t just stop with the length:width ratio. They also checked all other ratios to see whether these consistently show a high probability of a theropod trackmaker or not, just to retest their initial identification.

  With the publication of this study in 1988, dinosaur ichnologists had a quantitative checklist they could apply to three-toed dinosaur tracks. Of course, whether all dinosaur ichnologists actually read this paper, applied its methods, or tested their applicability to dinosaur tracks other than the ones they studied is another matter. Regardless, numbers produced by such a study also could be combined with non-numerical observations to test whether or not a fearsome carnivore or a peaceful herbivore had made a given series of three-toed dinosaur tracks. One example of such an observation is whether a track has sharp clawmarks or not. This feature is present in theropod tracks, whereas ornithopods tend to have more rounded or blunt ends to their toes.

  Just to make a long number-laden story a little shorter, Romilio and Salisbury calculated threshold values based on every parameter they could measure from the eleven big “theropod” tracks. Once these numbers were compared to those that Moratalla and his colleagues figured for their Spanish dinosaur tracks, the numbers fell into the “ornithopod” range rather than “theropod.” Accordingly, Romilio and Salisbury then concluded that the original trackmaker had been wrongly identified as a theropod and proposed that it must have been a big ornithopod.