Dinosaurs Without Bones (11 page)

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 analyses, 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.

To further back up their claim, they pointed toward a possible perpetrator,
Muttaburrasaurus langdoni
. Skeletal remains of this ornithopod were discovered in 1963 near the small town of Muttaburra in central Queensland, northeast of Lark Quarry, and like many dinosaur finds in this part of Australia a rancher, Doug Langdon, spotted its bones. Still, it wasn’t named officially until 1981, when the paleontologists took care to honor Mr. Langdon’s discovery by naming the species after him. Based on its body parts,
Muttaburrasaurus
was a big dinosaur. Its femur alone was about a meter (3.3 ft) long, which meant its hip height would have been close to the calculated value of 2.5 meters for the Lark Quarry trackmaker. Hence, it seemed a perfectly reasonable dinosaur to pick as a possible large three-toed trackmaker for Early to mid-Cretaceous rocks in this part of Australia.

In contrast, bones of only one large predatory theropod have been found in the same area and rocks of similar age:
Australovenator wintonensis
, lovingly nicknamed “Banjo” after Australian poet and writer of “Waltzing Matilda,” Andrew Barton “Banjo” Paterson. Despite the unabashed wickedness of this dinosaur—gorgeous big-clawed hands attached to long grasping arms, a mouth full of nasty pointed teeth, and a lightweight frame which suggested that it was a speedy predator—this theropod seemed a bit too small to match the trackmaker at Lark Quarry: more
Allosaurus
and less
Tyrannosaurus
. Otherwise, body parts of tyrannosaur-sized theropods are unknown in Queensland, but also in much of Australia with the exception of a few isolated bones in Victoria. Nearly all that is known about big Cretaceous theropods of this entire continent comes from their tracks, which Thulborn and others described from Western Australia in the 1990s, and a few that I and others found in Victoria in 2007–2008.

When Australian paleontologists Tony Thulborn and Mary Wade proposed the “dinosaur stampede provoked by a large stalking theropod” idea in 1979, this was nine years before the publication of Moratalla and his colleagues’ study and thirty years before the world learned about
Australovenator
. So Thulborn and Wade can’t
be faulted for what they didn’t yet know, and they were using the best science known then to interpret the tracks. Also, like I said before (and it bears repeating), they did fantastic work. I consider their 1979 paper a classic in dinosaur ichnology, which they also followed up in 1984 with a considerably more detailed 105-page report.

Nonetheless, it’s certainly possible they made a mistake in identifying the maker of the large three-toed tracks. At the time, they tentatively identified these on the basis of their resemblance to large theropod tracks known elsewhere, which had been allied with tyrannosaur-like tracemakers. Interestingly, objections to this identification, voiced formally by Romilio and Salisbury, were not new, as a few dinosaur paleontologists questioned it soon after Thulborn and Wade’s second article came out in 1984. Romilio and Salisbury were just the first to rigorously test the original hypothesis using statistical methods as opposed to just the scientific equivalent of name-calling.

Backtracking the Walking Dead

So the hypothesis is dead; long live the new hypothesis! This means the popular story of Lark Quarry needs to be revised, and the tour guides there must alter their spiels or at least present “both sides” of the argument. Furthermore, the
Gallimimus
-panicked-by-
Tyrannosaurus
scene in the movie
Jurassic Park
needs to be redone with an alternate version, showing the
Gallimimus
running for some other, unspecified reason. And, without a doubt, a stage play is absolutely necessary: to do anything less would be irresponsible. (I say the last facetiously, but a small troupe of creative folks actually did write and perform a short musical number retelling the original story of the dinosaur stampede, shown at the 2012 Museum Australia National Association meeting in Adelaide. Could a Broadway production be far behind, but now with the anticlimactic ending inserted?)

Not so fast. There’s one little problem with such simple and definitive declarations. What if Romilio and Salisbury were also
wrong? What if they had neglected a few clues in their study, or carried out the research in a less-than-careful way? For instance, we’ve seemingly forgotten all about one of the cast of characters in this drama, which was the medium-sized ornithopod that first left tracks at the site, before the arrival of the terrorized flock and the big … well, whatever. Were its tracks studied in the same way, to test whether it was also an ornithopod or not? Also, how about revisiting
Australovenator
as a possible trackmaker?