Sunday, January 9, 2011

Who Made the Three-Toed Dinosaur Track?

In paleontology, occasionally a scientific hypothesis rules for several decades or more and enters the public realm, becoming part of popular lore. Nonetheless, science is always changing, which means that what we took for granted as a “true” story can be upended in a way that surprises everyone, perhaps even the paleontologists doing the revising. Such is the situation with the “giant-stalking-theropod-dinosaur-causing-a-dinosaur-stampede” story of the Lark Quarry dinosaur tracksite in Queensland (Australia). This tale has been reigning for more than 30 years and is known worldwide by paleontologists and laypeople alike, but now faces a makeover in the light of new evidence.

Here are some dinosaur tracks from Queensland, Australia. The dinosaurs that made them had three toes on their rear feet, and walked only on those two feet (bipedally). So were these from theropod dinosaurs or ornithopod dinosaurs? Don’t know? Then I guess you’ll have to read more, won’t you? Display is at the Museum of Tropical Queensland in Townsville, Queensland (Australia). Note the stylish sunglasses (lower right) for scale.

As mentioned last week, the previous story of Lark Quarry is now challenged in a recent study by Anthony Romilio and Steven Salisbury of the University of Queensland, and published last month in the journal Cretaceous Research. In this study, these paleontologists say the “giant stalking theropod” (= bloody big carnivore with small-dinosaur tucker on its mind) that supposedly made big, three-toed tracks was actually a “giant walking ornithopod” (= still bloody big, but sooky and looking for a salad bar instead). This dinosaurian equivalent of wearing clown shoes (or, more aptly, Bigfoot shoes) is a quite a shocker for anyone who has grown up hearing the tale of Lark Quarry, perhaps bordering on heresy.

In science, though, we like slaughtering sacred cows, then making burgers out of them, adding a couple of slices of bacon, and putting a fried egg on top. (You know, the lot.) As mentioned last week, we do not prove in science, we disprove. This means that hypotheses are only accepted conditionally, then they may be tested later to 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 knocked down. And if they get knocked down, it’s often because someone found data that better supports an alternative hypothesis, or “another story.”

For example, think of how a young Einstein challenged facets of Newtonian physics with his general theory of relativity, or how Alfred Wegener provided the initial evidence for continental drift (which formed the foundation of modern plate-tectonic theory). In both instances, it took a while for the rest of the scientific community to discern the same phenomena and test it further, we waved goodbye to the previous hypotheses and unceremoniously threw them into the trash. (Although “recycling” sometimes happens too, especially with scientists who get a little too attached to a pet hypothesis, perhaps long after it’s dead.)

So just what was the evidence supporting the previous hypothesis – big theropod caused a panic because it was ready to 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 – about the dinosaur tracks at Lark Quarry? It all comes down to a common dilemma in dinosaur ichnology, and one that has been around for going on for more than 200 years: How do we distinguish three-toed dinosaur tracks from one another, and interpret who made the tracks?

Fortunately, the evidence backing up the new hypothesis for Lark Quarry is spelled out in a detailed and well-done study, and I say this as a professional ichnologist (with “professional” defined as when someone buys me a beer to hear what I have to say about ichnology). This study involved:

(1) A close look at the dinosaur tracks as they are preserved today at Lark Quarry;
(2) Studying casts made of the dinosaur tracks soon after they were excavated in the 1970s; and
(3) Lots of statistics, which I will try to explain simply to any of my non-scientist (yet admirably geeky) audience who might need it.

The heart of Romilio and Salisbury’s interpretation is a focused reexamination of the large dinosaur tracks, which are among the 3,300 tracks at Lark Quarry. Although these large tracks dwarf the others at Lark Quarry, they are relatively few in number, with only 11 such footprints recorded on the tracksite surface. That’s right: the key plot element of the original story of Lark Quarry hinges on a sequence of only 11 tracks, and identifying who made those tracks. So this was certainly a good place to start if someone felt like challenging the previous story, er, I mean, hypothesis.

When you look at a big three-toed dinosaur track, like this one at Lark Quarry in Queensland, you should always ask yourself: “cow” (ornithopod) or “raging bull” (theropod)? Although it’s never a good idea to underestimate the potential ferocity of cows, especially if armed

Why is identifying the maker of these big tracks so difficult? Mostly because the tracks show only three toes (digits) – a trait also known as tridactyl. These tracks are also symmetrical on either side of the middle toe, or mesaxonic. Furthermore, such tracks were made by two-legged (bipedal) dinosaurs, as trackways normally show an alternating right-left-right diagonal-walking pattern consistent with just the rear feet (pedes) doing the walking. Now that you know all of these neat terms, be sure to show them off in daily conversations, such as, “Wow, your emu leaves some of the best tridactyl mesaxonic tracks in a bipedal trackway I’ve ever seen. You must be proud!”

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 (tetradactyl = four toes, pentadactyl = five toes). Well, except for stegosaurs, which had tridactyl rear feet, but as far as we know, stegosaurs never walked bipedally. (Paleo-artists, though, should take note of that, and have fun.)

Typical trackway patterns for dinosaurs, and notice the similarity between the theropod and ornithopod ones if you took out the small front footprints. Figure stolen brazenly from a dinosaur textbook, Introduction to the Study of Dinosaurs, by A.J. Martin (2006), who also happens to be the same person writing this blog, so don’t take it too seriously.

So now you're thinking, "ornithopod or theropod" for your three-toed tracks, but notice also how in the trackway diagram the ornithopod is leaving small front-foot impressions. This is because some  ornithopods also walked quadrupedally sometimes, and bipedally at other times. Theropods, on the other hand (sorry) almost always moved on just two legs, although a few rare instances of hand impressions have shown up in their trace fossils made where they stopped to sit briefly.

So let’s say you found some three-toed dinosaur tracks like the ones shown in the first photo (the ones that were in the Museum of Tropical Queensland), 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 wide, whereas ornithopod tracks are wider than long. There, did it, done. The tracks shown in the first photo are from ornithopods. No worries, mate!

Oh, if only ichnology were so simple, where we could all be so satisfied with our results, saying, “Oh, look at me! I'm making people happy! I'm the Magical Man from Happy Land, in a gumdrop house on Lollipop Lane!” Well, time for an exercise in humility, Mr./Ms. Big-Note Oneself. Let’s run through a few questions and see how you do:

• 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” (track posterior) 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 you also think of how the substrate drying out might have changed a track outline before it was fossilized?
• Do you feel clueless yet? (If so, welcome to my world.)

Fortunately, an article written by three Spanish paleontologists – Josè J. Moratalla, Josè L. Sanz, and Santiago Jimenez – took some of this guesswork out of distinguishing theropod and ornithopod tracks. In this article, which they published in 1988 in the journal Geobios, they used a sample of 66 Early Cretaceous tridactyl dinosaur tracks from Spain that had been definitely identified as either ornithopod or theropod tracks. With these tracks, they measured nearly every parameter they could imagine: digit lengths, digit widths, angles between digits, widths on the foot between digits, and much more.

Diagram showing what could be (and should be) measured on a tridactyl dinosaur track; measurements can be then made into ratios and generate lots of numbers that can be put into statistical analyses. And you thought dinosaur ichnology would be easy? From Moratalla et al. (1988), Geobios, v. 21(4): 395-408.

Moratalla and his coauthors then compared ratios of these parameters to see which ones were significantly different from one another (statistically speaking). From these analyses, they figured out “threshold” values 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 is an 80% probability the track belongs to a theropod, but you don't just stop there. You also check the other ratios to see whether they consistently show a high probability of a theropod trackmaker, just to retest your initial identification.

With the publication of this study in 1988, dinosaur ichnologists then had a quantitative checklist they could apply to three-toed dinosaur tracks. (Whether all dinosaur ichnologists actually read this paper or applied its methods is another matter, but we won’t get into that persnickety subject this week.) These numbers also could be combined with other general, non-numbered observations to test whether a fearsome carnivore or a peaceful herbivore made a series of three-toed dinosaur tracks. One example of such an observation is the presence of sharp clawmarks in a track, which are in theropod tracks but absent in ornithopod tracks (ornithopods tend to have more rounded or blunt ends to their toes). Sharp clawmarks even show up in modern bird tracks, which we all know now are also theropod dinosaurs.

Comparison of a large modern flightless bird track (right) with an equivalent-sized theropod dinosaur track (left), with a few differences in overall form, but both have three forward-facing digits and sharp clawmarks in common, among other traits. The bird track is from a greater rhea (Rhea americana) in Patagonia, Argentina; scale = 15 cm (6 in). The dinosaur track is definitely from an Early Jurassic theropod, although I’m not sure which one, as it’s an epoxy resin cast of an original from the western U.S. The really cool looking photo of the dinosaur-track replica is courtesy of Tylight™, taken by Ty Butler in Atlanta, Georgia (USA).

As you may recall from previous entries (with your memory perhaps dependent on how much eggnog or other adult beverages you’ve had over the holidays), I told how the original hypothesis for the Lark Quarry tracksite – “dinosaur stampede provoked by a large stalking theropod” – was first proposed by Australian paleontologists Tony Thulborn and Mary Wade in 1979. This was nine years before the publication of Moratalla and his colleagues’ study, so Thulborn and Wade can’t be blamed for what they did not yet know; they were using the best science known then to interpret the tracks. And, 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 with a much more detailed 105-page report in 1984.

Nonetheless, where they could have made a mistake was with the identification of the large “theropod” tracks. They identified on the basis of their resemblance to dinosaur tracks known elsewhere, which had been allied with tyrannosaur-like tracemakers. Objections to this identification voiced by Romilio and Salisbury were not new, either, as a few dinosaur paleontologists questioned this affiliation 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 dissing.

What went wrong with the original study of the Lark Quarry tracks? Well, to go into that topic would require a discussion of the necromancy that ichnologists call ichnotaxonomy, which is the naming of trace fossils. And because that would require much more verbosity (and you’ve all been such a good audience today), we will take a closer look at that subject next week.

So for now, let us reflect on the benefits of skepticism, and how our reexamination of old stories can generate new perspectives and insights, similar to those caused by a young Albert Einstein as he grew up in rural Australia.

Rare, archival documentary footage of a young Albert Einstein, taken immediately after his invention of rock and roll music in Tasmania. What, you don’t believe me? Then be a real scientist and disprove it!

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