Monday, December 20, 2010

Footprints in the Sand

This story is about tracks, specifically dinosaur tracks. Footprints can be very informative, giving so much more information than just the shape of the foot. They provide information on species, body posture, locomotor ability, sociality, preferential environments, and stratigraphic and geographic faunal diversity.

As you might guess, it isn't always easy to identify fossilized dinosaur tracks. Sure, some species are easier than others like sauropods, stegosaurs, and ceratopsians. The difficulty arises in distinguishing bipedal theropods (carnivores walking on two legs) and ornithopod dinosaurs ("bird-hipped" dinos, grazers walking/running on two legs). You can see how they might look pretty similar.

A new, in press, paper in Cretaceous Research takes a look at the Dinosaur Stampede National Monument at Lake Quarry Conservation Park in central-western Queensland, Australia. This monument contains thousands of fossilized footprints of dozens of dinosaurs from the mid-Cretaceous. Since the 1970's, the popular hypothesis regarding this area is that an Allosaurus-sized dinosaur chased a mixed herd of small-bodied dinosaurs, causing a stampede. Previous studies have identified this "predatory protagonist" as a large tridactyl (three-toed) dinosaur, a theropod (likely a Tyrannosauropus). This particular taxon has a bit of a checkered history, scientists have argued about it since the early 1920's. It has also been proposed that the tracks on the monument are attributable to a hadrosaurid ornithopod (a duck-billed dinosaur that walked on two legs).

Figure 1. The Winton Formation at Lark Quarry.
These authors applied a multivariate analysis to discriminate between theropod and ornithopod tracks in order to identify the track maker. Multivariate analysis allows you to analyze more than one statistical variable at a time and therefore you are able to test multiple dimensions while taking into account several variables. Why mention the statistical test used? Some people like statistics. No really, I mentioned it because this is the first time that multivariate analysis has been used to identify tracks on this valuable piece of biological history. To perform this analysis the researchers measured all of the tracks using published line drawings, catalogued photographs, latex peels, museum casts, and actually measured the real ones themselves. The variables for the analysis included track length and width, total digit lengths, basal digit length and width, middle digit widths, and the distance from heal to the interdigital (hypex) point.

The results showed that a majority of these measurements fell within the threshold expected for ornithopod dinosaurs. In fact, the authors conclude that of the known types of theropods from this region, none of the body fossils adequately match the measurements and analyses that they did.

So what type of ornithopod was the track maker? Well, the footprints are slightly longer than they are wide, they have symmetrical toes, have claws, and have a V-shaped central digit. Of the known large Cretaceous ornithopods, previous studies have concluded that it could be either Ambyldactylus, Caririchnium or Iguanodonipus. Of these, only the iguanodontian dinosaur, Amblydactylus, shares the distinctive features exhibited in the Lark Quarry tracks. The tracks are remarkably similar to ornithopod tracks from Canada named Amblydactylus gethingi and so the authors suggest re-naming the Lark Quarry tracks Amblydactlyus cf. A. gethingi. In all likelihood, the footprints were made by a large ornithopod standing approximately 2.5 meters tall at the hips, likely a more primitive member of the group, probably similar to Muttaburrasaurus langdoni. This species specifically because it has been found in similarly aged rocks only a few hundred kilometers from Lark Quarry. The stampede of the smaller dinosaurs also represented in these rocks was probably due to the approach of this larger dinosaur. A large herbivore spooking a group of smaller dinosaurs is not exactly as exciting as a predator chasing them down but, in this case, is more accurate.

The authors state that if this identification is correct then it removes any published evidence that a large theropod dinosaur existed in the Australian Cretaceous. However, I do recall posting an article about a new theropod discovered in Australia from 110 million years ago - which I'm pretty sure is the Cretaceous period (144 to 65 million years ago?) - so I'm not sure about this wiping out all evidence thing. Just sayin'.

This is the paper:
Romilio, Anthony and Steven W. Salisbury (2010). A reassessment of large theropod dinosaur tracks from the mid-Cretaceous (late Albian-Cenomanian) Winton Formation of Lark Quarry, central-western Queensland, Australia: A case for mistaken identity. Cretaceous Research: in press. (DOI: 10.1016/j.cretres.2010.11.003)

And here's some more reading material on the subject:
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