How to Turn a Tyrannosaur Into a Iguanodont

Fossilized dinosaur tracks can be exceptionally informative traces of prehistoric life, but figuring out what dinosaur made a particular set of footprints can be tricky. Unless an animal literally dies in its tracks, the best we can do is to match the skeletal anatomy of dinosaur feet with the anatomical clues left in the impressions they left behind. Even then, however, the relationship between a given track and the potential trackmaker is subject to change. Through a reinvestigation of tracks from Australia, a pair of paleontologists has just pulled off the impressive feat of turning a charging theropod into an iguanodont out for a stroll.

The new research, by Anthony Romilio and Steven Salisbury, will appear in Cretaceous Research. The objects of their attention were tracks preserved at the approximately 100-million-year-old Lark Quarry site in Queensland, Australia. Believed to have been left by a large theropod dinosaur, the tracks were referred to the footprint type Tyrannosauropus. (Tracks are given their own unique names since it is often impossible to tell the exact genus of dinosaur that made them, especially since there are so many dinosaurs yet to be discovered!) This identification is probably incorrect, but to explain why, Romilio and Salisbury first untangled some of the confusion about this particular track type.

The problems started with large, three-toed tracks illustrated in a 1924 Natural History article by William Peterson about dinosaur footprints found in the roofs of Utah coal mines. Some of these tracks were proposed to have been made by Tyrannosaurus rex and were given the name Tyrannosauripus (with an "i") in 1955. Since this proposed name had not formally applied to any of Peterson's tracks, though, the name Tyrannosauripus was still available and was re-applied to a more definitive track of a giant theropod from New Mexico described in 1994.

Here's where things really got complicated. Peterson's 1924 article also contained illustrations of a second track type, and this different track variety was used to establish the name Tyrannosauropus (with an "o") in 1971. These, too, were thought to have been the footprints of a tyrannosaur, but all the Utah tracks later turned out to have been made by hadrosaurs. Given all this confusion, it seemed possible that the Australian Tyrannosauropus tracks had also been misidentified.

Using line drawings, photos, casts, and examinations of the original tracks, Romilio and Salisbury re-analyzed the dimensions of the Lark Quarry footprints. Altogether, the footprints were of the size and shape expected for an ornithopod dinosaur—the group containing hadrosaurs, iguanodonts, and their close relatives—and were inconsistent with the kind of tracks made by large theropods. In fact, only the recently described theropod Australovenator was in the right place at the right time to be the potential trackmaker, but it was far too small and did not match the Lark Quarry prints.

So what dinosaur actually left the three-toed tracks at Lark Quarry? It is difficult to say for sure, but Romilio and Salisbury note their close resemblance to other three-toed tracks from other localities given the name Amblydactylus. The features visible on these tracks—especially the slightly-pointed hoofs—would indicate that the animal was probably an iguanodont, and the overall best match in terms of anatomy, size and time period is Muttaburrasaurus. Through a bit of fossil sleuthing, Romilio and Salisbury turned a rampaging carnivore into an herbivore.

This change in identification drastically alters the story behind the Lark Quarry tracksite. It was originally thought that the big, three-toed tracks had been made by a predator that sparked a stampede of smaller dinosaurs in an attempt to ambush its prey. As presented on an Australian Broadcasting Corporation website about the tracks:

These footprints preserved in stone give us a glimpse of what happened in one moment millions of years ago when the large flesh-eating dinosaur approached the edge of a lake where about 150 small dinosaurs were drinking. All the small dinosaurs ran away from the lake’s edge toward the big predator in a desperate bid to escape. One might have been captured as it ran past, but there are no signs of struggle recorded at the site.

Now the story has to be revised. The trackways still record how a large group of small carnivorous and herbivorous dinosaurs scattered, but we can no longer envision a large predator bursting out of the trees to run after the tiny prey animals. What caused these small dinosaurs to run off remains a mystery, but the place can still rightly be called Dinosaur Stampede National Monument.

References:

Romilio, A., & Salisbury, S. (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 DOI: 10.1016/j.cretres.2010.11.003

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.