Limping With Dinosaurs

One of my favorite roadside stops is down a dirt track off Utah’s state road 191. Provided you don’t miss the turnoff around mile marker 148.7, and the soil hasn’t turned to a sucking mire by rain, the rough road will lead you through the desert scrub to a little parking lot with a Bureau of Land Management signboard at the start of a short trail. It’s not far from there. Hike up onto the tan stone and you’ll soon find yourself standing among the footsteps left more than 150 million years before.

There are other places to spot dinosaurs in the Moab area. Their bones and tracks seem to be everywhere. But I like this spot, called Copper Ridge, because the stone has preserved something rarely seen in annals of dinosauriana. Running diagonally along the exposed surface are the three-toed tracks of an ancient carnivore, but there’s something a little bit off about them. The stride is a little shorter on one side than the other. This dinosaur was limping.

Fossil track experts Martin Lockley and Adrian Hunt briefly described the seven-step trackway in 1995, and Moab-based paleontologist John Foster just published a more detailed look at the trace fossils. The set of seven tracks, attributed by Foster to an “ichnogenus” of footprint originally named from Europe called Hispanosauropus, were made by a theropod dinosaur that was about six feet tall at the hip and was moving at about 4.5 miles per hour.

Exactly what species of dinosaur left the trackway is unknown. This is the “Cinderella problem” in paleontology. Unless an animal literally dies in its tracks, there’s only so far trace fossils can take you in making a species-specific identification. In this case, there were at least three large theropod dinosaurs living in the area that could have made Hispanosauropus tracks. Torvosaurus, Ceratosaurus, and Allosaurus are all candidates, and, Foster points out, all three also occur in the European rocks where this particular track form was originally found.

Allosaurus has a bit of an edge, though. Not only does the dinosaur’s foot compare well with the petrified “slipper,” but it was the most abundant predatory dinosaur of its time. Chances are good that an Allosaurus was the dinosaur responsible, and these limping tracks dovetail with what we know about the carnivore’s rough-and-tumble lifestyle.

Being an Allosaurus wasn’t easy. Multiple skeletons—including “Big Al,” “Big Al II,” and the National Museum of Natural History specimen—show multiple injuries ranging from infected toe bones to broken ribs. Whether any of these injuries happened in the pursuit of a hot meal or represent the Jurassic equivalent of stubbing one’s toe is difficult to resolve, but the sheer number of bumps and scrapes and breaks these dinosaurs suffered—19 injuries on Big Al alone—shows that Allosaurus suffered their share.

And that’s why I love the quiet little trail hidden off the Utah byway. The Copper Ridge tracks represent a few of painful moments safeguarded by the rock. Walking alongside them, trying to imitate a Jurassic hobble, gets me a little closer to a Mesozoic mindset than simply staring at a skeleton. The poor Allosaurus wasn’t an impervious ancient monster. This dinosaur knew pain, and that feeling brings me just that much closer to a creature lost to time.

Reference:
Foster, J. 2015. Theropod dinosaur ichnogenus Hispanosauropus identified from the Morrison Formation (Upper Jurassic), Western North AmericaTheropod dinosaur ichnogenus Hispanosauropus identified from the Morrison Formation (Upper Jurassic), Western North AmericaTheropod dinosaur ichnogenus Hispanosauropus identified from the Morrison Formation (Upper Jurassic), Western North America. Ichnos. doi: 10.1080/10420940.2015.1059335