Walking With Raptors
A little more than a year ago, paleontologists working in Niger announced the discovery of Spinophorosaurus, a sauropod dinosaur with a wicked tail club. Its bones were not the only traces of dinosaurs to be found in the desert area. About three hundred feet from the exceptionally well preserved sauropod skeleton was a trackway containing more than 120 impressions from an unknown predatory dinosaur, and those tracks are the focus of a new study just published in PLoS One.
Described by Alexander Mudroch, Ute Richter and colleagues, the tracks were left by a dinosaur that walked on two toes and strode along the shore of a small lake or river between 176 and 161 million years ago. No body has been found, but since tracks receive their own distinct titles, the scientists have named these tracks Paravipus didactyloides. Their peculiar anatomy indicates that they were left by a raptor.
Among dinosaurs, members of only one group—the deinonychosaurs—are known to have supported themselves on two toes while having a retractable second toe that only barely touched the ground. Given the size of the tracks, the authors of the new study propose that the dinosaur that left them was about the size of Deinonychus from North America.
This is not the first time such tracks have been found. In 2008, a team of scientists described tracks made by a similar kind of dinosaur in the Early Cretaceous rock of China. Designated Dromaeopodus shandongensis, these tracks differed by having a distinct pad where the toe carrying the sickle claw touched the ground. The tracks from Niger have only a small impression in the same position, which indicates that this dinosaur lacked the additional foot pad.
While they can be difficult to interpret, the new tracks may also tell us something about the behavior of this yet-unknown dinosaur. There appear to be at least five different trackways, Mudroch and co-authors state, which were made at three different times. Two sets of early tracks were overlain by another pair of tracks of about the same size. This might indicate that two animals were moving together in one direction and then turned around, stepping on their own tracks. In fact, the pattern of one of these sets appears to indicate that one of the animals abruptly changed speed to avoid running into the other, and if this is true it is further evidence that some of the raptors were social. Days to weeks after this pair left the area, another individual crossed their tracks and left its own behind. At the moment, though, it is difficult to reconstruct this scene without the discovery of bodies. With any luck, a skeleton of the dinosaur that left the Paravipus tracks will soon turn up.
Li, R., Lockley, M., Makovicky, P., Matsukawa, M., Norell, M., Harris, J., & Liu, M. (2007). Behavioral and faunal implications of Early Cretaceous deinonychosaur trackways from China Naturwissenschaften, 95 (3), 185-191 DOI: 10.1007/s00114-007-0310-7
Mudroch, A., Richter, U., Joger, U., Kosma, R., Idé, O., & Maga, A. (2011). Didactyl Tracks of Paravian Theropods (Maniraptora) from the ?Middle Jurassic of Africa PLoS ONE, 6 (2) DOI: 10.1371/journal.pone.0014642