This week scientists at the University of Arkansas revealed that they have discovered and been studying a large area of dinosaur footprints in the southwest of the state. They have not revealed the exact location as people have a nasty habit of digging up dinosaur footprints and selling them to private collectors, which destroys them as a resource for scientists; even if they can be recovered they are far less useful when out of position. The research is being lead by Stephen Boss of the Department of Geosciences at the William J. Fulbright College of Arts and Sciences and is funded by the National Science Foundation.
Some of the dinosaur footprints.
The prints so far exposed cover an area of over 10 000 m³, and date from the early Cretaceous between 115 and 120 million years ago. There are a large number of prints, which is slightly surprising as they were made in fairly hostile conditions, salt pans around a drying sea or lagoon, similar to those seen in the modern Persian Gulf. At first this seems slightly puzzling, but it is unlikely that the prints were all laid down at the same time; the field may well be showing accumulated trails built up over hundreds or even thousands of years.
A modern salt pan - few animals choose to spend much time here.
It is not considered possible to identify dinosaurs from their footprints alone, however in some cases it is possible to make an informed guess. The track field includes prints from both large therapod and sauropod dinosaurs, and there are unlikely to have been many different candidates for producing these.
A single large therapod dinosaur is well known from the Early Cretaceous of the Southwest United States, and analogy with modern ecosystems suggests that such an animal would have been unlikely to tolerate many rivals. This is Acrocanthosaurus atokensis, an 11 m allosauroid with a distinctive crest, thought to have weighed between 6000 and 7000 kg. Several sets of trackways in Texas have already been attributed to Acrocanthosaurus.
An artists impression of Acrocanthosaurus atokensis, by restoration artist Mineo Shiraishi.
There are two known sauropod dinosaurs from the region during the Early Cretaceous, and again this is likely to be the limit. No modern environment supports two species of elephants, and only one (the savanahs of eastern and southern Africa) two species of rhino. Thus it is likely that the sauropod prints belong to one or both of these dinosaur species. These are Plueurocoelus nana and Paluxysaurus jonesi two similar 15 m brachiosaur-like dinosaurs, which are the former and current official state dinosaurs of Texas respectively. The similarity, both in anatomy and distribution of these dinosaurs makes them hard to tell apart, though in certain features they are distinctive; it is possible that they represent an example of sexual dimophism in a dinosaur. It is also possible that Astrodon johnstoni, a similar dinosaur from the Early Cretaceous of the northwestern United States.
As well as making plaster casts of the footprints which can be taken back to the lab, the team have been making detailed 3D laser scanned images of the whole site, this not only gives an accurate three dimensional images of the footprints within a computer, it also maps the prints on the ground accurately enabling scientists to study the foot-spacing and therefore gait of the printmakers. Footprints do not always accurately represent the foot-shape of the printmaker. In muddy terrains footprints are often distorted by the viscosity of the mud, but on this sort of terrain this is less likely. More likely is that the prints are in fact underprints; when an animal steps on a layered terrain it crerates prints not just at the surface, but in the layers beneath, as sediments are pressed down through one-another. These underprints are often preserved while the overprints are destroyed at the surface, but they are less accurate representations of the footprint maker.
How underprints are formed, different prints from the same foot, and how tracks are measured.