Caught in their tracks

How a theropod would have left a deep track

Some dinosaurs moved their feet in much the same way as modern birds according to scientists who have studied 210-million-year-old fossilised footprints in Greenland.

The US researchers analysed the impressions preserved in the rock to model the operation of the creatures' lower limbs.

Their study, published in the journal Nature, will add to the debate about the evolution of birds and the idea that they are descended from theropods, the carnivorous dinosaurs that walked around on their hind legs.

The team from the Universities of Brown, Harvard and Pennsylvania even ran a turkey through wet soil to compare the marks left in mud by a modern ground-dwelling bird with those caught in the Greenlandic prints.

And although it did show up some significant differences in the position of the big toe, foot posture and leg movement, the scientists still believe in the evolutionary relationship between dinosaurs and birds.

"Living birds retain many features from their theropod ancestors, but hind limb anatomy and function did change," says lead scientist Stephen Gatesy, a Brown assistant professor of ecology and evolutionary biology.

"Birds do not move exactly like 210-million-year-old dinosaurs, but they are the closest thing alive today."

Recorded in the mud

The dinosaur tracks were found on exposed rock near the Fleming Fjords in eastern Greenland. When the therapods ran through this area it would have been more tropical than arctic and probably covered by a network of shallow lakes.

The dinosaurs were probably walking through mud alongside shallow lakes

Where their feet landed on relatively firm ground, the therapods left three-toed impressions - the marks from joints and skin are clearly visible. Where the dinosaurs ran through sloppier muds, their feet sank deeper into the ground leaving behind unusually long, four-toed prints.

But whereas shallow prints reveal little about foot and leg movement, the deep "wounds" in the ground, the scientists realised, amounted to a three-dimensional recording of locomotor behaviour that could even tell them what the individual toes were doing.

In the tracks of both Triassic theropods and modern ground-dwelling birds, the foot plunges down and forward into the muck. The toes then collapse and come together as the foot is lifted up and forward.

Turkey experiment

Gatsey and his colleagues checked the comparison by running a modern turkey through sloppy mud and looking at its tracks.

"At first it was just a way of working out what was going on," Stephen Gatesy told BBC News Online. "Once we learned more, then it was obvious we could look for the subtleties of what is similar and what is different. We sent the turkey through various depths and various consistencies to see how the substrate properties might be affecting the tracks."

As with birds, the toes converge below the surface and emerge together from the front of the track

The study threw up several inconsistencies between birds and dinosaurs. The first toe - like our "big" toe - is reversed in most birds, allowing the foot to perch or grasp prey.

The deep Triassic prints bear an impression of the first toe that also points backwards, suggesting the existence of an as yet undiscovered theropod that could also perch.

However, the new research shows the backward pointing imprint is just an illusion created by the motion of the foot and the folding of the mud.

"The first toe doesn't have to be reversed, which is the way it would be if you read the print literally," Stephen Gatesy says. "We've learnt that the footprint can, in some ways, be deceiving - it can look more like a bird print than it actually should."

This revelation means an opposable first toe is likely to have evolved much later in history.

The study also suggests the theropods moved their hind legs about the hip, as in alligators. In contrast, walking birds primarily move by bending their knees, which raises the sole of the foot before it can leave a print.

Gatesy conducted the research with Kevin Middleton, Farish Jenkins Jr and Neil Shubin.