Dinosaur tracks show walking and running. (Paleontology).

A single trail of dinosaur footprints in a quarry northwest of London preserves a record of two different walking styles in the same animal, a tantalizing clue that some lumbering, bipedal dinosaurs could also run.

The trail of three-toed imprints is in a 163-million-year-old layer of white limestone. The footprints were probably made by a Megalosaurus, a 9-meter-long, meat-eating dinosaur first described in 1826, says Julia J. Day, a paleontologist at the University of Cambridge in England.

In a 35-m section of the trackway, the footprints are spaced about 3 m apart and pointed slightly away from the line of the animal's path. The fossilized imprints in this segment of the trail form an almost straight line, a clue that the animal was placing its feet almost directly beneath its body as it ran, Day notes. Then, within a half-dozen steps, the spacing between footfalls drops to about 1.3 m, the imprints become pigeon-toed, and the tracks become more distant from the center of the animal's path.

Using formulas that relate the length of the dinosaur's foot to its height, Day and her colleagues estimate that the animal's hips were about 2 m off the ground. That, plus measurements of the creature's stride length, enabled the scientists to compute the animal's speed at different points along its fossilized jaunt. When running, the Megalosaurus sprinted along at about 29 kilometers per hour. At the slower gait, it walked at a mere 7 km/hr, just a little faster than people walk. The researchers report their findings in the Jan. 31 Nature.

The simultaneous changes in footprint orientation and spacing preserved in the trackway suggest that Megalosaurus and its cousins could shift gears and run more efficiently when necessary, says Day. Because the imprints immortalized only a short portion of the creature's dash, it remains unclear just how long a large, bipedal dinosaur could sustain a sprint. --S.P.