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A second line of argument that supports the idea of dinosaurs as social creatures is the vast trackways that have been uncovered in both North America and Asia. Hundreds of sauropods -- Apatosaurus and its kin -- would evidently travel in herds across the late Jurassic landscape, leaving footprints as they went; similar trackways have been discovered for Triceratops and Maiasaura. The tracks of the theropods, the aggressive predatory group that includes T. rex, are often found in multiple sets, a strong clue that they traveled, and presumably hunted, in packs.
Footprints can tell scientists more than that, though. Their depth and spacing also give testimony about dinosaurs' size, weight and speed. All the evidence suggests that dinosaurs in general were strong and efficient walkers, capable of maintaining a brisk pace. Theropods, in particular, observes paleontologist James Farlow of Indiana University-Purdue University at Fort Wayne, "put their feet almost one in front of another. They had a gait very similar to a human being's." They almost never dragged their tail, as out-of- date museum exhibits would have people believe; instead they probably used them for balance. "Hardly ever do you see tail marks," explains Farlow. "I sometimes envision theropods as big animated seesaws with one end that can bite you."
Theropod tracks he has studied in Texas convince Farlow that the predators moved along briskly. "Their walking pace was somewhere between three and six miles an hour," says Farlow. He has also studied trackways that were probably made by running theropods. Top speed: between 15 and 20 m.p.h. "That's not as fast as an ostrich or a good racehorse," he says, "but it's faster than anything a human can do."
The most significant thing about dinosaur tracks, says Martin Lockley, a geologist at the University of Colorado, is that "they're so abundant relative to bones. Every animal has only one skeleton, but it can leave thousands of prints." Example: 80 years of quarrying at Dinosaur National Monument in Utah and Colorado have turned up evidence of only 80 individual dinosaurs. "We went in for three years to look for tracks," recalls Lockley, "and found footprints from 240 individual dinosaurs."
Lockley has found a similar wealth of tracks in South Korea and, to his surprise, discovered many prints belonging to baby apatosaurs (a.k.a. brontosaurs). Boasts Lockley: "The conventional wisdom was that baby brontosauruses were hard to find." Now he is tackling the question of why some dinosaurs limped, alternating short steps with long ones. "We're finding that those are quite common among both quadrupedal and bipedal dinosaurs," Lockley reports. It could be because of injury -- yet why do so many different species show the same limp? Is it a trotting gait? Is the animal carrying a burden, like its young? Is it staggering away from a fight with a predator hanging onto (or biting into) its side? The answer, he hopes, will eventually be found in the tracks.
THE BIRD CONNECTION