nullTwo titanosaurs walked side by side across this former lake bed in Bolivia 68 million years ago.

Martin Lockley

Eighty-four-year-old Sheldon Johnson never imagined that once he began digging, it would be so difficult to stop. In February 2000, he climbed into his trackhoe and drove up to a 40-foot hill on his small farm in southern Utah. The rusty sandstone mound did not match the level of the adjacent new city road, and the retired optometrist simply wanted to level it. Johnson busily went to work hauling out 15-foot-long rectangular slabs of the red rock. Then the trackhoe flipped one of the slabs over, and Johnson saw them: pristinely preserved dinosaur footprints. “It was unmistakable. I could see knuckles, claws, scales, and three big toes. No one hardly believed me at first,” he says.

Johnson immediately began turning over more layers of sandstone, breathlessly checking their underbellies for tracks. To his delight, nearly every one had some of the monstrous prints. He called around to state offices and universities, and within a matter of weeks hundreds of curious spectators—children, government officials, paleontologists—began flocking to the farm. Over the next few years, thousands of tracks were unearthed at the location now known as the St. George Dinosaur Discovery Site at Johnson Farm. Johnson had stumbled onto one of the world’s most important dinosaur trackways.

Once dismissed by most paleontologists as mere curiosities, trackways are increasingly being recognized as vital pieces of evidence that record otherwise unknowable details of daily life millions of years ago. Fossil bones are wonderful for understanding anatomy, but they are inherently static. Footprints and other impressions, on the other hand, are snapshots of a creature in action. “When you hold a bone of an extinct animal, you’re holding a remain,” says University of Manchester paleontologist Phillip Manning. “Trackways are from when the animal is still breathing.”


Martin Lockley, a University of Colorado paleontologist, has spent nearly three decades analyzing ancient prints. “Tracks are very dynamic,” he says. “They show things like speed, individual behavior, social behavior, and animals starting to run. They’re a quick way to get a lot of information.” A track site representing several types of dinosaurs can reveal which major groups cohabited, indicate the proportion of juveniles to adults, and offer a general census of the populations in the area.

So to Lockley it is not the tangible bones but the intangible footprints that contain the real stories. Only trackways—the negative spaces that the animals left behind—can tell how the dinosaurs hunted, dined, and interacted during the Mesozoic.

A DAY IN THE LIFE

One day some 198 million years ago, a dilophosaurus or similar dinosaur—a half-ton carnivorous lizard at least six feet tall and around 20 feet long—squatted in the mud by the side of Lake Dixie, a large Jurassic freshwater body in what is now southwestern Utah. (Since there are no fossils to match to the tracks, paleontologists can identify the creature only approximately, based on its anatomy.) Perhaps the fearsome beast surveyed the land as it sat. Maybe it was searching for its next meal. What we do know is its precise posture and movements. It sank its powerful haunches and small-clawed hands into the muck. Then it shuffled its feet and dragged its tail as it stepped forward into the arid Jurassic heat.

This scene is preserved at the St. George Dinosaur Discovery Site museum, less than 100 yards from where Johnson made his initial find. Soon after the dinosaur passed through, the water level in the lake rose and sediments quickly washed in, filling the tracks and preserving them in sandstone. The resulting traces of a crouching meat-eating dinosaur, found in 2004, are the only ones in the world that show clear hand impressions. The rarity of such markings indicates that the animals rarely sat down. “But when they did, you can see their posture,” says Andrew Milner, head paleontologist at the Dinosaur Discovery Site. “Modern birds like emus, or even little perching birds, fold the legs up underneath the body. These theropod dinosaurs—bipedal and carnivorous—did exactly the same thing.”

The similarity in the poses supports the popular theory that birds evolved from meat-eating dinosaurs. The traces at St. George also tell how these dinosaurs grasped their prey. In dinosaur reconstructions that often persist in museums today, the hands of meat-eating dinosaurs are turned down in front of the body. “When you look at the bones of these animals, you see they’d have to totally dislocate their shoulders in order to get their hands in that kind of position,” Milner says. “These impressions show the animals were able to grab their prey and hold on while biting at it.” The prints thus provide a glimpse of these early theropod dinosaurs as they ate their meals. The evidence also suggests that the birdlike arrangement of bones in the dinosaurs’ arms evolved more than 75 million years before the oldest evidence of that in the fossil record.

Those big, carnivorous dinos were not the only creatures roaming Lake Dixie’s clay-rich shores. The same sediment that filled the dinosaurs’ footprints created a snapshot of an entire ecosystem that was teeming with life. Tadpole nest impressions rest beside tracks of tiny amphibians stretching out in their muddy niche. Fish fins, beetles, branches, and even raindrops left their mark as well. And there was more than just one set of dinosaur footprints—a lot more.