DINOSAUR SOCIETY

Thousands of claw marks, some of them preserved down to skin impressions of the cuticle, reveal that hundreds of dinosaurs swam parallel to the early Jurassic shore of Lake Dixie. It is difficult to tell how far apart in time different prints were made, but there are ways. So far Milner and his team have found 25 layers of silt, each containing tracks from a different time period. Some of these tracks are separated by as much as two million years. But in the soft clay depression marking Lake Dixie, Milner determined that the swim tracks were made rapidly, within hours of each other. Fine sand quickly washed into the scratch marks and filled them, preserving the impressions before the lake current had a chance to wash them away. Not only could the dinosaurs swim, but they did it as part of a social outing: They were swimming in groups.

Many types of dinosaurs created such tracks as they plunged through the water. In deeper areas of the lake, a set of smaller coelophysid dinosaurs—known for their long, thin necks and tails—kicked their feet so they buoyed upward, their toes striking the muddy bottom, according to a 2006 paper by Milner. Impressions also reveal that a larger Dilophosaurus-like dinosaur, presumably accustomed to wading in shallow waters, lost its grip along the lake’s edge. “We can see the metatarsals, the lower part of the foot, and how the foot came to rest on the bottom. The animal had flipped sideways. More than 100 parallel scratch lines made by the animal’s scales follow the same direction as the toe pads. It was slipping,” Milner says.

Dinosaur swim tracks have been controversial in the past; a swimming animal may touch down on a lake floor while fully or partially afloat, leaving irregular foot and stride patterns that make it difficult to say which animal made the markings. But Lake Dixie’s exceptionally preserved tracks unambiguously reveal skin and the cuticles of claw tips along with the scale scratches. The creatures swimming here were undeniably theropod dinosaurs, Milner notes, and the tracks provide rare insight into how these fearsome reptiles behaved when they hit the water.

Most likely the theropods came to Lake Dixie to eat. The big surprise is what was on the menu. There are very few prints of plant-eating dinosaurs in the lake bed, Milner says, so abandon Jurassic Park-like images of savage hunters chasing down giant, plodding prey. Carnivores most likely took a dip in the warm waters for one key purpose: to fish. Thousands of fossilized semionotid fish (pdf) (ray-finned fish extinct since the Cretaceous) have been found in the area; their hard, enamel-coated scales would account for the wear found on dinosaur teeth recovered nearby. Diving, kicking up sand, and bouncing off the lake’s bottom, meat-eating dinosaurs of all sizes splashed around in the balmy waters of Lake Dixie, angling for a sushi dinner.

Other dinosaur track sites contain their own remarkable hints about dinosaur lifestyles. In southern Bolivia, just outside the country’s capital, Sucre, sites encompass more than 5,000 footprints representing 465 different dinosaurs from the late Cretaceous, near the end of the age of dinosaurs. On one hill at a site called Humaca, 11 juvenile sauropods (relatives of the enormous dino commonly known as a brontosaurus) appear to be moving in tandem. Their footprint trails veer in unison, implying that the animals turned together as a herd, somewhat like modern elephants or migrating geese. “That regular spacing is analogous to birds’ flying in formation. They had a sense of space between them. These large animals were gregarious,” Lockley says.

Nearby, footprints crisscross a wall at a Bolivian limestone quarry known as Cal Orck’o. Some 68 million years ago, herds of titanosaurs (more sauropods) apparently flocked to a prehistoric lake in this area in search of food and freshwater. Today, geologic movements have tilted those tracks into a rock face angled at a precarious 70 degrees. One set of footprints stretches 200 feet up the wall. Lockley and Christian Meyer, a Swiss paleontologist who led the mapping of this region from 1998 to 2003, hung from ropes to document the imprints exquisitely preserved here. Their fear was that a sudden earthquake, or just the gradual seeping of rain into rock layers at the quarry, would destroy the precious formation. Safeguarding the entire wall would cost millions of dollars.

Lockley and Meyer did their best with their limited funds, attempting to protect the face with clay, plastic, and even Gore-Tex-like webbing; nevertheless, a major section of Cal Orck’o collapsed in February, taking with it at least 200 footprints made by two titanosaurs, beasts that may have measured 40 feet long. Still, the park remains one of the world’s largest dinosaur trackway sites.

ORIGIN OF THE SPECIES

Some 50 million years before titanosaurs stomped through southern Bolivia, delicate roadrunner-like birds darted among the Cretaceous dinosaurs in what is now China’s Shandong province. These animals, too, left their prints behind. Fossil bones don’t clearly show whether modern-type birds fluttered about during the Cretaceous, but the treads in Shandong do, painting an improbable scene: Animals much like today’s roadrunners were in fact scampering beside two-legged, plant-eating dinosaurs. “According to the fossil record of bones, roadrunners didn’t appear until very recently, in the last million years during the Ice Age. These footprints are 100 million years old,” says Lockley, who wrote a study on the find, which is a telling example of how prints can fill in major gaps in the evolutionary record.

Footprints in southern Germany, for example, may extend the entire dinosaur lineage back four to five million years. The tracks were formed 240 to 245 million years ago by a cat-size reptile called Rotodactylus, known from its footprints alone; no bone evidence of dinosaurs dates this far back, says paleontologist Hartmut Haubold of Martin Luther University in Halle-Wittenberg, Germany. Rotodactylus is believed to be a dinosaur or a surviving dinosaur ancestor that lived just after dinos and crocodilians split into separate branches. The German trackways therefore offer a unique look at how the earliest dinosaurs differed from their rivals and evolved into the creatures that dominated the planet for the next 175 million years.

Like Rotodactylus, the earliest members of the dinosaur lineage were small, with fragile, chickenlike bones that rarely lasted long enough to form fossils. With so few skeletal remains, paleontologists are increasingly turning to trackways to close major gaps in that early chapter in dinosaur history. The lack of corroborating fossils makes it hard to be positive that Rotodactylus was an early member of the dinosaur line (and not, say, a closely related archosaur), but if Haubold is correct, dinosaurs emerged earlier and took longer to dominate than scientists believed.

“Footprints don’t preserve a lot of anatomy, but sometimes they lead to substantial revisions of a group’s evolutionary history,” says Steve Brusatte, a paleontologist at the American Museum of Natural History in New York City. An extreme example of such revision came this past January, when the cover of Nature featured footprints discovered in the Holy Cross Mountains in southern Poland. Those prints were made by the earliest tetrapod (four-legged) land vertebrate ever found. Nearly 400 million years old, they are 18 million years more ancient than the oldest known tetrapod bones. The find is forcing scientists to reassess their thinking about when and how fishlike creatures made the transition onto land.

Brusatte and Grzegorz Niedzwiedzki, the Warsaw University paleontologist who led the research, are examining several dinosaur track sites scattered in central and southern Poland. Some of these sites formed during the Triassic era, when dinosaurs first emerged. Large clay pits there preserved a vast accumulation of footprints from dinosaurs and their near relatives, yielding a nearly unbroken record of this ecosystem—everything from insects to branches to vertebrates—covering 75 million years. The tracks at these sites could possibly extend the dinosaur’s evolutionary line millions of years further, to an age even before Rotodactylus. Brusatte and Niedzwiedzki plan to return to the sites this summer to take a census of the footprints.

“The ultimate goal is to see when these early dinosaurs took off. We want to know whether they were very rare, when they became more common, and how big they were,” Brusatte says. “Bones can tell you that, but a single organism leaves millions of footprints and only one skeleton.”

SIMULATING TRACKWAYS

Fossil footprints are found all over the planet, but figuring out who took those steps requires careful detective work. So Simon Jackson, a paleontologist working with researchers at the University of Sheffield, recently ran a simulation showing how prints are distorted and preserved in dry, moist, and very wet sandy surfaces. In the image to the right, a model dinosaur foot was stamped in watery sand. As the foot pulled out, sediment walls closed up, narrowing the print. “Sometimes you get what looks like a twisted toe, or webbed footing. It’s not there in the actual foot, it’s the nature of the sediment,” says Martin Whyte, a paleontologist who collaborated on the study. Prints formed in clay (like those at the Dinosaur Discovery Site) are preserved best, while those in drier sand often appear enlarged, giving the illusion of a bigger animal with shorter strides. “Once you’ve sorted out the sediment, you can better interpret the actual foot and what that tells you about the animal that made it,” Whyte says.