In a remote corner of southern Arkansas, scientists have found a rocky field full of dinosaur footprints -- lots of them, in an area about as large as two football fields.
The fossilized tracks probably date to the Early Cretaceous period, 115 to 120 million years ago. Researchers say the dinosaurs who left them probably included giant predators, such as Acrocanthosaurus atokensis, an early cousin of T. rex. There are also large, long-necked plant-eating dinosaurs such as Pleurocoelus and Paluxysaurus, who may have been easy prey.
Stephen Boss of the University of Arkansas, who leads the team working on the site, says the footprints are in excellent shape -- probably a good snapshot of what life in the Cretaceous may have been like there.
Back then, the field was probably a mud flat on the southern coast of what is now North America. Silt, covering it over a period of months or years, would eventually have hardened to preserve the footprints.
"This place was very warm, but also pretty arid," said Boss. "We found molds of salts, gypsum, the kinds of things you see on a seacoast. If you want a modern analogy, try to imagine what it's like on the shores of the Persian Gulf -- really hot and dry."
And yet the dinosaurs gathered there in large numbers. Some of the tracks, hundreds of feet long, crisscross each other.
"One of the mysteries is what they were doing there," said Boss. "It was a harsh environment."
A. atokensis, if that's what the researchers have found, was one of the largest predators ever to walk the earth. The dinosaur had three toes on each foot. The footprints are about two feet long and a foot wide.
To understand these very ancient creatures, the research team used high and low technology. They took plaster casts of footprints -- much as paleontologists would have a century ago -- and also brought in laser scanners to map the site. The dinosaurs themselves may be long gone (there were no fossilized bones at the site), but precise measurements of their footprints can offer clues as to how they moved.
Dinosaur tracks are surprisingly common, but not in spots this large. Scientists can examine the area to infer how much rain there was, and how quickly it evaporated. If they can reconstruct the climate of the Early Cretaceous, the university said, it may help in predictions about Earth's future climate.
The land is on private property in southwestern Arkansas, not far from the borders of East Texas and northern Louisiana. For now, the researchers are keeping the precise location a secret.