National Geographic Daily News
Dinosaur tracks picture.
The newfound tracks suggest A. atokensis' feet weren't webbed after all.

Photograph courtesy Russell Cothren, University of Arkansas

Christine Dell'Amore

for National Geographic News

Published October 7, 2011

Fossilized tracks of dinosaurs "stomping in the mud" have been discovered in southwestern Arkansas, scientists say.

Spanning the length of two football fields, the footprints hint that a giant predator was a bit pigeon-toed.

Several species, including the eight-ton Acrocanthosaurus atokensis—one of the largest predators ever to walk Earth—and sauropods, or long-necked plant-eaters, left their footprints in the 120-million-year-old Cretaceous limestone.

At the time, Arkansas was a broad mud flat, similar to the hot, dry, and salty shores of the modern-day Persian Gulf—not a particularly "pleasant place," said team leader Stephen Boss, a geoscientist at the University of Arkansas in Fayetteville.

Predators like Acrocanthosaurus were likely attracted to the site by sauropods and other prey species, but "what the sauropods are doing out there, who knows?" Boss said.

Though found elsewhere in North America, dinosaur trackways are rare in the southern U.S., he said. Indeed, most people tend to think of dinosaurs dwelling in the "classic" western lands of Colorado and Utah.

"They don't think this is a place that dinosaurs once roamed, but it is—and here's the proof."

(See "First Dinosaur Tracks Found on Arabian Peninsula.")

Dinosaur Tracks Reveal Pigeon-Toed Predator?

A private citizen recently found the tracks, which were possibly exposed after a rainstorm scoured away a thin layer of shale. The shape of the footprints and the age of the limestone leaves "no doubt" that they were left by dinosaurs, said Boss, whose new research has not yet been published.

"The photographs seem to make it clear that they are indeed theropod dinosaur tracks," vertebrate paleontologist Thomas R. Holtz, Jr. said via email. Theropods, which included T. rex, were two-legged predators.

"Acrocanthosaurus tracks are already well known from Texas, and we have fossils of Acrocanthosaurus and closely related forms from Texas, Oklahoma, and Maryland, so almost certainly it lived in Arkansas, too," added Holtz, of the University of Maryland.

The tracks were likely left by multiple dinosaurs and must have been filled in fairly quickly—if they'd been exposed for long, the prints would have eroded beyond recognition, team member Boss said.

Set Lasers to "Discover"

Boss and colleagues scanned the trackway with a laser at a high resolution. The scan digitally preserved the tracks so that the scientists could analyze them and "walk across that surface in cyberspace," he said.

For example, looking more closely at Acrocanthosaurus's 2-foot-long (0.6-meter-long) footprint will help answer key questions such as "What did this thing look like when it had meat on it?" Boss said.

(See picture: "'Polar Predator' Dino Tracks Found.")

Already, the scans have revealed that the three-toed Acrocanthosaurus didn't have webbed feet—a discovery that wouldn't have been possible with just bones for evidence.

Researching the tracks digitally may also show scientists precisely how the dinosaur walked. "One of the things that surprised me [from early analyses]," Boss said, "is the feet turned inward—sort of pigeon-toed."

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