Dinosaur "Dance Floor" Found in Arizona

Rebecca Carroll
for National Geographic News
October 21, 2008
At least four types of Jurassic dinosaurs left more than a thousand footprints and tail-drag marks at a remote site in northern Arizona, according to a new study.

About 190 million years ago, during the Jurassic period, the area was likely an oasis surrounded by a vast desert, researchers said.

"All these footprints at a watering hole might tell us something about the social life of the dinosaurs," said Marjorie Chan of the University of Utah.

Chan wonders which species might have mingled at this "dinosaur dance floor."

She co-authored the study with University of Utah graduate student Winston Seiler, who found the site in 2006.

(Related: "First Dinosaur Tracks Found on Arabian Peninsula" [May 20, 2008].)

But the research has received mixed reactions from area paleontologists, with some doubting the tracks' authenticity.

Their work appears this month in the journal Palaios.

Crossing Dino Paths

Chan noted that the prints appear in what was once very wet, soft sand, which allowed for deep impressions.

"You can see the mounds of the sand going around their toes," she said.

Dinosaur footprints are named according to their shape and can't be linked conclusively to an exact species and genus without other evidence, such as bones. But some of a creature's habits can be guessed from its tracks.

The site's 16-inch (40-centimeter), three-toed Eubrontes tracks, for instance, are believed to come from an upright-walking meat-eater that was smaller than Tyrannosaurus rex, the scientists say.

Smaller tracks could be smaller species or younger dinosaurs. "Sometimes it's really hard to tell," Chan said.

Other large prints may have come from unknown sauropods, Chan said, referring to the long-necked herbivores that eventually evolved into Earth's most gigantic creatures.

(Test your dino IQ.)

Not Convinced

Alan Titus, a paleontologist at Grand Staircase-Escalante National Monument in nearby Kanab, Utah, has not seen the site in person but is not convinced that these are really dinosaur tracks.

"I've observed thousands of [Eubrontes] tracks in early Jurassic rocks of the Colorado Plateau and have never seen one that looked like the one in the news release," he said by 0email, noting that he would need to see the site before drawing a firm conclusion.

Andrew Milner, a paleontologist at St. George Dinosaur Discovery Site at Johnson Farm in Utah, also felt the photos were not enough evidence.

"What they're showing here look nothing like Eubrontes in my opinion," Milner said.

"If they do turn out to be tracks, it's really an interesting site—showing what they're doing behavior-wise would be interesting," he said.

"To be honest, I really want to go have a look."

Utah's state paleontologist Jim Kirkland also has not been to the site personally, but he has seen many of the researchers' photos and agrees with some of their conclusions.

"Some of the [imprints] are definitely tracks," he said. "I'm a little more leery about [the] tail-drags."

Tail-drag marks are rare and can be difficult to verify.

Kirkland said he's not convinced any of the prints are from sauropods, because "there's no fossil record in North America of sauropods that old." But, he added, "it's not impossible."

(Related: "Dinosaur Tracks Shed Light on Sauropod Evolution" [May 29, 2002].)

In any case, he said, the find could reveal a wealth of new information about dinosaur behavior.

"Someone needs to take this to the next level," Kirkland said. "It's an exciting site."

Alternative Explanations

The study rejects several alternative explanations for the unusual collection of pothole-like impressions in the rock.

"An initial interpretation of the impressions might be that they are potholes, the product of modern erosion," Seiler said.

"However," Seiler said, "upon close inspection many of the impressions are ringed by mounds of displaced sand that had to be formed when the sand was soft, before it was turned to rock."

The research team also "started seeing repeating patterns … footprints and, in some cases, three clear toes," he said.

Such evidence for dinosaur tracks outweighs evidence for the other possibilities, he said.

No Bones?

Chan noted one lingering mystery: "Why aren't there any dinosaurs bones? We still don't really know the answer to that."

Bones were not found at other track sites in the region, the scientists say, so it might just be that the environmental conditions weren't conducive to preserving them.

Until further research is done, what really happened at the dance floor is anyone's guess.

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