Rewind the clock to 95 million years ago, and imagine this scene: At the edge of a watering hole, more than a hundred dinosaurs have gathered. They are small, the size of chickens to ostriches.
They are drinking peacefully, when a giant meat-eating dinosaur suddenly tears out of the brush. Its teeth and claws flash as it charges in for a meal. The small dinosaurs flee, leaving evidence of their frantic escape in footprints pressed into soft mud.
That is what many scientists believe happened in northeastern Australia, at a site known as Dinosaur Stampede National Monument. Fossilized footprints lay scattered across a giant slab of rock.
"This event that you can see here right before your eyes–you cannot find anywhere else in the world," says tour guide John Taylor.
He points to a single line of big footprints that he says were left by the large meat-eating dinosaur. The rest of the rock is covered by tiny footprints, made by the little dinosaurs that ran away.
Scientists discovered these tracks in the 1970s, and it took them years to piece together what happened.
The story they tell–of the world's only known dinosaur stampede–attracts tourists from all over the world to this stretch of the Outback. It has also inspired an Australian children's song.
Watch a performance of "Dinosaur Stampede" at the Museums Australia 2012 National Conference.
But one scientist has now raised questions about what really happened at Dinosaur Stampede National Monument.
A fresh look
"Perhaps it's not a stampede," says Anthony Romilio, a paleontology graduate student at the University of Queensland in Brisbane.
Romilio did not set out to challenge the traditional interpretation of the dinosaur footprints. The goal of his research was to study the fossils to figure out how dinosaurs moved their limbs.
But he found himself hitting a brick wall. To his eye, the footprints did not fit with the standard view of what had happened there.
"It took me about six months of pure confusion, going, 'Look, my research–it's just going nowhere,'" he says. "My supervisor said, 'Just imagine you're the first scientist to come to the scene. How would you interpret it?'"
So Romilio took a fresh look at all those footprints and concluded that the scientists before him had misinterpreted the evidence.
He has two basic contentions.
First, that big dinosaur that thundered onto the scene? Romilio does not think it was a carnivore at all.
When Romilio studied the large footprints, he found that they had short, broad toes–evidence, he says, of a plant-eating dinosaur. "The footprints that belong to meat-eating dinosaurs [had] long toe impressions that were quite narrow," he explains.
If the large dinosaur was a plant eater, the small dinosaurs would have had little reason to run away.
Second, when Romilio looked at the little footprints, it seemed to him that the small dinosaurs were not running at all–they were swimming, pushing themselves along in a stream as the water buoyed their bodies.
Romilio displays a three-dimensional rendering of one of the little footprints on his computer. Such a small part of the foot made an impression, he says, that there is not much to it.
"Some of these animals, when they're being fully buoyed by the water, they're only able to just touch the river bottom with the tips of their toes," he says.
He suspects the little dinosaurs left their footprints while swimming in a stream over days or even weeks, not fleeing from a predator in a panic.
These conclusions have sparked a lively debate among paleontologists. Some scientists think Romilio has raised valid objections, and they too are questioning whether a stampede happened at all.
The role of mud
But Romilio has a lot of critics, including the chief dinosaur expert at the Queensland Museum, Scott Hocknull.
"Just simply looking at the footprints isn't enough," Hocknull says. He argues you have to consider the mud those footprints were left in.
For instance, if you look at the prints of the large dinosaur, they do appear to show big, round toes–as if from a plant eater. "[But] that's the mud being squished out from below the toes, between the toes, as the animal's slamming its foot into the mud," he says.
To demonstrate this point, he takes a cast of one of the big footprints and shines a light on it from a low angle to reveal its subtle contours.
"See this triangular piece?" he says, pointing at one of the toes. "That's the claw mark. There's only one type of animal that makes those sorts of footprints, and that's a meat-eating dinosaur."
Hocknull asserts that Romilio did not appropriately account for how the mud interacted with the dinosaur's feet.
Hocknull also argues against the contention that the little dinosaurs were swimming, not running. He says the footprints could not have been made underwater, because they would not have lasted.
"When you put mud underwater, you put the foot in, you pull the foot out, and it forms what looks like a footprint," he says. "[But] give it ten minutes, come back, and see if you can actually even see the footprint. What happens is it all collapses."
Hocknull says if these dinosaurs had been swimming, they would not have been able to leave any impressions behind, deep or shallow. And he has new research in the pipeline (which he says he cannot yet discuss publically) that he believes will clearly show the new interpretation to be wrong.
Romilio's supporters may not be so easily convinced, however.
"Not only do I think Anthony [Romilio] is on to something," wrote paleontologist Andrew Milner–curator at St. George Dinosaur Discovery Site in Utah–in an email, "but I believe he has cracked it by clearly demonstrating that many of the 'stampeding' dinosaur tracks are indeed dinosaur swim tracks."
Back at Dinosaur Stampede National Monument, tour guide John Taylor is skeptical of the theory that the small dinosaurs were swimming, but he says he is willing to change his tune if necessary.
"If they have the evidence to back that up, then that's what we'll start telling people," he says. "We're here as interpretive guides. We're not here to tell people our own personal beliefs."