How does an animal as big as a semitruck move around? That’s a question scientists have long asked about sauropods, the largest known dinosaurs, which may have weighed as much as 70 tons. Did they walk like a giraffe, picking up their two left and right legs in a dignified march? Or like an elephant, lifting and placing one front foot, then the opposite back foot to amble along?
Neither, according to a new analysis of fossilized footprints. Instead, the giant beasts kept one foot down on each side while leisurely swinging their diagonal legs, much like a beaver or a hedgehog does today.
The approach of analyzing trackways could be used to reconstruct the gaits of other dinosaurs as well, says Cary Woodruff, a sauropod paleontologist at the Great Plains Dinosaur Museum and Field Station, who was not involved with the study. “The way they did this tells us a lot about how these animals were moving,” he says. “It’s really neat.”
Fossilized bones can reveal the shape of an extinct animal’s body, but paleontologists look to footprints and other evidence to deduce things like behavior and locomotion. Past studies posited that sauropods, a group of long-necked dinosaurs that lived between 201 million and 66 million years ago, walked with a “pace gait,” like a giraffe, where the left or right legs hit the ground at once.
But that motion didn’t make sense to Jens Lallensack, a paleontologist at Liverpool John Moores University. If a sauropod walked this way, he says, it would risk falling over as it shifted its gargantuan weight to each side. “If a 30 to 40-ton animal falls, it’s going to be dead.”
So Lallensack and fellow paleontologist Peter Falkingham, also at John Moores, took a new tack. They chose three trackways—lines of footprints made by a single sauropod—uncovered near gypsum quarries in southwestern Arkansas in 1989 and 2018.
First, they measured the distances between each imprint, noting whether the tracks were made by the animal’s front or back and left or right foot. Then the researchers calculated how various limb phases, or measurements of the time lag between forefeet and hindfeet, fit onto the trackways. Each animal’s gait is extrapolated from this measurement: For example, a pace gait with a limb phase of 0% means both front and back legs on one side swing in sync.
The limb phase can be mapped on to a hypothetical sauropod’s body by measuring its trunk, or the distance between its shoulders and hips. For each potential limb phase along the trackway, this trunk length might vary between sets of prints. But the actual track maker had a uniform body. “The animal isn’t going to get longer or shorter as it walks,” Lallensack explains.
For all the possible limb phases, the researchers calculated which one corresponded to a trunk length that changed the least. For each trackway, they found the sauropod walked in a “diagonal couplet” pattern: always keeping one foot down on each side and picking up diagonal limbs nearly in sync, the team reports today in Current Biology.
Lallensack was surprised. “I thought they might walk like an elephant,” lifting and setting each foot one by one, he says.
The researchers tested their method on trackways left by modern animals including dogs, horses, camels, elephants, and even a raccoon, checking their work against videos of live animals walking. Each time, their analysis predicted the correct gait.
It’s a “reasonable” conclusion, says Purdue University, Fort Wayne, paleontologist James Farlow, who notes he’s “always thought diagonal made more sense than a pace gait” for a sauropod.
But both Farlow and Woodruff say that sauropods were diverse, varying in size and possibly in locomotion. “I’m not sure that what this particular beast in Arkansas was doing is the only thing this fauna would have done,” says Farlow, who has worked on nearby sauropod tracks in Texas.
Lallensack agrees and plans to apply the analysis to those of other sauropods and dinosaurs.
“A track is an intimate moment in time for these animals,” Woodruff says. “It helps us understand them in ways we can rarely do.”