The proof is in the footprints the dinosaurs left behind. Some sets of sauropod footprints include only the front feet.
The brachiosaur shown above is an example of a sauropod.
These prints have been a real puzzle for scientists, partly because sauropods were so massive. A typical adult sauropod might have weighed 100 metric tons, which is equivalent to the weight of about 70 cars.
Part of this weight was due to the animals' enormous tails. So it's not too difficult to imagine these dinosaurs standing on just their hind legs. They might have reared up, like a horse, to defend themselves. Or they might have stood on their back legs to reach leaves in the treetops.
But how did these hefty animals manage to walk on their front legs, keeping their tails off the ground? And why did they do it?
Donald Henderson, a scientist at the University of Calgary, used computer simulations and Jeffrey Wilson and Daniel Fisher of the University of Michigan used plastic models of sauropods to find the answers. They concluded that the dinosaurs could balance on their front legs only if they were wading in shallow water.
In some sauropod species, the rear of their bodies—along with their tails and hind legs—would float, while their front feet would dig into the underwater mud. In other sauropod species, the hind legs might also dig into the mud—but much less than the front feet.
Over time, the shallower, rear footprints would tend to disappear, while the deeper, front footprints would remain visible. All that would be left are hints of an amazing dinosaur feat and a tantalizing puzzle for researchers to solve.—S. McDonagh
Perkins, Sid. 2003. Bob, bob, bobbin' along: Dinosaur buoyancy may explain odd tracks. Science News 164(Oct. 25):262. Available at http://www.sciencenews.org/20031025/fob7.asp .
You can learn more about sauropods at www.zoomdinosaurs.com/subjects/dinosaurs/glossary/Sauropod.shtml (Enchanted Learning Software).