Bob, bob, bobbin' along: dinosaur buoyancy may explain odd tracks.

Some of the heftiest four-legged dinosaurs ever to walk the Earth occasionally left sets of footprints that include only the imprints of their front feet. New laboratory and computer studies may explain what those animals were doing with their hind legs.

The sauropod group of dinosaur species consisted of large herbivores, some weighing up to 100 metric tons. These behemoths spent most of their time on all fours but may have reared up on their hind limbs to defend themselves of browse on high foliage. That posture can't explain the trails of sauropod footprints with no traces of hind feet.

Adding water to the equation, however, may solve the puzzle. Computer analyses of sauropod buoyancy conducted by Donald M. Henderson, a paleontologist at Canada's University of Calgary, suggest that floating sauropods of some species could indeed have made forefoot-only trackways.

Henderson's model divides a sauropod's body into many thin slices and calculates both the downward-acting weight and the upward-acting buoyancy of each slice. The model also accounts for body cavities, such as the lungs, and for appendages, such as the neck, tail, and limbs.

Brachiosaurus and Camarasaurus, sauropods that had relatively long front limbs and a balanced weight distribution, floated with their forefeet deeper than their back feet, Henderson found. So, they could have left prints of only their front feet as they moved through shallow water. However, Diplodocus and Apatosaurus, sauropods that had long tails and carried most of their weight over their rear legs, floated with their hind feet deeper than their front feet. That makes it almost impossible for them to produce forefoot-only trackways while partially floating, says Henderson. He presented results of his analyses last week at the annual meeting of the Society of Vertebrate Paleontology in St. Paul, Minn.

However, there still may have been a way for even those sauropods to have left no-hind-foot tracks, argue Jeffrey A. Wilson and Daniel Fisher of the University of Michigan in Ann Arbor. By placing 1/40-scale models of various sauropods on sensitive balances, the scientists measured the changes in weight borne by each creature's front and rear feet as the models were immersed in slowly rising water. When it reached wading depth for the sauropods, the water partially buoyed the models' tails and bodies. That would have shifted the animals' weights toward their front feet, Wilson says.

At certain water depths, all the sauropod models that Wilson and Fisher analyzed--both those with balanced weight distributions and those that were hip-heavy--exerted footprint pressures with their front feet that were more than twice those exerted by their rear feet. Therefore, says Wilson, it's possible that some sediments would record only the imprints of a wading dinosaur's front feet. He presented these findings at last week's meeting in St. Paul.