Feeling green. Floating sauropods would have easily tipped sideways.

Bobbing for Sauropods

ST. PAUL, MINNESOTA--The first reconstructions of sauropod dinosaurs depicted them submerged in lakes, eating soft aquatic plants. Later, scientists realized that sauropods really were land-lubbers. The giant dinosaurs got their feet wet again--but just a little--at the annual meeting of the Society for Vertebrate Paleontology on 18 October, where researchers proposed that some strange footprints were left by sauropods that were wading. Don't picture full-immersion, though: Another study suggests that floating sauropods would have been prone to capsizing.

One of the initial findings that plunked sauropods in lakes was the discovery of strange trackways that had only the marks of front feet. With its butt bobbing, the story went, a sauropod pulled itself along with its forelimbs. Then in the late 1980s, some scientists argued that the dinosaurs had had all fours on land. All that was preserved were the traces of the front foot, or manus, which had pushed further into the sediment.

But paleontologist Jeff Wilson of the University of Michigan (UM), Ann Arbor, was a little bothered by the fact that most sauropods have their center of mass toward their back end, which might lead to greater pressure on the hind limbs. So he and Dan Fisher of UM measured the force exerted by the limbs of plastic models of various sauropods. When they added water, the tail raised. All four limbs stayed on the bottom of the tank, but the front limbs began to bear a greater share of the load--up to 20 times more for a sauropod called brachiosaurus. That could explain the manus-only trackways, they say.

According to a computer model by Don Henderson of the University of Calgary, however, the trackways imply just two legs on the bottom. Once a brachiosaurus was in shoulder-deep water, its tail would rise and its hind limbs would lose contact. In much deeper, and all fours would float off the bottom. At that point, Henderson's model indicates, these long, narrow animals would have been prone to capsizing. "The slightest wave would tip it sideways," Henderson says, who thinks sauropods would have had trouble swimming. Others suspect they probably could have. But no one sees sauropods heading back in the deep again.

The studies "give us a little window on some possible behaviors that we didn't think about before because we were so against sauropods in the water," says paleontologist Kristi Curry Rogers of the Science Museum of Minnesota, in St. Paul.

Related site
Classic illustrations of snorkeling sauropods