If you pick up a dinosaur picture book written before the 1970s, you'll probably see lots of swimming dinosaurs. Giant dinosaurs like Apatosaurus and Diplodocus used to be depicted as water dwellers. Their bodies were so big, scientists argued, that their legs couldn't have supported their weight on land. In this view, massive sauropods -- four-legged, herbivorous dinosaurs -- spent their lives buoyed by seas and swamps. They used their long necks to dive for aquatic plants and to raise their heads out of the water to breathe.
This theory portrayed sauropods a little more like wallowers than swimmers -- and it turned out to be wrong. In the 1950s, K.A. Kermack studied the effects that water pressure would have had on sauropods' breathing. According to his analysis, the pressure of the surrounding water would have crushed the thorax of a deeply submerged sauropod, cutting off its air supply [source: Fastovsky et al]. In the 1960s, researchers explored the fossilized remnants of an Apatosaurus habitat. They determined it to be a woodland, not a swamp [source: Rajewski].
With their long bodies and flipperlike fins, prehistoric animals like plesiosaurs and mosasaurs were definite swimmers -- but they weren't dinosaurs. Dinosaurs were, by definition, land animals. Since most land animals can swim at least a little if they suddenly find themselves in water, it seems logical that dinosaurs could, too. However, concrete evidence of this has been hard to come by.
If paleontologists find a dinosaur's body in the bottom of what used to be an ocean, that doesn't mean it died there. Predators, currents and even natural events like landslides could have moved the body to deeper water. For this reason, researchers look for trace fossils, not bones, when determining whether dinosaurs could swim. Next, we'll take a look at why finding trace fossils of a swimming dinosaur is so challenging and why partial prints offer the best clues of swimming dinosaurs.