Trackways preserve information about dinosaurs the same way the sand does as you walk down the beach -- if you look behind you, you can see where you've been. If you stopped to pick up a sand dollar or turn over a shell, you can see that, too. But the sand doesn't do a good job of keeping up with you if you decide to go for a swim. You might see your tracks going into the water or an occasional mark where your hand or foot touched the bottom, but you won't get a clear picture like you can with your footprints on the shore.
Similarly, most of the evidence that dinosaurs could swim comes from partial footprints and incomplete trackways. In the past, researchers interpreted trackways that preserved only a quadruped's front footprints, known as the manus, as the traces of a swimming sauropod. In this interpretation, the sauropod was using its front feet to push off the bottom while its back feet floated behind. Today, some paleontologists believe such trackways are too regular and predicable to represent a swimming dinosaur. Instead, these tracks may be underprints, impressions made very deep in the ground by a massive dinosaur walking on land [source: Vila et al].
The tracks most commonly interpreted as the prints of swimming dinosaurs are erratic. They suggest the marks of an animal that was adjusting to changing currents and depths. An early example is a report by W.P. Coombs in 1980. Coombs published findings of a set of Lower Jurassic scratch marks he interpreted as theropod swim tracks in the journal Science [source: Coombs]. This started to dispel the common idea that carnivorous dinosaurs stayed away from the water, so prey could escape them by swimming.
A 50-foot (15-meter) Cretaceous Period trackway found in Cameros Basin, La Roija, Spain, shows 12 irregular prints of an animal's back feet, or pes. The prints are in rippled ground, indicating that there was a current overhead. The left prints are claw marks moving in a parallel direction, and the right prints veer off at an angle. A team led by paleontologist Rubén Ezquerra interprets these prints as those of a swimming theropod -- a bipedal, often carnivorous dinosaur -- struggling against the current [source: Ezquerra et al]. Since the prints consist only of claw marks, researchers can't tell what type of dinosaur made them.
In Wyoming, a team led by Deborah Mickelson found a set of tracks that appear to be those of a bipedal dinosaur heading into the water for a swim. The prints start out clear, then gradually become less complete as they enter deeper water. These tracks are from the Middle Jurassic Period, but it's unclear exactly which dinosaur may have made them [source: Mickelson].
Other trackway finds combine claw marks, drag marks and partial prints, adding up to the suggestion of a floating or swimming dinosaur. In 2006, a team led by Andrew C. Milner reported a large collection of tracks found in St. George, Utah. The team was able to identify the tracks -- after a fashion. Because dinosaurs had muscles, fat and skin around their bones, it can be impossible to match a set of tracks with the skeleton of the dinosaur that made them. For this reason, scientists give tracks their own names and classifications. The Utah swim tracks have been identified as Characichnos, Grallator and Eubrontes. Characichnos denotes claw marks, while Grallator and Eubrontes are the tracks of bipedal dinosaurs.
These finds relate to bipedal dinosaurs -- so far, researchers haven't reported a lot of quadruped swim tracks. This doesn't mean that quadrupeds like Apatosaurus and Diplodocus never swam, though. Clear evidence that they did may someday be found.