The original view most scientists held of dinosaurs were as water or swamp dwellers that needed the buoyancy of water to support their huge bodies because they were just too big to live on land. Quadrupeds such as Apatosaurus and Diplodocus were often portrayed as wallowing in the depths of rivers and swamps similar to a modern hippopotamus. Apatosaurus and other Sauropods were thought to use their long necks to graze the water for aquatic vegetation.
It was not until the 1950's that this picture began to change with the work of K.A. Kermack of Columbia
University. Kermack studied how large Sauropods would have been able to breathe as water dwellers and found that the pressure of the water would have crushed their thorax and suffocated them.
Later in the 1960's further studies were carried out on the habitat of Apatosaurus and it was found they were more likely forest rather than water dwellers, showing that dinosaurs covered a more diverse habitation than had previously been suspected and were not restricted to water.
Most modern land animals will swim if they have to, and since most dinosaurs lived near sources of water it was assumed they could also swim but proof was need to establish this. Although many species had hollow bones which might have helped them to float a little, their narrow bodies could have been unstable in the water but this did not necessarily preclude them from swimming.
Some interesting finds have since bought more evidence to bear on the issue. In 2003 a set of fossilized track marks from a Cretaceous river bed in the Cameros Basin
in Spain provide some indication that dinosaurs could in fact swim. The track marks are fifteen metres long and show S-shaped claw marks which suggest a bipedal dinosaur dragged its feet along the lake bed as it swam.
The track way is well preserved in sandstone and consists of twelve prints with two to three scratch marks. These marks suggest the dinosaur was buoyed by the water and was swimming with a pelvic paddle motion something like a modern aquatic bird. Ripple marks around the dinosaur tracks indicate that it was trying to swim against the current in three metre deep water and trying to keep a straight line.
Other similar track ways have also been found. In Wyoming
a set of tracks from the Jurassic Era were discovered that appear to show foot prints of a dinosaur at the edge of a river gradually entering the water. The marks become fainter as the dinosaur enters deeper water. In Utah
track marks from various dinosaurs were found in 2006, which were made up of scratches, claw marks and partial prints which also suggest a dinosaur being buoyed up by a water source as it swam.
Although not all Palaeontologists believe such track marks demonstrate that dinosaurs could swim, and that they could have been made on land under certain conditions, evidence does seem to be accumulating that at least some dinosaurs were able to take to water. There has yet to be found any indication that large Sauropods could swim but this may come in time.