It was a thought in the 50's that a number of dinosaurs were no more than swamp dwellers, staying in water and swamps to support the weight of their bodies due to their size. It was thought that they could not have supported the weight without the help of water. Studies that were done by K. A. Kermack assumed that the pressure of deep water would have crushed their thorax causing them to drown due to their size and the pressure that water would have caused on the lungs but it soon became evident that dinosaurs were land animals. It was easy to assume that some of the prehistoric animals could swim due to their elongated bodies and flipper like fins but these were not real dinosaurs. By the 1960's the paleontologist knew that dinosaurs lived on solid land not in swamps as first thought. It was also stated that dinosaurs did swim when the need arose but were basically land animals.
There have been incidents where dinosaur fossils have been found at the floor of what use to be an ocean or lake. Was this evidence that dinosaurs did swim? The simple answer is no, due to the reality that a landslide or some other quirk of nature deposited the fossil there. Due to this reason researchers had to look for trace fossils, not bones, in their determining if the dinosaur could swim. This would prove to be quite a challenge.
Partial prints would supply the greatest clues as to whether the dinosaur could swim or not and this would be found in trackways. Researchers knew that track ways would be the principal thing to look for. They would leave marks equivalent to those left when walking along a beach. You can observe where you have been and the sand will verify the identical story but once you step into the water all is lost and there is no way to track yourself. You may leave a track at the bottom of the ocean floor but you can't identify it like you could on the shore. This is what researchers were looking for. Tracks that may have been left in the ocean floor by the dinosaur, they were unquestionably heavy enough to leave some kind of mark if they had entered the water.
Evidence that help scientist lean towards the belief that dinosaurs could swim, came from just such track ways. They found partial footprints and partial track ways. This was interpreted as track ways that conserved only a quadrupeds front footprint which is recognized as a Manus as the traces of a swimming sauropod. They had the notion that the sauropod would shove off with its front feet while the back legs floated at the rear. At first Paleontologists was beginning to assume that such track ways were too ordinary and conventional to characterize a swimming dinosaur. They now think these very tracks may have been made by dinosaurs which left impressions in the ground as they pushed off to swim. Their size would unquestionably have left a very yawning print in the earth; this is called "under prints". It would be the equal to a very big dinosaur walking on land and leaving a footprint, particularly if it walked on soft earth.
The tracks were erratic. They told the story of an animal that was struggling with the changes of currents and depth as it moved through the water. In 1980 W. P. Coombs published his results in the Journal Science of a set of lower Jurassic scrape marks that he thought were that of a theropods swim tracks. This finally dispelled the suggestion that was believed up to now that the dinosaur stayed away from water.
Researchers found in the Camerous Basin, La Roija, Spain, a fifty foot, Cretaceous Period track way that showed 12 disproportionate prints of an animal's feet or what is referred to in the world of science as "Pes". The earth that the prints are found in is rippled which indicates there was a current overhead. The left prints are those of claw marks that moved in a parallel path with the right prints veering off at an angle. It can't be specific what kind of dinosaur made these tracks but they are understood to be those of a swimming dinosaur. These tracks were determined by paleontologist, Ruben Ezquerra and his team. It is assumed that these tracks did belong to a theropod-a bipedal, often a carnivorous dinosaur that was struggling in opposition to the current and while in the water would left marks on the bottom of the ocean.
A team led by Deborah Mickleson, in Wyoming found tracks that appeared to be those of a bipedal dinosaur heading into the water to swim. At first the prints are very distinguishing but grow fainter as it goes into the water. These tracks are from the Middle Jurassic Period but once again, it is not known what kind of dinosaur made them.
There are other track ways that have been found which comprise of claw marks, drag marks, and incomplete prints which all points to swimming dinosaurs. A team led by Andrew C. Milner reported a large quantity of tracks which they found in 2006 in St. George, Utah. This team was able to recognize the tracks due to dinosaurs having muscles and skin around their bones when they made them. Due to this, the scientist gave these tracks their own name and classifications. The Utah tracks have been acknowledged as Characichnos (Claw marks), Grallator and Eubrontes (Tracks of bipedal dinosaurs). All the finds relate to bipedal dinosaurs and researchers haven't reported a whole lot on these swim tracks. This does not imply that quadrupeds such as Apatosaurus and Diplodocus never swam but up to now there is no apparent evidence that they did.
There is progress being made every day to find out without a doubt if dinosaurs did swim. Given that almost every animal on earth can swim it is not a farfetched idea to assume that dinosaurs did swim if they needed to when given the right circumstances. As for definite, without a doubt proof, there is none but with scientist working so hard towards the truth we may know for sure sooner than we think.