This fossil was found locally at the brick pits in Kings Dyke in 1994. Scientists studied the bones and found that this was a unique beast: a new type of pliosaur not found before.
Pliosaurs were a group of plesiosaurs, skilled in ambush and the fast pursuit of their prey. They had thin bones which were designed to reduce the weight of their skeletons. Pachycostasaurus Dawnii was found to have elements of its skeleton which were thickened rather than thin. This has suggested to scientists that it would not have been a fast swimmer, but instead a slow stable swimmer.
When new types or species are found, the scientists that have described them also get the chance to name them. Pachycostasaurus dawnii was named after its discoverer; local palaeontologist, Alan Dawn.