The fossil was described in 1996 by Palaeontologists Arthur Cruickshank, David Martill and Leslie Noè, due to its distinct set of features not present in contemporary pliosaurids Liopleurodon and Simolestes.

This robust, barrel shaped torso, coupled with a relatively short neck and small flippers, indicate that it was not an agile swimmer.

The holotypic specimen possessed five stout and conical tooth pairs on the mandibular symphysis (the length of which is a clear characteristic of the genus in comparison to other contemporary Pliosaurids Simolestes and Liopleurodon).

The relative size and shape of its teeth indicate a diet ranging from soft teuthoids, such as belemnites, to bony fish, and perhaps some of the larger reptiles.

Pachycostasaurus has a lightly constructed skull, a short, rather weak jaw symphysis, and ventral ballast for stability which would have resisted roll.