The type species, P. dawni, is represented by a single near complete specimen (PETMG R338) which was approximately 3.1 metres in length.
It had a robust barrel shaped torso, a relatively short neck and small flippers, indicating 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 such as Simolestes and Liopleurodon).
Its head was about 20% of overall body length, small for most Pliosaurids.
The specimen had a cervical vertebral count of at least 13, these vertebrae having shortened centra and an enlarged neural canal, which were not heavily ossified except in the region of the neural spine (the neural spines themselves were not fused to the centra, which suggests that the specimen was not fully grown; however, this could also be a Paedomorphic trait.)
The cervical ribs were shortened and thickened, as well as double headed.

The pectoral centra of the specimen are triangular in outline, and the dorsal vertebral centra are heavily ossified, with indistinct subcentral foramina and flat anterior and posterior faces.
The anterior and posterior Zygapophyses on the dorsal vertebrae are reduced, however the neural spines thickened, with midline anterior and posterior excavations.
The dorsal ribs, as well as the gastralia, are thickened and pachyostotic (a characteristic of the genus) with concave heads, a round cross-section and a ‘sausage-like’ profile.
Based on histological
sections taken of the pachyostotic elements, it was determined that the bones are osteosclerotic as well (combined, represents the condition pachyosteosclerosis).
This is one of the main characteristics of the genus.
It has been suggested that even though the holotypic individual appears to be a juvenile or sub-adult, its age is probably not solely responsible for these unusual bone structures.

The humeri of Pachycostasaurus are short, small compared to its body size, and lightly constructed.
The type species, P. dawni, is represented by a single near complete specimen (PETMG R338) which was approximately 3.1 metres in length. It had a robust barrel shaped torso, a relatively short neck and small flippers, indicating 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 such as Simolestes and Liopleurodon). Its head was about 20% of overall body length, small for most Pliosaurids. The specimen had a cervical vertebral count of at least 13, these vertebrae having shortened centra and an enlarged neural canal, which were not heavily ossified except in the region of the neural spine (the neural spines themselves were not fused to the centra, which suggests that the specimen was not fully grown; however, this could also be a Paedomorphic trait.) The cervical ribs were shortened and thickened, as well as double headed.
The pectoral centra of the specimen are triangular in outline, and the dorsal vertebral centra are heavily ossified, with indistinct subcentral foramina and flat anterior and posterior faces. The anterior and posterior Zygapophyses on the dorsal vertebrae are reduced, however the neural spines thickened, with midline anterior and posterior excavations. The dorsal ribs, as well as the gastralia, are thickened and pachyostotic (a characteristic of the genus) with concave heads, a round cross-section and a ‘sausage-like’ profile. Based on histological
sections taken of the pachyostotic elements, it was determined that the bones are osteosclerotic as well (combined, represents the condition pachyosteosclerosis). This is one of the main characteristics of the genus. It has been suggested that even though the holotypic individual appears to be a juvenile or sub-adult, its age is probably not solely responsible for these unusual bone structures.
The humeri of Pachycostasaurus are short, small compared to its body size, and lightly constructed.
Pachycostasaurus