There are two depressed regions of the skull piece, one along the bottom left side and another cut into the back end, interpreted as representing a site for jaw muscle attachments at the edge of the temporal fenestra and for the pineal foramen, respectively.

[1][2] Only the very front of the mandible of Pentasaurus is known, mostly consisting of the fused mandibular symphysis (including the splenial) and part of the left dentary and angular, although the tip of the beak is missing.

Features such as the shape of the beak and lateral dentary shelf likely influenced the feeding style of dicynodonts, although the significance of the unusual morphology in Pentasaurus is currently unknown.

The known portion of ulna is badly worn, but the roundness of its tip suggests that Pentasaurus had a separated olecranon process of the elbow, similar to Placerias and Ischigualastia but unlike some specimens of Stahleckeria.

Although similar in construction to sauropodomorph dinosaurs, the internal structure visible at the broken end shows extensive trabecular bone, characteristic of Triassic dicynodonts.

[1] The only known fossil remains of Pentasaurus were collected in the late 19th century by amateur palaeontologist Alfred "Gogga" Brown, an Englishman recluse who lived for much of his life in Aliwal North in the Eastern Cape Province, South Africa.

After initially corresponding with and sending a crate of fossils to the geologist Sir Roderick Murchison in London, Brown's relationships with English and later Parisian scientists soured.

The fossils of Pentasaurus were part of a lot accessioned in 1876 that was determined to have been collected from a single locality in the Norian aged lower Elliot Formation, based on the shared brown-grey colour of the bones and brown sandstone matrix surrounding them, as well as the association with fossils of the exclusively lower Elliot dinosaur Eucnemesaurus.

[1] Prior to the recognition of Pentasaurus body fossils, dicynodonts were inferred to have been present in the lower Elliot Formation from preserved footprints and track ways attributed to them.

[3] Similar trace fossils were identified on other continents, including South and North America, where they were more confidently attributed to large dicynodonts.

[1][5] Re-examination of Brown's collections in the 21st century revealed that a number of bones belonged to a large dicynodont, and in 2018 they were described by palaeontologist Christian Kammerer as a new genus and species, Pentasaurus goggai.

Furthermore, a number of features of the lower jaw and humerus support its position in the stahleckeriid subfamily Placeriinae, and so as a close relative of the North American Placerias, Moroccan Moghreberia and the giant Polish Lisowicia.

The robustness and position of the lateral dentary shelf far forward on the lower jaw is also unique to Pentasaurus among dicynodonts, as well as the thickness of its distal humerus.

[1] The lower Elliot Formation consists mostly of red-purple mudstones and thick, stacked layers of sandstone deposited by meandering river channels on a floodplain.

The stability of the channels suggests that riparian forests were present along the banks of these rivers, which is further supported by the relatively high abundance of fossilised wood fragments found in the lower Elliot Formation.

The coexistence of Pentasaurus and large sauropodomorphs in the lower Elliot Formation suggests that this may not be the case, and that the two groups may have instead been segregated by habitat preference, and may have fed upon different types of vegetation.