The only known specimen was discovered in a 1966–1967 expedition in Transkei District of Cape Province, but was not described until 2012 when Paul Sereno named it as the new taxon Pegomastax africana.

The front of the jaws would have supported a keratinous bill, and wear on the teeth suggests that Pegomastax had a herbivorous diet, possibly of tougher plant matter than relatives that lacked the canine.

It would have lived alongside an abundance of sauropodomorph taxa like Massospondylus, the ornithischian Lesothosaurus, and crocodylomorphs like Protosuchus, as well as the theropod Megapnosaurus and synapsids like Diarthrognathus and Pachygenelus.

A partial disarticulated skull of a new taxon of Heterodontosauridae was discovered in the 1966-1967 expedition to the Transkei District of Cape Province, South Africa, found in the upper Elliot Formation at the Voyizane (alternatively Voisana[3]) locality.

The specimen is preserved in a small sandstone block of matrix, and further preparation of CT scanning may allow for the identification of more intermediate bone fragments.

It can be distinguished from other heterodontosaurids by having a very deep predentary that projects forwards and down, a curved primary ridge on the dentary teeth, and a concave margin to the crown on either side of the middle denticle.

[1] Originally identified as an intermediate undescribed member of Heterodontosauridae, the 2012 description of Pegomastax and its placement within the family was supported by a phylogenetic analysis by Sereno.

Pegomastax was recovered as a derived heterodontosaurid in the subfamily Heterodontosaurinae, united with all other African members of the family and the South American Manidens, which was identified as its closest relative based on the asymmetrical shape and curvature of their tooth crowns.

Tianyulong confuciusi Echinodon becklesii Fruitadens haagarorum Manidens condorensis Pegomastax africana Heterodontosaurus tucki Abrictosaurus consors

[9] Heterodontosaurids have traditionally been assumed to be herbivorous, alongside all other ornithischians, with the possibilities that the caniniforms were a part of sexual dimorphism based on their absence in Abrictosaurus, which was suggested to be a young or female version of another taxon.

[3] Norman et al. suggested in 2011 alternatively that the canines were unlikely to have been used for display or cropping and rooting vegetation, as there was no wear from use present and no strong evidence for sexual variation.

A predominantly or exclusively herbivorous diet in Pegomastax could be directly supported by the extensive tooth-to-tooth wear facets it has, which show that it possessed the same adaptations for propalinal (forwards and backwards) jaw movement and processing plant material as other ornithischians.

[4] Most of the named sauropodomorphs have been found in the middle of the upper Elliot Formation where Pegomastax likely originates from, including Aardonyx, Antetonitrus, Arcusaurus, Ignavusaurus, Ledumahadi, Massospondylus and Pulanesaura.

[3] However, estimations for age of uncertain discoveries suggests that Pegomastax was older than all other heterodontosaurids, and the only other ornithischian it coexisted with was the index taxon Lesothosaurus that can be found throughout the entire upper Elliot Formation.

Plants are represented by the horsetails Equisetum and Equisetites, the cycadeoid Otozamites, the conifers Sphenolepidiurn and Pinus, the seed ferns Phoenicopsis and Dicroidium, algal mat Spirogyra, and fossil wood and pollen of Agathoxylon, Podocarpoxylon, Araucarioxylon, Lacrimasporonites, Uvaesporites and Cyathidites.