Superbly preserved skeletal elements of the new taxon Oromycter dolesorum from the Lower Permian deposits in the Dolese Quarry, near Richards Spur, Oklahoma, record the earliest known evidence of a member of Caseidae. Although found as isolated elements, there is strong circumstantial evidence that the maxillae, premaxillae, lacrimal, and dentary belonged to a single individual. Isolated axial and appendicular elements from this locality are also tentatively attributed to this new form.
Caseasaur autapomorphies of Oromycter include the presence of the premaxillary rostral process, indirect evidence for an external shelf on the nasal, and lateroventrally tilted external maxillary surface. Caseid autapomorphies include the anterodorsally oriented maxillary process, the short, tall lacrimal, typical caseid ornamentation on the lacrimal, procumbent anterior dentition, largest teeth located anteriorly on each bone, and spatulate, lingually curved marginal dentition. However, the available evidence indicates that Oromycter is a sister taxon to all other caseids because it lacks the leafy serrations on the teeth, has a relatively long lacrimal bone, has a larger number of marginal teeth than most other caseids, lacks an external narial shelf on the maxilla while one is poorly developed on the lacrimal. The presence of this early, basal member of the Caseidae at the Dolese Quarry is consistent with the hypothesis that the initial evolutionary history of this group of herbivores occurred away from the typical deltaic, floodplain depositional environments that preserve the majority of the known Permo-Carboniferous terrestrial vertebrate communities.