A remarkably well-preserved primitive horned brontothere skull from the Wiggins Formation of western Wyoming is described. Noteworthy characters of this specimen include an extensive system of large paranasal sinuses, a distinct vomerine plate, and a confluence of the anterior lacerate foramen and foramen rotundum. A nearly complete endocranial cast allows for a description of the brain and many of the cranial nerves. Perhaps the most important structures described are the large openings on the basicranium, here dubbed “sphenoidal apertures,” which may be unique among the Brontotheriidae. Among North American brontothere taxa, the skull is closest in morphology to the holotype of Protitanotherium emarginatum Hatcher, but has some important differences that make assignment to that genus and species uncertain. If reference to the genus Protitanotherium is correct, then the name P. australis (Wilson) may apply, although this may be a junior synonym of P. superbum Osborn. Given the uncertainties, the specimen is identified only as cf. Protitanotherium for the purposes of the present paper. Regardless of which name should be applied, characters displayed by the specimen suggest that it is the primitive sister genus to all other diplacodont brontotheres.