The distribution of neck muscles and the shape of the skull shows that Permian and Triassic dicynodonts may be classified into three categories which probably reflect feeding on low, medium and high vegetation. These are distinguished on the basis of the occipital index, the difference between the relative width and height of the occiput, which gives a measure of the relative importance of the lateral and the dorsoventral clusters of neck muscles. The basicranium is relatively shortened in Triassic forms, except in Vinceria, Shansiodon, Tetragonias, and Jachaleria, as has been noted before. Data on skull proportions indicates that the height of the parietal crest may be of little taxonomic use, and that the genus Tetragonias is not a clade. Dicynodonts with dietary preferences at the intermediate level correspond to major branching points in dicynodont phylogeny. High-level feeding adaptations among dicynodonts arose by the middle of the Tatarian (latest Permian) and lasted until the Late Triassic. Specialized low feeders existed only in the Middle Triassic.