Two of the four living species of tapir (genus Tapirus) possess a distinct sagittal crest, but the development of this crest follows an unusual path in one species, T. terrestris. In T. pinchaque, T. bairdii, and T. indicus, as in other mammals, temporal ridges form on either side of the braincase, and, as postnatal ontogeny proceeds, these ridges migrate dorsad and mediad. In T. pinchaque and other mammals with a true sagittal crest, the temporal ridges eventually meet at the dorsal midline of the skull. In T. terrestris, paired temporal ridges never develop; instead, the crest simply erupts from the skull midline early in postnatal ontogeny. The sagittal crest continues to heighten, resulting in the tall crest that is characteristic of this species. Development of the sagittal crest is generally thought to be linked to the development of the temporalis muscle. Dissections of one adult and one neonate of T. terrestris indicate that the temporalis is strongly developed in this species and unusually extensive in its origin at an early age. A lack of comparative data forbids any conclusive statements about the relative development of this muscle as compared to that in other tapirs, but the unusual pattern of crest development seen in T. terrestris may be a consequence of the development of the temporalis muscle in this herbivorous ungulate. The pattern of crest development observed in T. terrestris is unique among tapirs, and perhaps among mammals. This explanation of the unusual sagittal crest morphology of T. terrestris is consistent with current interpretations of skull morphology and the taxonomy of Pleistocene Tapirus.