The terraces of the Vaal River are well-known for producing abundant vertebrate fossils that span the Plio-Pleistocene and younger geological ages (Helgren, 1977). The tributaries of the Vaal River, such as the Modder and Riet Rivers, are equally rich in Quaternary fossil vertebrates and archaeological materials. The Modder and the Riet Rivers have lower-gradient longitudinal profiles than the Vaal River (Tooth et al., 2004), flowing over less-resistant bedrock and forming floodplain deposits controlled by igneous barriers, mostly dolerite. These floodplain deposits become eroded as the rivers incise, cutting through dolerite barriers, and in the process the fossil contents of the floodplain deposits are exposed in erosional areas, locally known as dongas. Such erosional areas can be extremely rich sources of vertebrate fossils and archaeological materials. On two adjacent farms on the Modder River, “Erfkroon” and Orangia” (Figure 1; Churchill et al., 2000; Lyons et al., 2014), extensive fossil exposures were discovered in June 1996 during routine reconnaissance by the Florisbad Quaternary Research Department of the National Museum, Bloemfontein. One of the first major discoveries on the farm “Erfkroon” was a skull and horn cores of a giant alcelaphine antelope, Megalotragus priscus (Broom, 1909). This find is unusually complete with a well-preserved braincase attached to the horn cores, which provides the opportunity of improved understanding of the skull morphology and affinities of the species. The aim of this contribution is to provide a description of this find, its sedimentary context and age estimates based on Electron Spin Resonance analyses (ESR). A reconstruction of the skull of M. priscus is proposed, which is based on the Erfkroon specimen and assisted by a complete mandible and a maxillary fragment from a pan site in the central Free State Province, known as “Mahemspan” (Figure 1). The fossil materials from Mahemspan were discovered and excavated in the late 1930’s and 1940’s by staff of the National Museum (Hoffman, 1953), and in this contribution we also provide a summary taxonomic list and ESR age estimates for this fossil occurrence.