Researchers currently recognize two species: "Sphenacodon ferox" (the type species) and "Sphenacodon ferocior". "Sphenacodon ferocior" can be up to 40% larger in overall size (at about long) compared to "Sphenacodon ferox" (at about ). In addition, the dorsal spines in "Sphenacodon ferocior" are proportionately 45% taller than in "Sphenacodon ferox". The recent discovery of a nearly complete skull of "Sphenacodon ferox" has helped clarify other distinctions between the two species, including the number of teeth in certain parts of the jaws and the size of the indented notch between the maxillary and premaxillary bones in the upper jaw. The two species occur together in some formations, but "Sphenacodon ferox" apparently survived later into the Early Permian.