the boss of modern muskoxen (Ovibos moschatus). The nasal horncore of adult Achelousaurus had an upward slant and its upper surface had correlates for a thick epidermal (outer layer of skin) pad that graded into correlates for a cornified sheath on the sides. A thick pad of epidermis may have grown from the V-shaped pitted notch at the tip of the nasal horncore. The growth direction of the nasal pad would have been towards the front. The supraorbital bosses may have had a thick pad of epidermis, which grew at a sideways angle similar to the curved horncores of Coronosaurus, as indicated by the orientation of the "fins" or ridges on the bosses. That the supraorbital bosses lacked a sulcus (or furrow) at their bases indicates that their horn pads stopped at the wrinkled edges of the bosses. The pitting might indicate a softer growing layer connecting the hard inner bone with the hard horn sheath. In addition, correlates for a rostral scale in front of the nasal boss and scale rows along the parietal midline and supraorbital-squamosal region were identified. Evolution Horner's hypothesis of anagenesis In 1992, the study by Horner et al. tried to account for the fact that within a limited geological period of time (about half a million years) there had been a quick succession of animal communities in the upper Two Medicine Formation. Normally, this would be interpreted as a series of invasions, with the new animal types replacing the old ones. But Horner noted that the newer forms often had a strong similarity to the previous types. This suggested to him that he had discovered a rare proof of evolution in action: the later fauna was basically the old one but at a more evolved stage. The various types found were not distinct species but transitional forms developed within a process of anagenesis. This conformed to the assumption, prevalent at the time, that a species should last about two to three million years. A further indication, according to Horner, was the failure to identify true autapomorphies – unique traits that prove a taxon is a separate species. The fossils instead showed a gradual change from basal (or ancestral) into more derived characters. The horned dinosaurs discovered by Horner exemplified this phenomenon. In the lowest layers of the Two Medicine Formation, below the overlaying Bearpaw Formation, "Transitional Taxon A" was present. It seemed to be