Redondasuchus reseri is a Late Triassic (Rhaetian) aetosaur from the Redonda Formation of eastern New Mexico. It differs from all other aetosaurs except Typothorax by its paramedian scutes with ventral keels. It is the only Rhaetian aetosaur currently known from the United States and is an index taxon for the Apachean land-vertebrate faunachron. A cladistic analysis of the aetosaurs, utilizing both characters derived from armor and other skeletal information, indicates that two distinct grades of aetosaurs exist. There is a basal grade consisting of the paraphyletic assemblage of aetosaurus and (Stagonolepis + Aetosauroides), and an advanced grade consisting of the clade Neoaetosauroides + Redondasuchus + ((Longosuchus + Desmatosuchus) + (Paratypothorax + typothorax)).