 Aetosaur phylogeny was first investigated in 1994 by paleontologist J. Michael Parrish. Aetosauroides, Aetosaurus, Desmatosuchus, Longosuchus, Neoaetosauroides, Stagonolepis, and Typothorax were included in the phylogenetic analysis. Aetosaurs were found to form a clade with rauisuchians, which Parrish termed Rauisuchiformes. Rauisuchiformes also included the superorder Crocodylomorpha, to which living crocodilians belong. Parrish found Aetosauria to be a monophyletic group and thus a true clade consisting of a common aetosaur ancestor and all of its descendants. To phylogenetically define Aetosauria, Parrish identified five synapomorphies, or shared characteristics. The first synapomorphy concerned the jaw, with the premaxilla at its tip being edentulous (toothless), upturned, and wide to form a "shovel". Moreover, the dentary bone in the lower jaw is also toothless, upturned, and broad. The reduced size and simple conical shape of the teeth was considered another synapomorphy. Two more synapomorphies of aetosaurs are shared with crocodylomorphs, but were not considered to be an indication of a close phylogenetic relationship; the body is covered in dorsal and ventral armor to form a complete carapace, and the paramedian osteoderms are much wider than they are long, with distinctive pitting. A final synapomorphy was found in the structure of the limb bones. In all aetosaurs, the limbs are very robust, with large muscle attachments such as the deltopectoral crest of the humerus, the fourth trochanter of the femur, the intracondylar ridge of the tibia, and the iliofibularis trochanter of the fibula.
