 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 . 
