 Although aetosaurs are known exclusively from the Late Triassic , their currently accepted position in archosaur phylogeny indicates that they originated from more basal pseudosuchian archosaurs in the Early or Middle Triassic . Given that aetosaurs are highly specialized with many anatomical features not seen in other pseudosuchians , the group 's evolutionary origins are poorly understood . The recent discovery of complete specimens of the Late Triassic pseudosuchian Revueltosaurus callenderi indicate that it may have been close to the ancestry of aetosaurs . Several phylogenetic analyses place it as the sister taxon or closest relative of Aetosauria . Like aetosaurs , Revueltosaurus has two rows of paramedian osteoderms along its back and , in the cheek region of the skull , a maxilla that fits into a groove of the jugal bone . One phylogenetic analysis places Turfanosuchus dabanensis , a Middle Triassic pseudosuchian , as the sister taxon of Revueltosaurus and Aetosauria , potentially making it the earliest known " stem aetosaur " ( " stem " meaning that it lies on the branch that includes aetosaurs , but is not itself an aetosaur ) . 
